Published Online: 24 September 2018

Proc Indian Natn Sci Acad 85 No. 1 March 2019 pp. 189-212  Printed in India. DOI: 10.16943/ptinsa/2019/49574

Review Article Emerging Roles of Arf-Like GTP-Binding Proteins: From Membrane Trafficking to Cytoskeleton Dynamics and Beyond RITURAJ MARWAHA, DEVASHISH DWIVEDI and MAHAK SHARMA* 1Laboratory of Membrane Trafficking, Department of Biological Sciences, Indian Institute of Science Education and Research (IISER) Mohali 140 306, India

(Received on 13 August 2018; Revised on 09 December 2018; Accepted on 10 December 2018)

Members of Rab and ADP-ribosylation factor (Arf) family of small GTP-binding (G) proteins regulate several aspects of intracellular transport and cytoskeleton organization. The phylogenetic analysis, combined with database mining approaches has led to the identification of Arf-like (Arl) G proteins as a sub-group of the Arf family with approximately 20 members in mammals. Arls are similar in structure to the Arfs, but exhibit immense diversity pertaining to their mechanisms of membrane recruitment, subcellular distribution and cellular functions. Only a few members of this sub-group are currently characterized, while information on the majority of Arl proteins remains scanty or is not known. In this review, we will cover our current understanding of the functions performed by Arl sub-family members, described under three broad categories: Arls involved in primary cilia formation and function, Arls engaged in secretory and endocytic transport, and Arls regulating microtubule and actin organization.

Keywords: GTPases; GTP-binding; Arf; Arf-like; Membrane Trafficking; Lysosome

Preface divided into five families, including Ras, Rho, Ran, Rab and Arf proteins with each family specialized to The Ras superfamily of proteins function as small play distinct functions in human cells (Goitre et al., monomeric G proteins, and regulate diverse cellular 2014). This review aims to provide a broad overview processes, including cell growth, differentiation, of the Arf-like (Arl) proteins, which are part of the proliferation, migration, establishment and maintenance Arf family and have more than 20 members in humans of cell polarity, vesicle and organelle transport, and (Burd et al., 2004; Donaldson and Jackson, 2011; cytoskeletal organization (Wennerberg et al., 2005). Gillingham and Munro, 2007). The term Arl refers to Using a simple biochemical strategy of alternating a protein only structurally similar to Arfs, however, between a GTP-bound (active) and a GDP-bound Arls are different from Arfs in many aspects such as (inactive) state, members of this superfamily transduce their mechanism of membrane binding and diversity information via signaling cascades. In the active state, of functions at the cellular level. We discuss these the G proteins recruit their interaction partners, also aspects of Arl biology from their discovery to function known as effectors, and perform downstream in the next section. functions until GTP hydrolysis returns them to their inactive state. Regulatory proteins control this switch Kennison and co-workers discovered Arl between active and inactive states, i.e. guanine proteins in 1991 where they identified a Drosophila nucleotide exchange factors (GEFs) catalyze the gene encoding a protein structurally similar to the Arfs exchange of GDP for GTP and GTPase-activating of yeast and mammals, but lacking the co-factor proteins (GAPs) accelerate hydrolysis of the bound activity for the ADP-ribosylation of adenylate cyclase GTP to GDP (Fig. 1A). Based on their structure, activator Gsá by cholera toxin (known as Arf activity). sequence and function, the Ras superfamily is further Thus, the identified protein was then named as Arf-

*Author for Correspondence: E-mail: [email protected] 190 Rituraj Marwaha et al.

Fig. 1: A) The GTPase cycle. Small GTP-binding proteins cycle between the inactive GDP and active GTP binding state regulated by GEFs (promotes GDP to GTP exchange) and GAPs (hydrolyzing GTP to GDP). In their active state, GTPases recruit their downstream effectors required for various cellular functions. B) Schematic representation of Rabs, Arfs and Arls membrane association. Rab GTP-binding proteins in their GTP-bound state associate with the membrane via C-terminal geranylgeranyl moieties, which get inserted into the lipid bilayer. Arfs contain an N-terminal amphipathic helix and their second amino acid residue is myristoylated that helps to associate with the membrane. All Arls have the N-terminal amphipathic helix, which is required for membrane association. Arl1 associates with the membrane in a manner similar to Arfs (Mechanism-1), whereas, in many of the other Arls, the second amino acid myristoylation signal is absent. Some Arls like Arl8a and Arl8b have acetylation instead of myristoylation, and it is speculated that these require a receptor binding for their association to the membrane (Mechanism-2) like (Arl) protein 1 (Arl1) (Tamkun et al., 1991). eukaryotes that regulates vesicle budding at the Subsequent studies revealed the existence of endoplasmic reticulum (ER) (Donaldson and Jackson, mammalian cDNAs encoding proteins closely related 2011). to Arfs including Arl2 (Clark et al., 1993), Arl3 (Cavenagh et al., 1994), Arl4 (Schurmann et al., 1994) Arl sub-family members are highly conserved and Arl5 (Breiner et al., 1996). Arls form a large in vertebrates and some members like Arl1, Arl2 and sub-family of the Arf family with more than 20 Arl18 are present in lower organisms, such as; yeast members in humans, in comparison; there are only 6 and metazoans. Some members of the Arl subfamily mammalian Arf proteins. The six Arf proteins are also have homologues in plants, including Arl1, Arl2, classified based on their sequence homology into three Arl3, Arl5, Arl6 and Arl8. Table 1 list different Arl classes, namely, Class I (Arf1, 2 and 3), Class II (Arf4 sub-family members with information on their and 5) and Class III (Arf6) (Kahn et al., 2006). Arf subcellular distribution, orthologs, paralogs and known family also includes Sar1, a G protein conserved in all functions. Emerging Roles of Arf-Like GTP-Binding Proteins 191 ., ., et et et al et al ., ., 2015 en ., 2011; ., 2011b); Khatter Y et al ., 1993 ., 2012 ., 2012; et al ., 2007; ., 2007; ., 2007; et al ., 2006; ., 2006; ., 2002 et al et al ., 1999; Engel et al et al ., 1996; ., 2003; et al et al et al ., 2014); Wiens et al et al ., 2011; et al ., 2011 ., 2010b; et al ., 2007; ., 2012a et al et al et al et al ., 2004 et al et al et al et al ., 2015a; Marwaha ., 2010; Zhang 2017 Reference(s) Lowe Munro, 2005 Setty 2002; Clark Hofmann Antoshechkin and Han, Grayson Lin Hofmann Hofmann Jacobs et al Li Li al Garg Houghton Houghton 2011; Kaniuk Rosa-Ferreira and Munro, 2011 — Bagshaw Rosa-Ferreira and Munro, Bagshaw Rosa-Ferreira al 2011a Liew 2006; Zhang Jin Antigen Actin Antiviral Adipogenesis; Autophagosome-lysosome fusion; -induced filament formation ysosome positioning and motility; ysosome motility Function(s) Traficking at the TGN; Endosome-to- Golgi trafficking receptors Maintenance of Golgi structure; Endosome- Microtubule dynamics; mitochondrial fusion Ciliary transport and localization of sensory to-Golgi transport; Signal transduction Cholesterol secretion; Microtubule dynamics; Maintenance of mitochondrial morphology and Cell morphological changes; Signal transduction remodeling; Nuclear dynamics membrane potential; Endosome-to-Golgi transport Salmonella Endosome-to-Golgi transport; L presentation; Cargo trafficking towards lysosomes; innate immune responses L Cilia biogenesis and signaling ysosome- ysosome; L ysosome Reported localization(s) Trans-Golgi Network (TGN) Cytosol; Nucleus; Cytosol; Microtubules; Ciliary tip Mitochondria Cortical actin Cytosol; Nucleus; Cytosol; Nucleus; Cortical actin Cortical actin Golgi TGN L related organelles No information available No information available L Cilia , , erio erio erio D. rerio and and and and , D. r , D. r , D. r A. thaliana A. thaliana and and A. thaliana A. thaliana A. thaliana Reported in other organisms S. cerevisiae, C. elegans, D. melanogaster S. cerevisiae, C. elegans, S. cerevisiae, C. elegans, and D. melanogaster and and D. melanogaster D. melanogaster D. rerio D. rerio D. melanogaster D. rerio S. cerevisiae, C. elegans, D. melanogaster — D. rerio D. rerio D. rerio C. elegans, D. melanogaster S. cerevisiae, C. elegans, D. rerio C. elegans, D. melanogaster D. melanogaster — — — — Known ARL4C ARL5C ARL5C ARL5B ARL8B ARL4D ARL4D ARL8A paralogs ARL4C and ARL4A and ARL4A and ARL5B and ARL5A and ARL5A and 400 402 403 379 10124 10123 26225 55207 84100 Human 221079 390790 127829 gene ID Arl sub-family members ARL7 ent RP55 Other GIE1; GIE2; ARL4 ARL8 BBS3; fer ARL5; ARL12 ARFL1 ARFL2 ARFL3 ARF4L name(s) ARFLP5 ARL10C ARL10B LAK; able 1: Dif T ARL family member ARL1 ARL2 ARL3 ARL4A ARL4C ARL4D ARL5A ARL5B ARL5C ARL8B ARL6 ARL8A 192 Rituraj Marwaha et al.

., Membrane association of Arl GTP-binding et al ., 2005 proteins et al

1 Arf proteins, including the Arls, have a conserved ., 2009; ., 2007; Barral mode of membrane binding that distinguishes them ., 2003b; Setty ., 2017 ., 2018 ., 201 et al ., 2011 et al from the other families of G proteins. All Arf family et al et al et al et al et al ., 2012; Casalou ., 2003; Shin members harbor an N-terminal amphipathic helix ang — — Arya — Duldulao 2014 Caspary et al Paul Zhao Y — Panic et al required for membrane binding (Antonny et al., 1997). Additionally, relative to the other G proteins, the length of intervening sequence between switch-I and switch-II (known as “inter-switch”) regions is much longer in Arf family members (Burd et al., 2004; Donaldson and Jackson, 2011). Switch-I and switch-II are regions within the minimal G domain of all Ras superfamily members that form the nucleotide-binding pocket and also interact with the GEFs and GAPs. In the GDP- bound state, switch-I and switch-II regions of the Arf family members orient such that the inter- switch region is retracted, forming a pocket holding the N-terminal amphipathic helix. Once Apoptosis; Macrophage activation; Signal transduction Cilia formation and maintenance; Recycling of cargo; Cell migration Antigen presentation Signal transduction Signal transduction Endosome-to-Golgi transport the GDP is exchanged for GTP, there is a two- residue shift in the inter-switch region pulling the switch regions up, and facilitating the switch to active conformation by releasing the amphipathic helix for membrane anchorage. The nucleotide- dependent structural change phenomenon unique to the Arf family members has been termed as No information available No information available Cytosol; Nucleus; Cortical actin No information available No information available Cilia; Recycling endo- No information available No information available some; Cortical actin Multi-vesicular bodies Golgi Cytosol No information available No information available Golgi the “inter-switch toggle” mechanism (Kahn et al., 2006; Pasqualato et al., 2002). In case of Arl1, erio D. rerio D. rerio Arl2, Arl3 and Arfs (such as Arf1 and Arf6), the and and

, D. r detailed working of inter-switch toggle mechanism have been described based on structural analysis D. rerio of either the GTP- or GDP-bound states of the and proteins (Antonny et al., 1997; Pasqualato et al., D. rerio A. thaliana 2002). D. rerio and D. rerio D. melanogaster S. cerevisiae, C. elegans, C. elegans, D. melanogaster C. elegans, D. melano gaster and D. rerio D. rerio D. melanogaster — S. cerevisiae, C. elegans, D. melanogaster Another feature that contributes to the membrane association of all Arf proteins and few — — — — — — Arls (such as Arl1) is “myristoylation” - a lipid ARL9 ARL10 ARL13B ARL13A modification that occurs co-translationally and requires the presence of “glycine” at the second (+2) amino acid position. Thus, upon GTP-binding, 80117 54622 10139 285598 115761 132946 200894 392509 339231 both the N-terminal amphipathic helix and 100506084 myristoyl group are inserted into the membrane TS1 — — (Amor et al., 2001; Panic et al., 2003a). Notably, ARF7 JBTS8; ARL13 ARL ARL2L1 ARL17B ARL10A ARFRP2 ARFRP1 most Arl proteins either lack this modification or ARL17A; the signal for myristoylation is not conserved. For instance, Arl2 contains the N-myristoylation motif but is reported to lack the modification (Sharer et ARL10 ARL11 ARL9 ARL13B ARL13A ARL14 ARL15 ARL16 ARL17 ARL18 Emerging Roles of Arf-Like GTP-Binding Proteins 193 al., 2002). Arl8 paralogs, Arl8a and Arl8b, have unconventional functions of different Arl proteins. “isoleucine” and “leucine” as their particular second amino acids, which are required for acetylation and Arls in Cilia and Ciliogenesis might bind to membranes through putative acetylation Cilia are slender, microscopic, hair-like microtubule- receptors (Hofmann and Munro, 2006; Khatter et al., based structures emanating from the surface of most 2015b). Given the different mechanisms, a common eukaryotic cells that act as organelles for motility and scheme of Arls association with membranes cannot sensation. Based on their function, cilia are categorized be described yet. Further structural studies need to into two types, i.e., motile cilia that have a rhythmic be performed with individual Arls to know more about waving motion mediating fluid flow in the extracellular the mechanism of their membrane association. milieu, and non-motile or primary cilia that act as cell’s Although Arls and Arf might differ in the N-terminal antenna and harbor many sensory receptors and modification, one common feature is their close signaling molecules (Spasic and Jacobs, 2017). contact with the membrane, as compared to other G Primary cilia regulate transduction of various signaling protein subfamilies including Ras, Rho and Rabs that pathways, such as Sonic hedgehog (Shh), Wnt and have a long C-terminal linker at which the lipid Platelet-derived growth factor (PDGF), which are modification takes place (Fig. 1B). Arfs and Arls thus function close to the membrane and recruit effectors that induce membrane curvature (such as coat proteins) or modify lipids (such as phosphoinositide kinases) or tether closely apposed membranes. As the mode of membrane binding for several Arls remains unknown, so is the identity of the GEFs and GAPs for Arl proteins. This is in contrast to Arfs where the identification of GEFs and GAPs has been aided by the presence of signature catalytic domains, i.e. all Arf-specific GEFs contain a conserved Sec7 domain, and all Arf-specific GAPs contain a conserved zing-finger catalytic domain (Donaldson and Jackson, 2011). It is not known whether the Arf-specific GEFs and GAPs can also act on Arl proteins and will be a crucial question to be addressed in future studies. To summarize the current literature on Arl proteins, we have classified Arls into three broad Fig. 2: Schematic representation of the subcellular categories according to their cellular localization and distribution of the Arl sub-family members. Arl sub- function: Arls involved in primary cilia formation and family members localize to different compartments function, Arls engaged in secretory and endocytic within cells. For example, Arl1, Arl5, Arl15 and Arl18 transport, and Arls regulating cytoskeletal organization. localize to the Golgi, regulating trafficking to and from this compartment. Arl4 and its paralogs localize Like other G proteins of the Ras superfamily, Arls to actin filaments beneath the plasma membrane. function by recruiting effector proteins to specific Arl11 exhibit nucleo-cytoplasmic distribution and can subcellular locations (Fig. 2). However, some of the also be found on cortical actin structures and strategies or mode of functioning are unique to Arl regulates ERK1/2 signaling. Arl14 has been reported proteins and do not have a parallel example in Rabs to localize to MVBs in dendritic cells regulating MHC-II trafficking. Multiple localizations have been and Arfs, such as their function as a GEF for another reported for Arl2 that includes the cytoplasm, G protein or function as guanosine dissociation mitochondria and microtubules. Arl2 localize to inhibitors (GDIs)-displacement factors for allosteric mitochondria and is crucial for its function and release of lipidated proteins from their respective morphology. Arl2 has been shown to be a key GDIs or function in regulating microtubule biogenesis. regulator of microtubule dynamics. Arl8b localizes to lysosomes and regulates its positioning and fusion In the next section, we review the conventional and with other organelles 194 Rituraj Marwaha et al. crucial for embryonic development and tissue pattern interaction between the intraflagellar transport complex formation (Goetz and Anderson, 2010). Evolutionarily (IFT) and ciliary kinesin motor (Kif17), thus regulating conserved microtubule motor-mediated bidirectional intraciliary transport (Evans et al., 2010; Wright et transport by kinesin (anterograde transport) and al., 2011). Arl3 interacts with and regulates the axonemal dynein (retrograde transport) maintains the localization of Kif7, a ciliary tip kinesin that in turn sensory transduction activity of primary cilia. A wide regulates Shh signaling from cilia. Further, Arl3 in range of genetic defects like Bardet-Biedl syndrome, complex with Arl13b modulates the association Joubert syndrome, nephronophthisis, Meckel-Gruber between IFT subcomplexes (IFT-A and IFT-B) and syndrome, autosomal dominant polycystic kidney regulates intraciliary transport (Li et al., 2010). disease, and autosomal recessive polycystic kidney Recent studies have suggested a mode of action disease have already been shown to be related to for Arl3 in the cilia, where Arl3 is proposed to regulate defects in cilia biogenesis/signaling, collectively the delivery of lipidated ciliary cargo to the appropriate referred to as “ciliopathies” (Reiter and Leroux, 2017). ciliary membrane (Ismail, 2017; Ismail et al., 2012; Earlier studies have shown that amongst the Arl sub- Ismail et al., 2011). Inside the cilium, Arl3 is converted family members, Arl3, Arl6 and Arl13b play distinct to its GTP-bound form by its GEF, Arl13b, a member roles in cilia formation and function (Fig. 3). The next of the Arl sub-family that localizes to cilia (Fig. 3). section describes the role of these Arl proteins in cilia Arl3 associates with its interaction partners formation and function. Phosphodiesterase 6 delta subunit (PDE6δ) and Uncoordinated-119 protein (UNC-119), which act as Arl3 guanosine dissociation inhibitors (GDIs)-like factor for The role of Arl3 in the context of cilia biology was the shuttling of prenylated and myristoylated cargo first appreciated with the observation that Arl3 ortholog protein, respectively (Constantine et al., 2012; Linari in Leishmania donovani (LdARL-3A) regulates et al., 1999; Veltel et al., 2008; Zhang et al., 2012). flagellum biogenesis (Cuvillier et al., 2000). In this cascade of events, Arl3 functions as a GDI Furthermore, comparative genomics analysis had displacement factor or GDF to promote the release revealed that Arl3 and Arl6 are present only in the of the lipidated cargo from PDE6δ and UNC-119 ciliated organisms (Avidor-Reiss et al., 2004; Grayson (Ismail et al., 2012; Ismail et al., 2011). The process et al., 2002). More direct evidence came from the is regulated by RP-2, a GAP for Arl3, which is immunofluorescence studies showing that Arl3 localized at the base of the cilia and likely promotes localizes to the cilia in mammalian photoreceptors and GTP hydrolysis of Arl3 upon its exit from the cilium furthermore, mouse model of germ line Arl3 deletion (Gotthardt et al., 2015) (Fig. 3). Interestingly, both showed a multi-organ ciliopathy phenotype GEF (Arl13b) and GAP (RP-2) of Arl3 are mutated characterized by impaired photoreceptor development in Joubert syndrome (JS) and Retinitis Pigmentosa, and cyst formation in the kidney, liver and pancreas respectively (Cantagrel et al., 2008; Chapple et al., (Grayson et al., 2002; Schrick et al., 2006). 2000). These observations indicate that altering the Interestingly, overexpression of constitutively active levels of Arl3-GTP in the cilium can lead to ciliopathies, Arl3 ortholog in C. elegans (ARL-3 Q72L) and L. emphasizing the importance of this G protein in donovani (LdARL-3 Q70L) caused ciliogenesis regulating cilia function. defects, suggesting that Arl3 acts as a negative regulator of ciliogenesis (Cuvillier et al., 2000; Li et Arl6 al., 2010). In agreement with these studies, the Arl3 Arl6, also known as BBS3, was the first member of knockout in worms or its transient depletion in Arl sub-family associated with an autosomal recessive mammalian cells did not affect overall cilia morphology. ciliopathy, Bardet-Biedl syndrome (BBS), which is However, in mammalian cells, Arl3 depletion led to characterized by obesity, mental retardation, renal defects in ciliary transport and sensory receptor anomalies, polydactyly, retinal degeneration, and localization (Lai et al., 2011; Li et al., 2010). Further hypogenitalism (Chiang et al., 2004). Arl6 is one of studies in human retinal pigment epithelial cells have the seventeen genes implicated in BBS till now, and shown that Arl3 forms a complex with its GAP, mutations that lead to premature truncation and affect Retinitis pigmentosa-2 (RP-2) and mediates the GTP-binding of Arl6 have been reported in BBS Emerging Roles of Arf-Like GTP-Binding Proteins 195

Fig. 3: Pictorial representation of Arls and their distribution in ciliated cells. Arl sub-family members like Arl3, Arl6 and Arl13b have been reported to regulate cilia biology. Arl13b interacts with tubulin and is essential for the localization of sensory receptors. Arl13b acts a GEF for Arl3, thus activating it, which in turn associate with a carrier protein in complex with lipidated cargo. After Arl3 association, the lipidated cargo is released, and the remaining carrier protein is transported in cilia. RP2 regulates Arl3-GTP to Arl3-GDP conversion and once in the inactive state, Arl3 releases the carrier protein and comes out of the cilia. Also, Arl3 helps in regulating the localization of sensory receptors in ciliary tips. Arl6 is responsible for intraflagellar transport of BBSome-bound cargo patients (Kobayashi et al., 2009; Marion et al., 2012; can be dragged through the periciliary diffusion barrier Otto et al., 2010; Zaghloul and Katsanis, 2009). Arl6 into the cilia (Jin et al., 2010) (Fig. 3). Interaction of function is conserved across evolution, as evident from Arl6 with the BBSome complex is also highly the BBS-associated phenotypes observed inzebrafish conserved across evolution with residues implicated and mice lacking Arl6 expression (Yen et al., 2006; in binding of Arl6 to BBS1 conserved between Zhang et al., 2011). Chlamydomonasrein hardtii and humans (Lechtreck et al., 2009; Mourão et al., 2014). Notably, ciliary Besides Arl6, eight other BBS proteins, including GPCRs, smoothened (smo) and somatostatin receptor BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9 and 3 (Sstr3) activation trigger Arl6-dependent BBIP10 form a coat-like ciliary trafficking protein polymerization of BBSome, which is required for the complex (known as the BBSome) that is highly exit of activated GPCRs from the cilia (Ye et al., conserved across evolution and mediate trafficking 2018). The nucleotide-free form of Arl6 directly binds towards the primary cilium (Berbari et al., 2008; to IFT27 (intraflagellar transport 27) and favor exit Blacque et al., 2004; Nachury et al., 2007). GTP- of the BBSome complex from cilia. Also, Arl6 binding of Arl6 results in recruitment of the BBSome- regulates the retrograde transport of Shh and Wnt associated cargo complex to ciliary membranes that signaling receptors inside the cilia (Liew et al., 2014; 196 Rituraj Marwaha et al.

Wiens et al., 2010; Zhang et al., 2011). Taken together, (Cantagrel et al., 2008). The hennin mice that carry these pieces of evidence suggest that the symptoms a null mutation at the Arl13b gene locus resemble the of BBS upon Arl6 mutation might be due to the mutant phenotypes observed in JS patients. These reduced targeting of BBSome to the cilia, resulting in mice show coupled defects in cilia structure, defect loss of sensory receptors or molecules from cilia in Shh and BMP (Bone morphogenetic protein) (Mourão et al., 2014). In contrast to its role in signaling pathways and abnormal expression of Wnt regulating cilia signaling, Arl6 is dispensable for cilia ligands (Horner and Caspary, 2011; Larkins et al., formation, as evident by normal primary cilia 2011). Ultrastructure analysis of cilia in the hennin morphology and numbers in mice knock out for Arl6 mutant mouse revealed not only a defect in the ciliary (Zhang et al., 2011). length but also truncated ciliary axoneme with open B-tubules of the outer microtubule doublets (Caspary Notably, among all the Arls reported in et al., 2007). A defect in B-tubule closing was also ciliogenesis or ciliary functioning, Arl6 is the only G observed in Arl13 mutant C. elegans, indicating protein reported to move inside the cilia at typical IFT conservation of Arl13b function in cilia formation and rates, although Arl6 is not an integral IFT component functioning (Horner and Caspary, 2011; Larkins et (Fan et al., 2004). How Arl6 moves with IFT and al., 2011). A recent study has shown that Arl13b whether it regulates cargo motility is not yet known. interacts with Vangl2 (Vang (Van Gogh, Drosophila- A recent study has shown the role of Arl6 in trafficking like 2) to regulate cilia length (Song et al., 2016). of activated ciliary receptors by selective permeation through transition zone (Ye et al., 2018). How Arl6 Arl13b not only regulates cilia ultrastructure but regulates this selective permeation is still an open also governs localization of transmembrane ciliary question for further investigation. proteins and trafficking of sensory receptors inside cilia (Cevik et al., 2010; Li et al., 2010; Liem et al., Arl13b 2012). For instance, Arl13b regulates the localization and trafficking of Smoothened (Smo) receptor, a sonic Arl13 has two paralogs, Arl13a and Arl13b, conserved hedgehog signaling inhibitor, inside the cilia. GTP- in C. reinhardtii, C. elegans, D. rerio and mammals. bound Arl13b also directly interacts with the Sec5 The two paralogs share 43% sequence similarity in and Sec8 subunits of the exocyst complex and case of humans. Whereas the role of Arl13b in cilia regulates ciliogenesis. This interaction is required for biology is well established in different organisms (as functional cilia, as depletion of either Arl13b or subunits discussed below), the sub-cellular function of Arl13a of exocyst complex impair formation of functional cilia is yet to be explored. A recent study using zebrafish (Seixas et al., 2016). In neurons, Arl13b regulates (D. rerio) as the model organism recognizes Arl13a primary cilia-mediated signaling of ciliary GPCR and Arl13b to be having a separate temporal expression (Sstr3), which in turn, regulates synaptic connectivity in early zebrafish embryos, speculating different and the interneuron morphology (Ye et al., 2018). functions for the two paralogs (Song and Perkins, Notably, SUMOylation modification of Arl13b reported 2018). Arl13b was first identified in zebrafish as one in both C. elegans and mammalian cells regulated of the genes responsible for polycystic kidney disorder Arl13b function in mediating ciliary targeting of the and was closely related to Chlamy domonas genes sensory receptors but was not required for ciliogenesis encoding components of IFT particles (Sun et al., (Li et al., 2012b). A recent study has characterized 2004). A later study showed that zebrafish Arl13b is tubulin as a direct binding partner of Arl13b and this enriched in the cilium and is essential for cilia formation binding was required for the homogenous distribution in multiple organs. In agreement with these findings, of Arl13b along the ciliary membrane. Further, this Arl13b mutants in zebrafish showed cilia-associated study also suggests that correct distribution of signaling phenotypes, including cystic kidney and body curvature proteins also depends upon the interaction of Arl13b (Duldulao et al., 2009). Mutations in the G domain of with the microtubules of the ciliary axoneme Arl13b (R79Q) that significantly reduce its ability to (Revenkova et al., 2018) (Fig. 3). bind GTP have been linked to the classical form of JS, an autosomal recessive disorder arising due to Recently, Arl13b was identified as a GEF for coupled defects in cilia structure and cilia signaling Arl3, which as previously described, regulates the Emerging Roles of Arf-Like GTP-Binding Proteins 197 allosteric release of lipidated cargo inside the cilium from that compartment by recruiting their downstream (Ismail, 2017). The GEF activity of Arl13b is mediated effectors. Members of the Ras super-family of small by its G domain and additional helix present in the GTPases, i.e. Rabs, Arfs and Arls are known to unusually long C-terminal region of Arl13b. The C- regulate vesicular trafficking in cells (Khatter et al., terminal region of Arl13b harbors a coiled-coil domain 2015b). As previously described, Arfs and Arls and a proline-rich domain (PRD). Indeed, both the effectors are constrained close to the membrane GTPase and the C-terminal domain of Arl13b were surface and therefore include proteins that induce required for cilia formation and functioning (Li et al., membrane curvature (such as coat proteins), or modify 2010) (Fig. 3). lipids (such as phosphoinositide kinases) or tether closely apposed membranes (Fig. 1B). Below we Interestingly, besides its function in regulating summarize the information on Arl proteins and their cilia structure and function, Arl13b colocalizes with effectors implicated in membrane trafficking. markers of recycling endosomes, Arf6 and Rab22a, and regulates the recycling of endocytosed cargo Arl1 (Barral et al., 2012). Arl13b colocalizes and interacts with the actin cytoskeleton and regulates cell migration Arl1 is the first Arl sub-family member to be identified by promoting the formation of circular dorsal ruffles and is one of the well-characterized Arl proteins. It is (Casalou et al., 2014). Non-muscle myosin heavy conserved from yeast to mammals and is also found chain IIA (also known as Myh9) was identified as an in plants and protozoa (Munro, 2005). Studies in Arl13b effector that was necessary for Arl13b and multiple model organisms, including yeast and fruit actin interaction, and consequently for circular dorsal flies, as well as in mammalian cells have shown that ruffles formation and cell migration (Casalou et al., Arl1 localizes to the trans-Golgi network (TGN) 2014). Other studies showing the defective migration (Lowe et al., 1996). The primary function of Arl1 in of interneurons in Arl13b knockout mice as well as in different organisms is to regulate the trafficking of fibroblasts from the hennin mice support the role of various cargos at the TGN. The effectors of Arl1 Arl13b in cell migration (Higginbotham et al., 2012; belong to three major categories: GRIP domain Pruski et al., 2016). proteins, Arfaptins, and Arf-GEF complex. The GRIP domain-containing proteins, Golgins act as tethering Arls in Secretory and Endocytic Transport factors during vesicle fusion and are required for Pathways maintaining the architecture of Golgi complex (Yu and Lee, 2017). In mammalian cells, Arl1 interacts with Eukaryotic cells contain an extensive endomembrane two GRIP domain-containing proteins, namely Golgin- system involved in material exchange both within the 97 and Golgin-245. Arl1 and Golgin-97 interaction have cell and with the extracellular environment. This been reported to regulate the transport of vesicles exchange of material is essential for maintaining containing E-cadherin and Interleukin (IL)-10 thus cellular homeostasis and takes place via three major modulating cell polarity and innate immunity (Lu et trafficking pathways: secretory, endocytic, and al., 2004). The GTP-bound Arl1 recruits Golgin-245 recycling. The cargo for exchange is packaged into to the Golgi membrane, which in turn regulates vesicles at the donor compartment, which then buds trafficking of TNF (Tumor necrosis factor)-á, IL-6 off and is released into the cytosol. The vesicle then and promotes lipid droplet and chylomicrons formation moves on the microtubule track and upon reaching (Panic et al., 2003a; Wu et al., 2004). the acceptor compartment; it fuses with the acceptor membrane to release its content. Various classes of Human Arl1 also plays an essential role in proteins including coat proteins, small GTPases, regulating protein secretion and does so through the molecular motors, tethering proteins and SNAREs are action of its downstream effectors like Arfaptin-1 and responsible for the tight regulation of membrane Arfaptin-2. Arfaptins are BAR domain-containing trafficking pathways (Huotari and Helenius, 2011; proteins, which interact with Arl1 through this domain Naslavsky and Caplan, 2018). Small GTPases are (Nakamura et al., 2012). Studies have shown that the master regulators of the compartment that they Arl1 interaction with Arfaptin-1 regulates insulin localize to, regulating the trafficking of cargo to and secretion from the pancreatic β-cells, and also 198 Rituraj Marwaha et al. regulates trafficking of acidic secretory protein Arl5 chromogranin-A in neuroendocrine cells (Cruz-Garcia et al., 2013; Gehart et al., 2012). Also, Arl1 forms a Arl5 is a Golgi-localized G protein, which is absent in multiprotein complex with inactive protein kinase D2 yeast but is conserved in higher eukaryotes, with two (PKD2), Arfaptin-2 and Arf1 to regulate the secretion paralogs in humans, namely Arl5a and Arl5b of the matrix metalloproteinase in pancreatic cancer (Houghton et al., 2012). Studies in mammalian cells cells (Eiseler et al., 2016). Arl1-Arfaptin interaction with stable over-expression of Arl5b showed is also reported in Drosophila where it has been enhanced trafficking of TGN38 cargo from implicated in the maintenance of neuronal growth and endosomes-to-Golgi, while the reverse phenotype was development (Chang et al., 2015). The fruitfly ortholog observed upon its depletion. These findings were of mammalian Arl1 was also found to be essential for further supported by the observation that Arl5b the centrosome cycle, wing development and salivary recruits the tethering factor GARP complex at the granule formation (Torres et al., 2014). TGN, known to regulate endosomes-to-Golgi trafficking (Houghton et al., 2012; Rosa-Ferreira et Previous studies have shown that Arl1 recruits al., 2015). Arl5b not only participates in the membrane Arf-GEFs, brefeldin-A inhibited guanine nucleotide- trafficking pathway but also plays a role in host antiviral exchange proteins (BIGs)-BIG1 and BIG2 in innate immune response by negatively regulating mammalian cells and their orthologs, Sec7 (ortholog melanoma differentiation-associated gene-5 (MDA- of BIG1) in yeast and Sec71 (ortholog of BIG1 and 5) signaling, which is required for the IFN (Interferon)- BIG2) in the fruitfly, to the TGN (Christis and Munro, β pathway activation (Kitai et al., 2015). Despite 2012; Richardson et al., 2012). It has been emerging new functions of Arl5, the identity of protein demonstrated in Drosophila that Arl1 and Sec71 partners regulated by Arl5b needs further investigation. recruit AP-1 (adaptor protein-1) and regulate normal salivary granule formation (Torres et al., 2014). In Arl8 yeast cells, Arl1 deletion is viable but shows defects Arl8 is the only known Arl protein primarily localized in potassium ion uptake, cell wall integrity and leads to lysosomes, the organelle responsible for degradation to hypersensitivity to salt, high temperature, increased of extracellular and intracellular cargo. It is highly pH, and other stress stimuli, suggesting a role of Arl1 conserved from protozoan to metazoans, lost in yeast in providing stress tolerance to the organism but reappeared in higher-order organisms. In (Maresova et al., 2012). Further, in yeast cells, Arl1 organisms such as Drosophila melanogaster, might have an overlapping function at the TGN with Trypanosoma cruzi and Caenorhabditis elegans, the tethering factor, GARP (Golgi associated a single gene encodes for Arl8, whereas in plants like retrograde protein) complex, as suggested by the Arabidopsis thaliana and Nicotiana tabacum, there synthetic lethality and physical interaction of Arl1 with are four Arl8-related genes. In mammals, two Arl8 Vps53, a subunit of the GARP complex (Panic et al., paralogs are present; Arl8a and Arl8b that share 91% 2003b). Arl1 in complex with Arl3 has also been sequence identity. We refer the readers to an extensive reported to regulate selective autophagy in yeast cells, review of Arl8 for details on its discovery, structure, where it modulates Atg9 trafficking at the TGN (Wang mechanism of lysosomal membrane association and et al., 2017a). functions (Khatter et al., 2015b). Like for other known small G proteins, a set of Initial reports on Arl8b suggested its localization specific GEFs and GAPs regulate the membrane on the mitotic spindle with a role in chromosome recruitment and GTP/GDP cycle of Arl1. In case of segregation during mitosis (Okai et al., 2004). Soon, yeast cells, the Arf-GEF Syt1 (Synaptotagmin 1) and these findings were refuted with the data showing Arf-GAP Gcs1 are characterized as regulators of lysosomal localization of both Arl8 paralogs and their Arl1 activation, whereas no GEFs or GAPs for Arl1 role in regulating lysosome motility on the microtubule are known in humans (Yu and Lee, 2017). Future tracks (Bagshaw et al., 2006; Rosa-Ferreira and studies will reveal how Arl1 is regulated in mammals Munro, 2011). Subsequent studies identified SKIP (Sif- and shed light on the detailed mechanism of their A and kinesin-interacting protein; also known as action. PLEKHM2) as the downstream effector of Arl8b Emerging Roles of Arf-Like GTP-Binding Proteins 199 that mediated kinesin-1 recruitment to promote cell studies, BORC complex did not show any GEF microtubule-based motility of lysosomes towards the activity towards Arl8b, while in C. elegans BORC cell periphery (Boucrot et al., 2005; Rosa-Ferreira subunit, SAM-4, was shown to have GEF activity and Munro, 2011). Accordingly, siRNA-mediated towards Arl8 and BORC-mediated activation of Arl8 depletion of both Arl8b and SKIP in mammalian cells was essential for the transport of SVPs (synaptic resulted in perinuclear clustering of lysosomes. Later vesicle precursor) in axons (Niwa et al., 2016). In studies have revealed that Arl8b-SKIP complex also neuronal cells, the BORC-Arl8-SKIP-Kinesin-1 regulatesmotility of lysosome-related organelles (lytic complex has been shown to regulate lysosome granules) in NK cells, and lysosome tubulation in movement specifically in axons and not in dendrites. macrophages and dendritic cells (Mrakovic et al., The axonal lysosomal movement was also shown to 2012; Tuli et al., 2013). Arl8b and kinesin-1 have been be essential for maintaining the growth cone and reported to promote the plus-end-directed movement turnover of autophagosomes in distal axons (Farias of late-endosomal protein complex p14-MP1 to focal et al., 2017). Findings from two recent studies suggest adhesions, leading to their disassembly and turnover, that under nutrient-rich conditions, BORC weakly thus helping the cells to migrate (Schiefermeier et associates with Ragulator, a scaffold complex that al., 2014). In C. elegans, where Arl8b effector SKIP regulates mTOR complex 1 (mTORC1). BORC is not present, Arl8 directly binds to the kinesin motor association with Ragulator recruits mTORC1 to protein Unc-104/Kif-1a and transports the pre- lysosomes, and simultaneously BORC promotes Arl8- synaptic vesicles in neurons (Klassen et al., 2010; kinesin recruitment to lysosomes, positioning them to Wu et al., 2013). As described in a recent study, Arl8 the cell periphery and leading to mTORC1 activation in its GTP-bound state is recruited to the membranes (Filipek et al., 2017; Pu et al., 2017). Interestingly, of synaptic vesicles and promotes the transition of BORC and Arl8 recruit two kinds of kinesin motors Unc-104 from an auto-inhibited state to active state, namely kinesin-1 (also known as KIF5B) and kinesin- thus facilitating the transport of vesicles on microtubule 3 (also known as KIF1A) for lysosome movement on tracks (Niwa et al., 2016). microtubule tracks (Guardia et al., 2016).

Lysosome positioning by Arl8b has been Apart from its role in lysosome positioning, Arl8b implicated in cell migration and cancer cell invasion. is also a crucial factor responsible for mediating fusion Depletion of Arl8b in prostate cancer cells led to the of lysosomes with other compartments. The set of juxtanuclear accumulation of lysosomes and resulted downstream effectors of Arl8b that facilitate fusion in reduced invasive growth and protease release to events are the HOPS (Homotypic Protein Sorting) degrade the extracellular matrix (Cabukusta and complex - a hexameric tethering complex, SKIP/ Neefjes, 2018; Dykes et al., 2016). Arl8b has also PLEKHM2 and PLEKHM1 (Khatter et al., 2015a; been shown to play an essential role in regulating the Marwaha et al., 2017) (Fig. 4). First evidence for trafficking of antigen-presenting molecules like CD1d Arl8’s role in cargo trafficking was shown in C. and MHC class II towards lysosomes (Garg et al., elegans where its loss led to an impaired fusion of 2011; Michelet et al., 2015). Together, these studies late endosomes and lysosomes (Nakae et al., 2010). provide strong evidence of Arl8’s role in regulating In mammalian cells, Arl8b directly binds to Vps41 lysosome positioning and motility in various cell types subunit of the HOPS complex, recruiting it to and conservation of this function among different lysosomal membranes that further facilitate the organisms (Fig. 4). assembly of the rest of the subunits, except for Vps39 which is brought onto these endosomes through its BORC (BLOC (Biogenesis of lysosome-related interaction with SKIP (Garg et al., 2011; Khatter et organelles complex) One-Related Complex), a multi- al., 2015a). In a recent study from our group, we subunit protein complex has been recently reported identified PLEKHM1, a known Rab7 effector, as an to regulate membrane association of Arl8b (Pu et al., interaction partner of Arl8b in mammalian cells 2015) (Fig. 4). Knockout of myrlysin, a BORC- (Marwaha et al., 2017; McEwan et al., 2015). specific subunit, led to dissociation of Arl8b from PLEKHM1 interacts with Arl8b via its N-terminal lysosomal membranes and lysosome clustering in the RUN domain and with Rab7 via its C-terminal perinuclear region (Pu et al., 2015). In mammalian pleckstrin homology (PH) and C1 domain, thus acting 200 Rituraj Marwaha et al. as a linker between the two GTPases. It forms a Arl8b and its effectors not only support cellular platform for the assembly of the fusion machinery physiology but are also targeted by the intracellular including HOPS complex, thus mediating fusion of pathogens, such as Salmonella typhimurium and lysosomes with endosomes and autophagosomes. Both Mycobacterium tuberculosis that exploits Arl8b (and PLEKHM1 and SKIP/PLEKHM2 interact with Arl8b its effectors) for its survival and replication in via their respective RUN domains, thus competing mammalian cells (Michelet et al., 2018; Sindhwani et with each other for binding to Arl8b and regulating al., 2017). Arl8b depletion was observed to hamper lysosome distribution in an opposing manner Salmonella-induced filament (Sif, a structure (Marwaha et al., 2017). It will be interesting to important for bacterial pathogenesis) formation determine the upstream signaling pathway and stimuli (Kaniuk et al., 2011). In a recent study from our group, that leads to preferential binding of one effector of we have reported that HOPS complex is recruited to Arl8b over the other. For instance, given the current the vacuolar survival and replicative niche of the knowledge about PLEKHM1 and SKIP/PLEKHM2 pathogen in an Arl8b-dependent manner that helps as effectors of Arl8b and their roles in autophagic the bacteria-containing vacuole to gain access to host cargo clearance and lysosome distribution, it is membrane and nutrition (Sindhwani et al., 2017) (Fig. interesting to speculate that Arl8b affinity for 4). PLEKHM1 versus SKIP might depend upon the nutrient status of the cell.

Fig. 4: Arl8 and its downstream effectors. Arl8b localizes to the lysosomal membranes in the presence of multi-subunit complex BORC. Once on the membranes, in its GTP-bound state, Arl8b binds to its downstream effectors like SKIP, HOPS complex, and PLEKHM1 to regulate lysosome positioning and cargo degradation. The Arl8b-SKIP-HOPS complex machinery is exploited by pathogens like Salmonella for establishing infection in host cells Emerging Roles of Arf-Like GTP-Binding Proteins 201

Earlier studies have documented that infection embryonic development, as its deletion in mouse with virulent M. tuberculosis causes necrosis of the embryos was lethal (Mueller et al., 2002; Zahn et al., macrophages, a form of cell death, by inflicting plasma 2006). Localization studies in yeast (Arl3 is the yeast membrane lesions, which in turn allows the bacteria ortholog of Arfrp1) and mammalian cells showed that to escape from their host macrophage and infect new Arfrp1 localizes to the trans-Golgi compartment and cells. In contrast, when macrophages are infected promotes the recruitment of Arl1 and its downstream with avirulent M. tuberculosis, these lesions are effectors; GRIP domain-containing proteins including rapidly resealed by a repair mechanism (termed as Golgin-97 and Golgin-245, to the Golgi (Panic et al., plasma membrane repair) dependent on lysosome 2003b; Setty et al., 2003; Shin et al., 2005). Arfrp1 recruitment (Behar et al., 2010; Divangahi et al., also interacts with ARF1 GEF cytohesin and regulate 2009). However, mechanisms of plasma membrane the ARF-dependent inhibition of Phospholipase D repair induced in response to M. tuberculosis (Schurmann et al., 1999). Arfrp1 also regulates the infection remain unknown. In a recent study, it has trafficking to and from TGN of cargo, such as; E- been reported that the presence of Arl8b is crucial to cadherin, GLUT4, Vangl2, VSVG and endosome-to- control the death outcome of the M. tuberculosis- Golgi delivery of Shiga toxin (Guo et al., 2013; Hesse infected macrophages, as silencing of the Arl8b gene et al., 2010; Shin et al., 2005; Zahn et al., 2008). significantly increased the level of necrotic cells upon Recent studies shed light on the role of Arfrp1 in avirulent M. tuberculosis infection (Michelet et al., modulating the lipid metabolism in cells. Arfrp1 2018). Together, these results provide mechanistic disrupted mouse models show reduced amounts of insight into how M. tuberculosis evades membrane brown and white adipose tissues and hampered repair to thrive inside the host cells. Also, in plants, chylomicron formation, indicating an essential role of Arl8 is shown to act as a crucial host factor required Arfrp1in lipid metabolism (Hesse et al., 2013; Hommel for Tomato Mosaic virus (ToMV) pathogenesis et al., 2010). (Nishikiori et al., 2011). These studies show that Arl8 is an essential factor regulating lysosome biology and Arls Involved in Cytoskeletal Organization further investigation of its diversity in function and Several of the Arl sub-family members have been effectors will help in understanding the underlying reported to interact with the microtubule (MT) and regulatory mechanisms of lysosome function. actin cytoskeleton and regulate their organization Arl14 within the cell. These include Arl2, Arl4 and its paralogs, and Arl11. This list will expand as the function Arl14 was identified as one of the candidates in a and interaction partners of Arl sub-family members genome-wide siRNA screening for host factors in are explored in the future. For instance, recently dendritic cells that regulates antigen presentation by Arl13b has been shown to interact with tubulin and MHC class II molecules (Paul et al., 2011). Arl14 this interaction was crucial for the correct distribution was reported to localize to the multi-vesicular bodies of signaling proteins in the cilium (Revenkova et al., and colocalized with PIP5K1A, a phosphatidyl-kinase. 2018). We summarize below the current The authors reported that Arl14 regulates the vesicular understanding on the role of Arl proteins in cytoskeletal transport of MHC class II on actin cytoskeletal tracks organization. by recruiting its effector ARF7EP, which in turn, binds and recruitsthe actin-based motor protein myosin-1E Arl2 for transport of these vesicles. It will be important to Arl2 was serendipitously discovered during cloning analyze Arl14 localization and interaction partners in of human Arfs and is a highly conserved Arl sub- other cell types apart from immune cells. family member with orthologs present from yeast to Arfrp1/Arl18 humans and even in plants (Clark et al., 1993; McElver et al., 2000). Subsequent studies showed that Arl2 Arfrp1, also referred to as Arl18, is conserved from binds with tubulin-folding cofactor D, which mediates yeast to mammals and its orthologs are also found in its interaction with á-tubulin, indicating a possible role plants and Dictyostelium. Initial studies on Arfrp1 for Arl2 in regulating MT assembly (Bhamidipati et suggested that it is an essential factor for the early al., 2000; Kahn et al., 2005). Arl2 role in MT 202 Rituraj Marwaha et al. polymerization was confirmed through genetic studies cargo to different intracellular locations. Arl2 exclusion conducted in multiple organisms and has established from cilia suggests that Arl3 performs a similar function Arl2 as a regulator of cofactors involved in áâ-tubulin for lipidated cargo targeted for delivery towards the folding and degradation (Antoshechkin and Han, 2002; cilium. What is unclear is that whether these functions Hoyt et al., 1990; Price et al., 2010; Radcliffe et al., of Arl2 are linked to its role in regulating MT dynamics 2000). MT polymerization and degradation is regulated and will need further exploration. by an evolutionarily conserved tubulin chaperone complex (TBC complex) comprising of five subunits Arl4 (TBCA-E) (Tian and Cowan, 2013). Arl2 interacts Arl4 form a sub-group within the Arl sub-family having with TBCC subunit of TBC complex and this three paralogs: Arl4a, Arl4c and Arl4d. They differ interaction strongly activates GTP-hydrolysis of Arl2, from other members of the Arl sub-family by an suggesting TBCC acts as a GAP for Arl2 (Mori and extended C-terminal region containing stretches of Toda, 2013; Nithianantham et al., 2015). Recent basic amino acids and a short insertion in the inter- studies have shown that binding of the Arl2-TBC switch region (Pasqualato et al., 2002). The unique complex to áâ-tubulin is GTP-dependent and is basic residues at the C-terminus of Arl4 proteins form required for MT polymerization (Francis et al., 2017a; the binding site for importin-á and function as nuclear Francis et al., 2017b). Disruption in Arl2 activity or localization signal for the Arl4 paralogs (Jacobs et depletion of tubulin-chaperone subunits results in loss al., 1999). Notably, gene expression of the Arl4 of MT dynamics, resulting in defects in cell shape, paralogs is developmentally regulated as well as with polarity, and severely impairs mitotic and meiotic the stages of differentiation and is tissue-specific (Lin spindle formation (Chen et al., 2016; Long et al., 2015; et al., 2000; Lin et al., 2002; Schurmann et al., 1994). Price et al., 2010; Zhou et al., 2017). The information Initial in situ hybridization experiments in rats revealed on the role of Arl2 in MT depolymerization is limited, Arl4a mRNA expression in germ cells of testes and and the role played by individual subunits of TBC also coincided with the development of the brain, complex in regulating the MT dynamics needs further suggesting its role in neurogenesis and somitogenesis investigation. Higher resolution structural studies on (Jacobs et al., 1998; Lin et al., 2000). Arl4a knockout Arl2-TBC chaperones in different GTP-hydrolysis mice showed reduced sperm count; however, the and áâ-tubulin bound states will reveal the nature of progeny size and frequency were not affected, biogenesis and degradation of MTs. A crucial question suggesting that mouse Arl4a is not essential for germ that remains unanswered is how do the levels of cell formation or development (Schurmann et al., soluble áâ-tubulin changes with the lifespan of cells 2002). and during intensive polymerization processes like cell division? How does in vivo soluble áâ-tubulin Interestingly, ectopic expression of the tagged concentration affect MT dynamics, and more versions of all three Arl4 paralogs revealed their pre- specifically how all these processes are regulated by dominant localization to the plasma membrane with Arl2-TBC chaperone? some nuclear accumulation for Arl4d. The plasma In addition to regulating MT dynamics, Arl2 also membrane localization of Arl4 protein required both; localizes to mitochondria and in association with its the recognition motif for myristoylation, and the stretch GAP, ELMOD2 (ELMO domain containing 2), of basic residues at their C-terminus (Hofmann et regulate mitochondrial fusion, motility and ATP levels al., 2007). Screening for Arl4 binding partners led to (Newman et al., 2017; Newman et al., 2014). In plants the identification of ARNO (Arf nucleotide-binding and worms, Arl2 orthologs are shown to regulate site opener), a GEF for small G protein Arf6, as an cytokinesis (McElver et al., 2000). Recent studies effector for Arl4 paralogs. Expression of Arl4a, Arl4c have also revealed the role of Arl2 along with closely and Arl4d were sufficient to recruit ARNO (also related subfamily member, Arl3, in trafficking of known as cytohesin-2) and its other relatives lipidated cargo by interaction with carrier proteins cytohesin-1, Grp1/cytohesin-3, and cytohesin-4 to the including PDE6ä and UNC-119 (Fansa et al., 2016; plasma membrane (Hofmann et al., 2007). Thus, Arl4 Ozdemir et al., 2018). It is likely that the major cellular paralogs function as the upstream regulator of ARNO function of Arl2 is regulating the trafficking of lipidated to promote activation of Arf6, which in turn regulates Emerging Roles of Arf-Like GTP-Binding Proteins 203 actin remodeling, endocytosis, and cell adhesion. biology was revealed in a large scale screening study Supporting the role of Arl4a as a critical regulator of where a higher frequency of truncated Arl11 actin dynamics, it was shown to interact with ELMO, (Trp149Stop) protein and down-regulation of Arl11 which further remodels actin cytoskeleton via the expression by hypermethylation of its promoter was DOCK-180 and Rac signaling pathway (Patel et al., explicitly found in the cancer cells (Yendamuri et al., 2011). A recent functional characterization study for 2007). In subsequent studies, a mutation in Arl11 gene Arl4a has also revealed that it partly localizes to the leading to either premature truncation (Trp149Stop) TGN and interacts with the Golgi resident protein or point mutation (Cys148Arg) was found in various GCC185 to regulate the Golgi organization and cancer types (Yendamuri et al., 2008). The Arl11 endosomes-to-Golgi trafficking (Lin et al., 2011). expression is down-regulated by promoter methylation in lung carcinomas, and restoration of Arl11 expression Arl4c (more commonly known as Arl7) is the in lung cancer cell lines by adenovirus transduction or closest relative of Arl4a with 71% identity at amino by treatment with a demethylating reagent led to up- acid level. An initial study on Arl4c suggested that it regulation of pro-apoptotic factors and tumor cell death is a nuclear protein as its EGFP-tagged version (Yendamuri et al., 2007). localized inside the nucleus (Jacobs et al., 1999). A subsequent study has characterized Arl4c as one of Although Arl11 was shown to be a tumor the key players in the cholesterol export pathway suppressor, not much was known about its cellular (Engel et al., 2004). Further experiments using gain- functions, mechanism of action or downstream of-function liver X receptor (LXR) systems, identified effectors. Expression profile data suggested Arl4c as a direct target of LXR that regulates expression of Arl11 in lymphoid tissues like spleen, cholesterol efflux (Hong et al., 2011). Interestingly, bone marrow and lymph nodes (Siltanen et al., 2013; Arl4c was also shown to interact with á-tubulin and Yendamuri et al., 2007). A recent study revealed that regulate transferrin receptor recycling from early Arl11 localizes in the nucleus, cytosol and cortical actin endosomes, suggesting a role in membrane trafficking structures (Arya et al., 2018). The authors found that pathway (Wei et al., 2009). Arl11 is a crucial factor for macrophage activation, Arl4d shares 59% sequence identity with Arl4c as its down-regulation led to low levels of pro- at amino acid level. Arl4d localize to the plasma inflammatory cytokine production by LPS-stimulated membrane in its GTP-bound state and promotes macrophages, and inability to control Salmonella recruitment of ARNO to the cell surface (Hofmann growth in macrophages. The mechanism described et al., 2007). However, the GTP-binding defective proposes that Arl11 interacts and colocalizes with form of Arl4d (T35N) localized to the mitochondria ERK1/2 (MAPK downstream of Toll-like receptor 4 and its expression dissipated the mitochondrial (TLR4) signaling) on the actin cytoskeleton and membrane potential, leading to fragmentation of the promotes ERK1/2 phosphorylation, which is required organelle (Li et al., 2012a). The authors also found for pro-inflammatory cytokine production upon LPS/ Arl4d localization to mitochondria under endogenous pathogen stimuli. Further, ectopic expression of Arl11 conditions, indicating that GDP-bound Arl4d might led to prolonged activation of the ERK1/2 pathway, have a function in regulating mitochondria morphology leading to upregulation of the apoptotic machinery and function. (Arya et al., 2018; Platko et al., 2018). This study is the first to report on the cellular function of Arl11. Arl11 With the current knowledge about Arl11, it holds a lot of potential to be further explored as a pro- Arl11, also known as ARLTS1 (ADP-ribosylation inflammatory, tumor suppressing candidate protein that factor-like tumor suppressor gene-1), was initially can be a crucial target for the treatment of various identified as a putative tumor suppressor gene located carcinomas and pathogen encounters. on the chromosome locus 13q14.3, genomic region often deleted in a variety of hematopoietic and solid Functions of Other Less Characterized Arls tumors (Calin et al., 2005). Arl11 is a 196 amino acids long protein with homologs in the fruit-fly, zebrafish, As mentioned in Table 1, Arl sub-family comprises plants and mammals. The first link of Arl11 to tumor nearly 20 members, and for many subfamily members 204 Rituraj Marwaha et al. no function has been ascribed so far or very little Arl16 information is available. The next section summarizes the limited information/functions available for such Arl16 was initially identified as an early stage signature Arl sub-family members. gene for the Alzheimer’s disease in a gene expression- profiling screen done on mouse models of Alzheimer’s Arl9 disease (Arisi et al., 2011). A single report till date on Arl16 identifies it as a negative regulator of RIG-1 Arl9 is a 187 amino acid long protein that has remained (retinoic acid inducible gene-1), a receptor for RNA uncharacterized to date. Bioinformatics analysis derived from viruses-mediated signaling (Yang et al., predicted that Arl9 might be affected in prostate 2011). The authors found that Arl16 interacts with cancer, but quantitative real time-PCR analysis of the C-terminal domain of RIG-1 and suppresses the prostate tumor samples showed no difference in Arl9 RNA-RIG-1 interaction. Depletion of Arl16 led to levels as compared to control samples (Louro et al., replication of VSVG and upregulation of virus-induced 2004). Arl9 was also reported to be one of the several IFN-â expression. The subcellular localization of over- neurogenic genes affecting developing Xenopus visual expressed Arl16 in cultured cells was observed to be system (Bestman et al., 2015). Future studies can cytosolic. The importance and diversity of physiological shed light on the localization and physiological functions roles played by Arl16 can only be appreciated with of this Arl sub-family member. efforts towards detailed characterization of Arl16 to know more about its tissue expression levels, Arl10 downstream effectors, GEFs and GAPs. Arl10 is another Arl protein subfamily member that has remained uncharacterized to date. Interestingly, Conclusion the only report based on a two-step genome-wide Arl proteins were discovered more than two decades association study (GWAS) suggests Arl10 is a gene ago, yet we have only begun to understand their associated with social conformity (Chen et al., 2018). physiological functions. There are around 20 members of this sub-family of proteins in mammals, but only a Arl15 few members such as Arl1, Arl2, Arl8b, and ciliary Arl15, also known as Arfrp2, is widely expressed in Arls-3, 6 and 13b have been studied in depth. Arls, human tissues with maximum expression levels in similar to Arf proteins, recruit effectors that function skeletal muscles (Richards et al., 2009). Various closely apposed to membranes. Examples of these GWAS have identified Arl15 as a gene influencing associations include coat adaptors (e.g., BBSome adiponectin levels and associated with metabolic recruited by Arl6), tethering factors (e.g., Golgins disorders such as childhood obesity, type II diabetes, recruited by Arl1 or HOPS complex recruited by and coronary heart disease (Breitfeld et al., 2012; Arl8b), and GEFs (e.g., ARNO recruited by Arl4 Dahlman and Arner, 2010; Glessner et al., 2010; paralogs). Growing evidence suggests that Arl proteins Richards et al., 2009). Population-specific genetic play unconventional roles to those generally attributed studies identified Arl15 as a non-HLA gene to the small G proteins, such as the recently-described influencing rheumatoid arthritis susceptibility in North role of Arl13b as a GEF for Arl3 or Arl2/Arl3 function Indians and Han population from Northwest China as GDFs to promote the release of GDI-bound (Negi et al., 2013; Wang et al., 2017b). Given the lipidated cargo to specific sub-cellular locations. high association of Arl15 with diabetes, a recent study Several Arl proteins including Arl2, Arl4, Arl11 and has shown that Arl15 is up-regulated and associated Arl13b bind to, and regulate microtubule and actin with the Golgi membranes upon insulin stimulation and cytoskeleton organization. The interaction of Arl depletion of Arl15 hindered the activation of the insulin proteins with actin cytoskeleton has been shown to signaling pathway (Zhao et al., 2017). It will be be important for processes such as cell migration and important to identify the upstream molecular players assembly of signaling complexes. Due to the lack of that promote membrane recruitment of Arl15 upon information on the majority of Arl sub-family members, insulin stimulation. it is not known whether regulating cytoskeleton organization and linking membrane-cytoskeleton will Emerging Roles of Arf-Like GTP-Binding Proteins 205 ultimately come out to be a general feature of this Acknowledgements subfamily. For most Arl proteins, we are yet to uncover their subcellular localization, interaction partners and R.M. and D.D. acknowledge financial support from regulatory factors such as GEFs and GAPs. the Indian Institute of Science Education and Research (IISER) Mohali and CSIR-UGC, respectively. M.S. In conclusion, Arl proteins form a large subgroup acknowledges financial support from the Wellcome within the Arf family with several members that are Trust/Department of Biotechnology (DBT) India highly conserved across evolution. The expansion of Alliance (Grant number: (IA/I/12/1/500523), Science this sub-family with increasing complexity of functions & Engineering Research Board (SERB)-Department at the cellular level, as in the case of higher of Science & Technology (Grant number: EMR/2017/ eukaryotes, suggests that like Rabs, Arl proteins have 002273) and IISER Mohali. M.S. is a recipient of a “characteristic” role to perform at diverse sub- Wellcome Trust/DBT India Alliance Intermediate cellular locations. As part of future studies, it will be Fellowship, INSA Medal for Young Scientist (2018) important to identify sub-cellular functions of the and NASI-Young Scientist Platinum Jubilee Award majority members of the Arl sub-familybefore a (2018). common role can be ascribed to the Arl proteins, as has been attributed to the Rab family of small G proteins.

References of lysosomes Biochem Biophys Res Commun 344 1186- 1191 Amor J C, Horton J R, Zhu X, Wang Y, Sullards C, et al. (2001) Structures of yeast ARF2 and ARL1: distinct roles for the Barral D C, Garg S, Casalou C, Watts G F M, Sandoval J L, et al. N terminus in the structure and function of ARF family (2012) Arl13b regulates endocytic recycling traffic GTPases J Biol Chem 276 42477-42484 Proceedings of the National Academy of Sciences 109 21354-21359 Antonny B, Beraud-Dufour S, Chardin P and Chabre M (1997) N-terminal hydrophobic residues of the G-protein ADP- Behar S M, Divangahi M and Remold H G (2010) Evasion of ribosylation factor-1 insert into membrane phospholipids innate immunity by Mycobacterium tuberculosis: is death upon GDP to GTP exchange Biochemistry 36 4675-4684 an exit strategy? Nat Rev Microbiol 8 668-674 Antoshechkin I and Han M (2002) The C. elegans evl-20 gene is Berbari N F, Lewis J S, Bishop G A, Askwith C C and Mykytyn a homolog of the small GTPase ARL2 and regulates K (2008) Bardet-Biedl syndrome proteins are required for cytoskeleton dynamics during cytokinesis and the localization of G protein-coupled receptors to primary morphogenesis Dev Cell 2 579-591 cilia Proc Natl Acad Sci U S A 105 4242-4246 Arisi I, D’Onofrio M, Brandi R, Felsani A, Capsoni S, et al. Bestman J E, Huang L C, Lee-Osbourne J, Cheung P and Cline H (2011) Gene expression biomarkers in the brain of a mouse T (2015) An in vivo screen to identify candidate neurogenic model for Alzheimer’s disease: mining of microarray data genes in the developing Xenopus visual system Dev Biol by logic classification and feature selection J Alzheimers 408 269-291 Dis 24 721-738 Bhamidipati A, Lewis S A and Cowan N J (2000) ADP Arya S B, Kumar G, Kaur H, Kaur A and Tuli A (2018) ARL11 ribosylation factor-like protein 2 (Arl2) regulates the regulates lipopolysaccharide-stimulated macrophage interaction of tubulin-folding cofactor D with native tubulin activation by promoting mitogen-activated protein kinase J Cell Biol 149 1087-1096 (MAPK) signaling J Biol Chem 293 9892-9909 Blacque O E, Reardon M J, Li C, McCarthy J, Mahjoub M R, et Avidor-Reiss T, Maer A M, Koundakjian E, Polyanovsky A, al. (2004) Loss of C. elegans BBS-7 and BBS-8 protein Keil T, et al. (2004) Decoding cilia function: defining function results in cilia defects and compromised specialized genes required for compartmentalized cilia intraflagellar transport Genes Dev 18 1630-1642 biogenesis Cell 117 527-539 Boucrot E, Henry T, Borg J P, Gorvel J P and Meresse S (2005) Bagshaw R D, Callahan J W and Mahuran D J (2006) The Arf- The intracellular fate of Salmonella depends on the family protein, Arl8b, is involved in the spatial distribution recruitment of kinesin Science 308 1174-1178 206 Rituraj Marwaha et al.

Breiner M, Schurmann A, Becker W and Joost H G (1996) Cloning asymmetric division J Cell Biol 212 661-676 of a novel member (ARL5) of the ARF-family of Ras- Chiang A P, Nishimura D, Searby C, Elbedour K, Carmi R, et al. related GTPases Biochim Biophys Acta 1308 1-6 (2004) Comparative genomic analysis identifies an ADP- Breitfeld J, Stumvoll M and Kovacs P (2012) Genetics of ribosylation factor-like gene as the cause of Bardet-Biedl adiponectin Biochimie 94 2157-2163 syndrome (BBS3) Am J Hum Genet 75 475-484 Burd C G, Strochlic T I and Setty S R (2004) Arf-like GTPases: Christis C and Munro S (2012) The small G protein Arl1 directs not so Arf-like after all Trends Cell Biol 14 687-694 the trans-Golgi-specific targeting of the Arf1 exchange Cabukusta B and Neefjes J (2018) Mechanisms of lysosomal factors BIG1 and BIG2 J Cell Biol 196 327-335 positioning and movement Traffic 19 761-769 Clark J, Moore L, Krasinskas A, Way J, Battey J, et al. (1993) Calin G A, Trapasso F, Shimizu M, Dumitru C D, Yendamuri S, Selective amplification of additional members of the ADP- et al. (2005) Familial cancer associated with a ribosylation factor (ARF) family: cloning of additional polymorphism in ARLTS1 N Engl J Med 352 1667-1676 human and Drosophila ARF-like genes Proc Natl Acad Sci Cantagrel V, Silhavy J L, Bielas S L, Swistun D, Marsh S E, et al. U S A 90 8952-8956 (2008) Mutations in the Cilia Gene ARL13B Lead to the Constantine R, Zhang H, Gerstner C D, Frederick J M and Baehr Classical Form of Joubert Syndrome American Journal of W (2012) Uncoordinated (UNC)119: coordinating the Human Genetics 83 170-179 trafficking of myristoylated proteins Vision Res 75 26-32 Casalou C, Seixas C, Portelinha A, Pintado P, Barros M, et al. Cruz-Garcia D, Ortega-Bellido M, Scarpa M, Villeneuve J, Jovic (2014) Arl13b and the non-muscle myosin heavy chain M, et al. (2013) Recruitment of arfaptins to the trans- IIA are required for circular dorsal ruffle formation and cell Golgi network by PI(4)P and their involvement in cargo migration Journal of Cell Science 127 2709 export EMBO J 32 1717-1729 Caspary T, Larkins C E and Anderson K V (2007) The graded Cuvillier A, Redon F, Antoine J C, Chardin P, DeVos T, et al. response to Sonic Hedgehog depends on cilia architecture (2000) LdARL-3A, a Leishmania promastigote-specific Dev Cell 12 767-778 ADP-ribosylation factor-like protein, is essential for Cavenagh M M, Breiner M, Schurmann A, Rosenwald A G, Terui flagellum integrity J Cell Sci 113 2065-2074 T, et al. (1994) ADP-ribosylation factor (ARF)-like 3, a Dahlman I and Arner P (2010) Genetics of adipose tissue biology new member of the ARF family of GTP-binding proteins Prog Mol Biol Transl Sci 94 39-74 cloned from human and rat tissues J Biol Chem 269 18937- Divangahi M, Chen M, Gan H, Desjardins D, Hickman T T, et al. 18942 (2009) Mycobacterium tuberculosis evades macrophage Cevik S, Hori Y, Kaplan O I, Kida K, Toivenon T, et al. (2010) defenses by inhibiting plasma membrane repair Nat Joubert syndrome Arl13b functions at ciliary membranes Immunol 10 899-906 and stabilizes protein transport in Donaldson J G and Jackson C L (2011) ARF family G proteins <em>Caenorhabditis elegans</em> The Journal and their regulators: roles in membrane transport, of Cell Biology 188 953 development and disease Nat Rev Mol Cell Biol 12 362- Chang L, Kreko-Pierce T and Eaton B A (2015) The guanine 375 exchange factor Gartenzwerg and the small GTPase Arl1 Duldulao N A, Lee S and Sun Z (2009) Cilia localization is essential function in the same pathway with Arfaptin during synapse for in vivo functions of the Joubert syndrome protein growth Biol Open 4 947-953 Arl13b/Scorpion Development 136 4033 Chapple J P, Hardcastle A J, Grayson C, Spackman L A, Willison Dykes S S, Gray A L, Coleman D T, Saxena M, Stephens C A, et K R, et al. (2000) Mutations in the N-terminus of the X- al. (2016) The Arf-like GTPase Arl8b is essential for three- linked retinitis pigmentosa protein RP2 interfere with the dimensional invasive growth of prostate cancer in vitro normal targeting of the protein to the plasma membrane and xenograft formation and growth in vivo Oncotarget 7 Hum Mol Genet 9 1919-1926 31037-31052 Chen B, Zhu Z, Wang Y, Ding X, Guo X, et al. (2018) Nature vs. Eiseler T, Wille C, Koehler C, Illing A and Seufferlein T (2016) nurture in human sociality: multi-level genomic analyses Protein Kinase D2 Assembles a Multiprotein Complex at of social conformity J Hum Genet 63 605-619 the Trans-Golgi Network to Regulate Matrix Chen K, Koe C T, Xing Z B, Tian X, Rossi F, et al. (2016) Arl2- Metalloproteinase Secretion J Biol Chem 291 462-477 and Msps-dependent microtubule growth governs Engel T, Lueken A, Bode G, Hobohm U, Lorkowski S, et al. Emerging Roles of Arf-Like GTP-Binding Proteins 207

(2004) ADP-ribosylation factor (ARF)-like 7 (ARL7) is (2010) A genome-wide study reveals copy number variants induced by cholesterol loading and participates in exclusive to childhood obesity cases Am J Hum Genet 87 apolipoprotein AI-dependent cholesterol export FEBS Lett 661-666 566 241-246 Goetz S C and Anderson K V (2010) The primary cilium: a Evans R J, Schwarz N, Nagel-Wolfrum K, Wolfrum U, Hardcastle signalling centre during vertebrate development Nat Rev A J, et al. (2010) The retinitis pigmentosa protein RP2 Genet 11 331-344 links pericentriolar vesicle transport between the Golgi Goitre L, Trapani E, Trabalzini L and Retta S F (2014) The Ras and the primary cilium Human Molecular Genetics 19 superfamily of small GTPases: the unlocked secrets 1358-1367 Methods Mol Biol 1120 1-18 Fan Y, Esmail M A, Ansley S J, Blacque O E, Boroevich K, et al. Gotthardt K, Lokaj M, Koerner C, Falk N, Gießl A, et al. (2015) (2004) Mutations in a member of the Ras superfamily of A G-protein activation cascade from Arl13B to Arl3 and small GTP-binding proteins causes Bardet-Biedl syndrome implications for ciliary targeting of lipidated proteins eLife Nat Genet 36 989-993 4 e11859 Fansa E K, Kosling S K, Zent E, Wittinghofer A and Ismail S Grayson C, Bartolini F, Chapple J P, Willison K R, Bhamidipati (2016) PDE6delta-mediated sorting of INPP5E into the A, et al. (2002) Localization in the human retina of the X- cilium is determined by cargo-carrier affinity Nat Commun linked retinitis pigmentosa protein RP2, its homologue 7 11366 cofactor C and the RP2 interacting protein Arl3 Hum Mol Farias G G, Guardia C M, De Pace R, Britt D J and Bonifacino J Genet 11 3065-3074 S (2017) BORC/kinesin-1 ensemble drives polarized Guardia C M, Farias G G, Jia R, Pu J and Bonifacino J S (2016) transport of lysosomes into the axon Proc Natl Acad Sci U BORC Functions Upstream of Kinesins 1 and 3 to S A 114 E2955-E2964 Coordinate Regional Movement of Lysosomes along Filipek P A, de Araujo M E G, Vogel G F, De Smet C H, Eberharter Different Microtubule Tracks Cell Rep 17 1950-1961 D, et al. (2017) LAMTOR/Ragulator is a negative regulator Guo Y, Zanetti G and Schekman R (2013) A novel GTP-binding of Arl8b- and BORC-dependent late endosomal protein-adaptor protein complex responsible for export positioning J Cell Biol 216 4199-4215 of Vangl2 from the trans Golgi network Elife 2 e00160 Francis J W, Goswami D, Novick S J, Pascal B D, Weikum E R, Hesse D, Hommel A, Jaschke A, Moser M, Bernhardt U, et al. et al. (2017a) Nucleotide Binding to ARL2 in the (2010) Altered GLUT4 trafficking in adipocytes in the TBCDARL2beta-Tubulin Complex Drives absence of the GTPase Arfrp1 Biochem Biophys Res Conformational Changes in beta-Tubulin J Mol Biol 429 Commun 394 896-903 3696-3716 Hesse D, Jaschke A, Chung B and Schurmann A (2013) Trans- Francis J W, Newman L E, Cunningham L A and Kahn R A Golgi proteins participate in the control of lipid droplet (2017b) A Trimer Consisting of the Tubulin-specific and chylomicron formation Biosci Rep 33 1-9 Chaperone D (TBCD), Regulatory GTPase ARL2, and Higginbotham H, Eom T Y, Mariani L E, Bachleda A, Hirt J, et al. beta-Tubulin Is Required for Maintaining the Microtubule (2012) Arl13b in primary cilia regulates the migration and Network J Biol Chem 292 4336-4349 placement of interneurons in the developing cerebral cortex Garg S, Sharma M, Ung C, Tuli A, Barral D C, et al. (2011) Dev Cell 23 925-938 Lysosomal trafficking, antigen presentation, and microbial Hofmann I and Munro S (2006) An N-terminally acetylated Arf- killing are controlled by the Arf-like GTPase Arl8b like GTPase is localised to lysosomes and affects their Immunity 35 182-193 motility J Cell Sci 119 1494-1503 Gehart H, Goginashvili A, Beck R, Morvan J, Erbs E, et al. Hofmann I, Thompson A, Sanderson C M and Munro S (2007) (2012) The BAR domain protein Arfaptin-1 controls The Arl4 family of small G proteins can recruit the secretory granule biogenesis at the trans-Golgi network cytohesin Arf6 exchange factors to the plasma membrane Dev Cell 23 756-768 Curr Biol 17 711-716 Gillingham A K and Munro S (2007) The small G proteins of the Hommel A, Hesse D, Volker W, Jaschke A, Moser M, et al. Arf family and their regulators Annu Rev Cell Dev Biol 23 (2010) The ARF-like GTPase ARFRP1 is essential for 579-611 lipid droplet growth and is involved in the regulation of Glessner J T, Bradfield J P, Wang K, Takahashi N, Zhang H, et al. lipolysis Mol Cell Biol 30 1231-1242 208 Rituraj Marwaha et al.

Hong C, Walczak R, Dhamko H, Bradley M N, Marathe C, et al. traffic and microtubule dynamics Biochem Soc Trans33 (2011) Constitutive activation of LXR in macrophages 1269-1272 regulates metabolic and inflammatory gene expression: Kaniuk N A, Canadien V, Bagshaw R D, Bakowski M, Braun V, identification of ARL7 as a direct target J Lipid Res 52 et al. (2011) Salmonella exploits Arl8B-directed kinesin 531-539 activity to promote endosome tubulation and cell-to-cell Horner V L and Caspary T (2011) Disrupted dorsal neural tube transfer Cell Microbiol 13 1812-1823 BMP signaling in the cilia mutant Arl13bhnn stems from Khatter D, Raina V B, Dwivedi D, Sindhwani A, Bahl S, et al. abnormal Shh signaling Developmental Biology 355 43-54 (2015a) The small GTPase Arl8b regulates assembly of Houghton F J, Bellingham S A, Hill A F, Bourges D, Ang D K, et the mammalian HOPS complex on lysosomes J Cell Sci al. (2012) Arl5b is a Golgi-localised small G protein 128 1746-1761 involved in the regulation of retrograde transport Exp Cell Khatter D, Sindhwani A and Sharma M (2015b) Arf-like GTPase Res 318 464-477 Arl8: Moving from the periphery to the center of lysosomal Hoyt M A, Stearns T and Botstein D (1990) Chromosome biology Cell Logist 5 e1086501 instability mutants of Saccharomyces cerevisiae that are Kitai Y, Takeuchi O, Kawasaki T, Ori D, Sueyoshi T, et al. (2015) defective in microtubule-mediated processes Mol Cell Biol Negative regulation of melanoma differentiation-associated 10 223-234 gene 5 (MDA5)-dependent antiviral innate immune Huotari J, Helenius A (2011) Endosome maturation EMBO J30 responses by Arf-like protein 5B J Biol Chem 290 1269- 3481-3500 1280 Ismail S (2017) A GDI/GDF-like system for sorting and shuttling Klassen M P, Wu Y E, Maeder C I, Nakae I, Cueva J G, et al. ciliary proteins Small GTPases 8 208-211 (2010) An Arf-like small G protein, ARL-8, promotes the Ismail S A, Chen Y X, Miertzschke M, Vetter I R, Koerner C, et axonal transport of presynaptic cargoes by suppressing al. (2012) Structural basis for Arl3-specific release of vesicle aggregation Neuron 66 710-723 myristoylated ciliary cargo from UNC119 EMBO J 31 Kobayashi T, Hori Y, Ueda N, Kajiho H, Muraoka S, et al. (2009) 4085-4094 Biochemical characterization of missense mutations in the Ismail S A, Chen Y X, Rusinova A, Chandra A, Bierbaum M, et Arf/Arl-family small GTPase Arl6 causing Bardet-Biedl al. (2011) Arl2-GTP and Arl3-GTP regulate a GDI-like syndrome Biochem Biophys Res Commun 381 439-442 transport system for farnesylated cargo Nat Chem Biol 7 Lai C K, Gupta N, Wen X, Rangell L, Chih B, et al. (2011) 942-949 Functional characterization of putative cilia genes by high- Jacobs S, Schilf C, Fliegert F, Koling S, Weber Y, et al. (1999) content analysis Mol Biol Cell 22 1104-1119 ADP-ribosylation factor (ARF)-like 4, 6, and 7 represent Larkins C E, Aviles G D G, East M P, Kahn R A and Caspary T a subgroup of the ARF family characterization by rapid (2011) Arl13b regulates ciliogenesis and the dynamic nucleotide exchange and a nuclear localization signal FEBS localization of Shh signaling proteins Molecular Biology of Lett 456 384-388 the Cell 22 4694-4703 Jacobs S, Schurmann A, Becker W, Bockers T M, Copeland N G, Lechtreck K F, Johnson E C, Sakai T, Cochran D, Ballif B A, et al. et al. (1998) The mouse ADP-ribosylation factor-like 4 (2009) The Chlamydomonas reinhardtii BBSome is an gene: two separate promoters direct specific transcription IFT cargo required for export of specific signaling proteins in tissues and testicular germ cell Biochem J 335 259-265 from flagella J Cell Biol 187 1117-1132 Jin H, White S R, Shida T, Schulz S, Aguiar M, et al. (2010) The Li C C, Wu T S, Huang C F, Jang L T, Liu Y T, et al. (2012a) GTP- Conserved Bardet-Biedl Syndrome Proteins Assemble a binding-defective ARL4D alters mitochondrial morphology Coat that Traffics Membrane Proteins to Cilia Cell 141 and membrane potential PLoS One 7 e43552 1208-1219 Li Y, Wei Q, Zhang Y, Ling K and Hu J (2010) The small GTPases Kahn R A, Cherfils J, Elias M, Lovering R C, Munro S, et al. ARL-13 and ARL-3 coordinate intraflagellar transport and (2006) Nomenclature for the human Arf family of GTP- ciliogenesis The Journal of Cell Biology 189 1039 binding proteins: ARF, ARL, and SAR proteins The Journal Li Y, Zhang Q, Wei Q, Zhang Y, Ling K, et al. (2012b) of Cell Biology 172 645 SUMOylation of the small GTPase ARL-13 promotes Kahn R A, Volpicelli-Daley L, Bowzard B, Shrivastava-Ranjan P, ciliary targeting of sensory receptors The Journal of Cell Li Y, et al. (2005) Arf family GTPases: roles in membrane Biology 199 589 Emerging Roles of Arf-Like GTP-Binding Proteins 209

Liem K F, Ashe A, He M, Satir P, Moran J, et al. (2012) The IFT- Marwaha R, Arya S B, Jagga D, Kaur H, Tuli A, et al. (2017) The A complex regulates Shh signaling through cilia structure Rab7 effector PLEKHM1 binds Arl8b to promote cargo and membrane protein trafficking The Journal of Cell traffic to lysosomes J Cell Biol 216 1051-1070 Biology 197 789 McElver J, Patton D, Rumbaugh M, Liu C, Yang L J, et al. (2000) Liew Gerald M, Ye F, Nager Andrew R, Murphy J P, Lee Jaclyn S, The TITAN5 gene of Arabidopsis encodes a protein related et al. (2014) The Intraflagellar Transport Protein IFT27 to the ADP ribosylation factor family of GTP binding Promotes BBSome Exit from Cilia through the GTPase proteins Plant Cell 12 1379-1392 ARL6/BBS3 Developmental Cell 31 265-278 McEwan D G, Popovic D, Gubas A, Terawaki S, Suzuki H, et al. Lin C Y, Huang P H, Liao W L, Cheng H J, Huang C F, et al. (2015) PLEKHM1 regulates autophagosome-lysosome (2000) ARL4, an ARF-like protein that is developmentally fusion through HOPS complex and LC3/GABARAP regulated and localized to nuclei and nucleoli J Biol Chem proteins Mol Cell 57 39-54 275 37815-37823 Michelet X, Garg S, Wolf B J, Tuli A, Ricciardi-Castagnoli P, et Lin C Y, Li C C, Huang P H and Lee F J (2002) A developmentally al. (2015) MHC class II presentation is controlled by the regulated ARF-like 5 protein (ARL5), localized to nuclei lysosomal small GTPase, Arl8b J Immunol 194 2079- and nucleoli, interacts with heterochromatin protein 1 J 2088 Cell Sci 115 4433-4445 Michelet X, Tuli A, Gan H, Geadas C, Sharma M, et al. (2018) Lin Y C, Chiang T C, Liu Y T, Tsai Y T, Jang L T, et al. (2011) Lysosome-Mediated Plasma Membrane Repair Is ARL4A acts with GCC185 to modulate Golgi complex Dependent on the Small GTPase Arl8b and Determines organization J Cell Sci 124 4014-4026 Cell Death Type in Mycobacterium tuberculosis Infection Linari M, Hanzal-Bayer M and Becker J (1999) The delta subunit J Immunol 200 3160-3169 of rod specific cyclic GMP phosphodiesterase, PDE delta, Mori R and Toda T (2013) The dual role of fission yeast Tbc1/ interacts with the Arf-like protein Arl3 in a GTP specific cofactor C orchestrates microtubule homeostasis in tubulin manner FEBS Lett 458 55-59 folding and acts as a GAP for GTPase Alp41/Arl2 Mol Long L M, He B F, Huang G Q, Guo Y H, Liu Y S, et al. (2015) Biol Cell 24 1713-1724, S1711-1718 microRNA-214 functions as a tumor suppressor in human Mourão A, Nager A R, Nachury M V and Lorentzen E (2014) colon cancer via the suppression of ADP-ribosylation Structural basis for membrane targeting of the BBSome by factor-like protein 2 Oncol Lett 9 645-650 ARL6 Nat Struct Mol Biol 21 1035-1041 Louro R, Nakaya H I, Paquola A C, Martins E A, da Silva A M, et Mrakovic A, Kay J G, Furuya W, Brumell J H and Botelho R J al. (2004) RASL11A, member of a novel small monomeric (2012) Rab7 and Arl8 GTPases are necessary for lysosome GTPase gene family, is down-regulated in prostate tumors tubulation in macrophages Traffic 13 1667-1679 Biochem Biophys Res Commun 316 618-627 Mueller A G, Moser M, Kluge R, Leder S, Blum M, et al. (2002) Lowe S L, Wong S H and Hong W (1996) The mammalian ARF- Embryonic lethality caused by apoptosis during like protein 1 (Arl1) is associated with the Golgi complex gastrulation in mice lacking the gene of the ADP-ribosylation J Cell Sci 109 209-220 factor-related protein 1 Mol Cell Biol 22 1488-1494 Lu L, Tai G and Hong W (2004) Autoantigen Golgin-97, an effector Munro S (2005) The Arf-like GTPase Arl1 and its role in of Arl1 GTPase, participates in traffic from the endosome membrane traffic Biochem Soc Trans 33 601-605 to the trans-golgi network Mol Biol Cell 15 4426-4443 Nachury M V, Loktev A V, Zhang Q, Westlake C J, Peranen J, et Maresova L, Vydareny T and Sychrova H (2012) Comparison of al. (2007) A core complex of BBS proteins cooperates the influence of small GTPases Arl1 and Ypt6 on yeast with the GTPase Rab8 to promote ciliary membrane cells’ tolerance to various stress factors FEMS Yeast Res biogenesis Cell 129 1201-1213 12 332-340 Nakae I, Fujino T, Kobayashi T, Sasaki A, Kikko Y, et al. (2010) Marion V, Stutzmann F, Gérard M, De Melo C, Schaefer E, et al. The arf-like GTPase Arl8 mediates delivery of endocytosed (2012) Exome sequencing identifies mutations in LZTFL, macromolecules to lysosomes in Caenorhabditis elegans a BBSome and smoothened trafficking regulator, in a family Mol Biol Cell21 2434-2442 with Bardet-Biedl syndrome with situs inversus and Nakamura K, Man Z, Xie Y, Hanai A, Makyio H, et al. (2012) insertional polydactyly Journal of Medical Genetics 49 Structural basis for membrane binding specificity of the 317 Bin/Amphiphysin/Rvs (BAR) domain of Arfaptin-2 210 Rituraj Marwaha et al.

determined by Arl1 GTPase J Biol Chem 287 25478-25489 13 405-410 Naslavsky N and Caplan S (2018) The enigmatic endosome - Pasqualato S, Renault L and Cherfils J (2002) Arf, Arl, Arp and sorting the ins and outs of endocytic trafficking J Cell Sci Sar proteins: a family of GTP-binding proteins with a 131 structural device for ‘front-back’ communication EMBO Negi S, Juyal G, Senapati S, Prasad P, Gupta A, et al. (2013) A Rep 3 1035-1041 genome-wide association study reveals ARL15, a novel Patel M, Chiang T C, Tran V, Lee F J and Cote J F (2011) The Arf non-HLA susceptibility gene for rheumatoid arthritis in family GTPase Arl4A complexes with ELMO proteins to North Indians Arthritis Rheum 65 3026-3035 promote actin cytoskeleton remodeling and reveals a Newman L E, Schiavon C R, Zhou C and Kahn R A (2017) The versatile Ras-binding domain in the ELMO proteins family abundance of the ARL2 GTPase and its GAP, ELMOD2, J Biol Chem 286 38969-38979 at mitochondria are modulated by the fusogenic activity of Paul P, van den Hoorn T, Jongsma M L, Bakker M J, Hengeveld mitofusins and stressors PLoS One 12 e0175164 R, et al. (2011) A Genome-wide multidimensional RNAi Newman L E, Zhou C J, Mudigonda S, Mattheyses A L, Paradies screen reveals pathways controlling MHC class II antigen E, et al. (2014) The ARL2 GTPase is required for presentation Cell 145 268-283 mitochondrial morphology, motility, and maintenance of Platko K, Lebeau P and Austin R C (2018) MAPping the kinase ATP levels PLoS One 9 e99270 landscape of macrophage activation J Biol Chem 293 9910- Nishikiori M, Mori M, Dohi K, Okamura H, Katoh E, et al. 9911 (2011) A host small GTP-binding protein ARL8 plays Price H P, Peltan A, Stark M and Smith D F (2010) The small crucial roles in tobamovirus RNA replication PLoS Pathog GTPase ARL2 is required for cytokinesis in Trypanosoma 7 e1002409 brucei Mol Biochem Parasitol 173 123-131 Nithianantham S, Le S, Seto E, Jia W, Leary J, et al. (2015) Pruski M, Rajnicek A, Yang Z, Clancy H, Ding Y-Q, et al. (2016) Tubulin cofactors and Arl2 are cage-like chaperones that The ciliary GTPase Arl13b regulates cell migration and regulate the soluble alphabeta-tubulin pool for microtubule cell cycle progression Cell Adhesion & Migration 10 393- dynamics eLife 4 405 Niwa S, Lipton D M, Morikawa M, Zhao C, Hirokawa N, et al. Pu J, Keren-Kaplan T and Bonifacino J S (2017) A Ragulator- (2016) Autoinhibition of a Neuronal Kinesin UNC-104/ BORC interaction controls lysosome positioning in KIF1A Regulates the Size and Density of Synapses Cell response to amino acid availability J Cell Biol 216 4183- Rep 16 2129-2141 4197 Okai T, Araki Y, Tada M, Tateno T, Kontani K, et al. (2004) Pu J, Schindler C, Jia R, Jarnik M, Backlund P, et al. (2015) Novel small GTPase subfamily capable of associating with BORC, a multisubunit complex that regulates lysosome tubulin is required for chromosome segregation J Cell Sci positioning Dev Cell 33 176-188 117 4705-4715 Radcliffe P A, Vardy L and Toda T (2000) A conserved small Otto E A, Hurd T W, Airik R, Chaki M, Zhou W, et al. (2010) GTP-binding protein Alp41 is essential for the cofactor- Candidate exome capture identifies mutation of dependent biogenesis of microtubules in fission yeast SDCCAG8 as the cause of a retinal-renal ciliopathy Nat FEBS Lett 468 84-88 Genet 42 840-850 Reiter J F and Leroux M R (2017) Genes and molecular pathways Ozdemir E S, Jang H, Gursoy A, Keskin O and Nussinov R underpinning ciliopathies Nat Rev Mol Cell Biol 18 533- (2018) Arl2-Mediated Allosteric Release of Farnesylated 547 KRas4B from Shuttling Factor PDEdelta J Phys Chem B Revenkova E, Liu Q, Gusella G L and Iomini C (2018) The 122 7503-7513 Joubert syndrome protein ARL13B binds tubulin to Panic B, Perisic O, Veprintsev D B, Williams R L and Munro S maintain uniform distribution of proteins along the ciliary (2003a) Structural basis for Arl1-dependent targeting of membrane J Cell Sci 131 homodimeric GRIP domains to the Golgi apparatus Mol Richards J B, Waterworth D, O’Rahilly S, Hivert M F, Loos R J, Cell 12 863-874 et al. (2009) A genome-wide association study reveals Panic B, Whyte J R and Munro S (2003b) The ARF-like GTPases variants in ARL15 that influence adiponectin levels PLoS Arl1p and Arl3p act in a pathway that interacts with Genet 5 e1000768 vesicle-tethering factors at the Golgi apparatus Curr Biol Richardson B C, McDonold C M and Fromme J C (2012) The Emerging Roles of Arf-Like GTP-Binding Proteins 211

Sec7 Arf-GEF is recruited to the trans-Golgi network by Salmonella exploits the host endolysosomal tethering factor positive feedback Dev Cell 22 799-810 HOPS complex to promote its intravacuolar replication Rosa-Ferreira C, Christis C, Torres I L and Munro S (2015) The PLoS Pathog 13 e1006700 small G protein Arl5 contributes to endosome-to-Golgi Song P, Dudinsky L, Fogerty J, Gaivin R and Perkins B D (2016) traffic by aiding the recruitment of the GARP complex to Arl13b Interacts With Vangl2 to Regulate Cilia and the Golgi Biol Open 4 474-481 Photoreceptor Outer Segment Length in ZebrafishArl13b/ Rosa-Ferreira C and Munro S (2011) Arl8 and SKIP act together Vangl2 Epistasis Regulates Photoreceptor Cilia to link lysosomes to kinesin-1 Dev Cell 21 1171-1178 Investigative Ophthalmology & Visual Science 57 4517- Schiefermeier N, Scheffler J M, de Araujo M E, Stasyk T, 4526 Yordanov T, et al. (2014) The late endosomal p14-MP1 Song P and Perkins B D (2018) Developmental expression of the (LAMTOR2/3) complex regulates focal adhesion dynamics zebrafish Arf-like small GTPase paralogs arl13a and arl13b during cell migration J Cell Biol 205 525-540 Gene Expr Patterns 29 82-87 Schrick J J, Vogel P, Abuin A, Hampton B and Rice D S (2006) Spasic M and Jacobs C R (2017) Primary cilia: Cell and molecular ADP-ribosylation factor-like 3 is involved in kidney and mechanosensors directing whole tissue function Semin Cell photoreceptor development Am J Pathol 168 1288-1298 Dev Biol 71 42-52 Schurmann A, Breiner M, Becker W, Huppertz C, Kainulainen Sun Z, Amsterdam A, Pazour G J, Cole D G, Miller M S, et al. H, et al. (1994) Cloning of two novel ADP-ribosylation (2004) A genetic screen in zebrafish identifies cilia genes as factor-like proteins and characterization of their differential a principal cause of cystic kidney Development 131 4085 expression in 3T3-L1 cells J Biol Chem 269 15683-15688 Tamkun J W, Kahn R A, Kissinger M, Brizuela B J, Rulka C, et Schurmann A, Koling S, Jacobs S, Saftig P, Krauss S, et al. (2002) al. (1991) The arflike gene encodes an essential GTP- Reduced sperm count and normal fertility in male mice binding protein in Drosophila Proceedings of the National with targeted disruption of the ADP-ribosylation factor- Academy of Sciences 88 3120-3124 like 4 (Arl4) gene Mol Cell Biol 22 2761-2768 Tian G and Cowan N J (2013) Tubulin-specific chaperones: Schurmann A, Schmidt M, Asmus M, Bayer S, Fliegert F, et al. components of a molecular machine that assembles the (1999) The ADP-ribosylation factor (ARF)-related alpha/beta heterodimer Methods Cell Biol 115 155-171 GTPase ARF-related protein binds to the ARF-specific Torres I L, Rosa-Ferreira C and Munro S (2014) The Arf family guanine nucleotide exchange factor cytohesin and inhibits G protein Arl1 is required for secretory granule biogenesis the ARF-dependent activation of phospholipase D J Biol in Drosophila J Cell Sci 127 2151-2160 Chem 274 9744-9751 Tuli A, Thiery J, James A M, Michelet X, Sharma M, et al. Seixas C, Choi S Y, Polgar N, Umberger N L, East M P, et al. (2013) Arf-like GTPase Arl8b regulates lytic granule (2016) Arl13b and the exocyst interact synergistically in polarization and natural killer cell-mediated cytotoxicity ciliogenesis Molecular Biology of the Cell 27 308-320 Mol Biol Cell 24 3721-3735 Setty S R, Shin M E, Yoshino A, Marks M S and Burd C G Veltel S, Kravchenko A, Ismail S and Wittinghofer A (2008) (2003) Golgi recruitment of GRIP domain proteins by Specificity of Arl2/Arl3 signaling is mediated by a ternary Arf-like GTPase 1 is regulated by Arf-like GTPase 3 Curr Arl3-effector-GAP complex FEBS Lett 582 2501-2507 Biol 13 401-404 Wang I H, Chen Y J, Hsu J W and Lee F J (2017a) The Arl3 and Sharer J D, Shern J F, Van Valkenburgh H, Wallace D C and Kahn Arl1 GTPases co-operate with Cog8 to regulate selective R A (2002) ARL2 and BART enter mitochondria and bind autophagy via Atg9 trafficking Traffic 18 580-589 the adenine nucleotide transporter Mol Biol Cell 13 71-83 Wang J, Qi X, Zhang X, Yan W and You C (2017b) [Genetic Shin H W, Kobayashi H, Kitamura M, Waguri S, Suganuma T, et polymorphisms of ARL15 and HLA-DMA are associated al. (2005) Roles of ARFRP1 (ADP-ribosylation factor- with rheumatoid arthritis in Han population from related protein 1) in post-Golgi membrane trafficking J northwest China] Xi Bao Yu Fen Zi Mian Yi Xue Za Zhi 33 Cell Sci 118 4039-4048 1681-1685 Siltanen S, Fischer D, Rantapero T, Laitinen V, Mpindi J P, et al. Wei S M, Xie C G, Abe Y and Cai J T (2009) ADP-ribosylation (2013) ARLTS1 and prostate cancer risk—analysis of factor like 7 (ARL7) interacts with alpha-tubulin and expression and regulation PLoS One 8 e72040 modulates intracellular vesicular transport Biochem Sindhwani A, Arya S B, Kaur H, Jagga D, Tuli A, et al. (2017) Biophys Res Commun 384 352-356 212 Rituraj Marwaha et al.

Wennerberg K, Rossman K L and Der C J (2005) The Ras al. (2007) Tumor suppressor functions of ARLTS1 in superfamily at a glance J Cell Sci 118 843-846 lung cancers Cancer Res 67 7738-7745 Wiens C J, Tong Y, Esmail M A, Oh E, Gerdes J M, et al. (2010) Yu C J and Lee F J (2017) Multiple activities of Arl1 GTPase in Bardet-Biedl Syndrome-associated Small GTPase ARL6 the trans-Golgi network J Cell Sci 130 1691-1699 (BBS3) Functions at or near the Ciliary Gate and Modulates Zaghloul N A and Katsanis N (2009) Mechanistic insights into Wnt Signaling Journal of Biological Chemistry 285 16218- Bardet-Biedl syndrome, a model ciliopathy The Journal 16230 of Clinical Investigation 119 428-437 Wright K J, Baye L M, Olivier-Mason A, Mukhopadhyay S, Zahn C, Hommel A, Lu L, Hong W, Walther D J, et al. (2006) Sang L, et al. (2011) An ARL3–UNC119–RP2 GTPase Knockout of Arfrp1 leads to disruption of ARF-like1 cycle targets myristoylated NPHP3 to the primary cilium (ARL1) targeting to the trans-Golgi in mouse embryos Genes & Development 25 2347-2360 and HeLa cells Mol Membr Biol 23 475-485 Wu M, Lu L, Hong W and Song H (2004) Structural basis for Zahn C, Jaschke A, Weiske J, Hommel A, Hesse D, et al. (2008) recruitment of GRIP domain golgin-245 by small GTPase ADP-ribosylation factor-like GTPase ARFRP1 is required Arl1 Nat Struct Mol Biol 11 86-94 for trans-Golgi to plasma membrane trafficking of E- Wu Y E, Huo L, Maeder C I, Feng W and Shen K (2013) The cadherin J Biol Chem 283 27179-27188 balance between capture and dissociation of presynaptic Zhang H, Constantine R, Frederick J M and Baehr W (2012) The proteins controls the spatial distribution of synapses prenyl-binding protein PrBP/delta: A chaperone Neuron 78 994-1011 participating in intracellular trafficking Vision Res 75 19- Yang Y K, Qu H, Gao D, Di W, Chen H W, et al. (2011) ARF-like 25 protein 16 (ARL16) inhibits RIG-I by binding with its C- Zhang Q, Nishimura D, Seo S, Vogel T, Morgan D A, et al. (2011) terminal domain in a GTP-dependent manner J Biol Chem Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model 286 10568-10580 reveals common BBS-associated phenotypes and Bbs3 Ye F, Nager A R and Nachury M V (2018) BBSome trains remove unique phenotypes Proceedings of the National Academy activated GPCRs from cilia by enabling passage through of Sciences 108 20678-20683 the transition zone The Journal of Cell Biology Zhao J, Wang M, Deng W, Zhong D, Jiang Y, et al. (2017) ADP- Yen H J, Tayeh M K, Mullins R F, Stone E M, Sheffield V C, et ribosylation factor-like GTPase 15 enhances insulin- al. (2006) Bardet-Biedl syndrome genes are important in induced AKT phosphorylation in the IR/IRS1/AKT retrograde intracellular trafficking and Kupffer’s vesicle pathway by interacting with ASAP2 and regulating PDPK1 cilia function Hum Mol Genet 15 667-677 activity Biochem Biophys Res Commun 486 865-871 Yendamuri S, Trapasso F and Calin G A (2008) ARLTS1 - a novel Zhou C X, Shi L Y, Li R C, Liu Y H, Xu B Q, et al. (2017) tumor suppressor gene Cancer Lett 264 11-20 GTPase-activating protein Elmod2 is essential for meiotic Yendamuri S, Trapasso F, Ferracin M, Cesari R, Sevignani C, et progression in mouse oocytes Cell Cycle 16 852-860.