Cell Science at a Glance 889

Intraflagellar transport observed as the bidirectional movement of human diseases such as retinal ‘particles’ along Chlamydomonas degeneration, polycystic kidney disease, at a glance reinhardtii flagella (Kozminski et al., Bardet-Biedl syndrome (BBS), Jeune Limin Hao and Jonathan M. 1993). Subsequently, time-lapse asphyxiating thoracic dystrophy, Scholey* fluorescence-microscopy assays in respiratory disease and defective transgenic Caenorhabditis elegans allowed determination of the left-right axis Molecular and Cellular Biology, University of California at Davis, Davis, CA 95616, USA observations of the transport of specifically (Blacque and Leroux, 2006). *Author for correspondence tagged IFT along sensory cilia (e-mail: [email protected]) (Orozco et al., 1999). IFT is now Numerous exciting and rapidly expanding

Journal of Cell Science 122, 889-892 understood to be highly conserved among topics that are relevant to IFT exist, such Published by The Company of Biologists 2009 a broad range of ; mutations in as the cellular and developmental functions doi:10.1242/jcs.023861 IFT proteins in these organisms disrupt of cilia and the pathogenesis of ciliary biogenesis (Absalon et al., 2008; diseases (Adams et al., 2008; Badano et al., (IFT) is the Avidor-Reiss et al., 2004; Blacque et al., 2006; Bisgrove and Yost, 2006; bidirectional transport of multisubunit 2008; Follit et al., 2006; Jekely and Arendt, Eggenschwiler and Anderson, 2007; complexes, called IFT particles, 2006; Nachury et al., 2007; Rosenbaum Fliegauf et al., 2007). Our focus here, along axonemal (MTs) and Witman, 2002; Scholey, 2003), however, is on summarizing recent work beneath the ciliary membrane. IFT plays although, in rare cases, flagella are on the composition and mechanism of essential roles in the assembly and function assembled in the using IFT- action of the IFT machinery, including the of cilia and flagella by contributing to cell independent mechanisms (Briggs et al., IFT-particle subcomplexes IFT-A and motility, sensory perception and cilium- 2004; Han et al., 2003; Sarpal et al., 2003). IFT-B, IFT motors, the BBSome, and other based signaling (Rosenbaum and Witman, Importantly, defects in IFT and newly identified polypeptides that are 2002; Scholey, 2003). IFT was first have been linked to various associated with IFT-B.

Intraflagellar Transport at a Glance Limin Hao and Jonathan M. Scholey

Intraflagellar transport IFT-particle components

Cell body Cilium BBSome

5 7 Golgi 1 9 2 GTP complex GTP 1 Recruitment 4 8 DYF-3 Rab8 Vesicle Rab8 and assembly 2 Anterograde IFT Rt5 20 Membrane 172 20 Transition protein DYF-11 GTP fiber 3 Turnaround I 140 20

Journal of Cell Science GTP 144 GTP 57 Rab8 A 88 80 Rab8 PtdIns 2 Rab8 BBS Rt5 122A 72 BBS 1 BBS Rt5 139 52 A B 3 81 20 A 81 B Rt5 43 B GTP 122B Rab8 27 74 IFT-B GDP Kinesin-II OSM-3 IFT-20 46 GDP IFT-A DYF-1 DYF-13 Rab8 A Rabin8 MT BBS GTP BBS IFT-A complex IFT-B complex Putative IFT-B B Chlamy* Worm† Chlamy* Worm† components

A IFT144 DYF-2 IFT172 OSM-1 DYF-1 4 B A B 6 A 5 IFT140 CHE-11 IFT88 OSM-5 DYF-3/Qilin BBS Rt5Rt5 BBS Rt5 BBS IFT139 ZK328.7 IFT81 IFT-81 B DYF-11 PCM1 Rab8 Rab8 IFT122A DAF-10 IFT80 CHE-2 DYF-13 CEP290 BBS GTP GTP IFT122B IFTA-1 IFT74/72 IFT-74 4 Turnaround II Pericentriolar 6 Disassembly 5 Retrograde IFT IFT43 ? IFT57 CHE-13 material IFT52 OSM-6 Key Active Active Active A IFT-A BBS BBSome Direction kinesin-II OSM-3 *Chlamydomonas IFT46 DYF-6 of IFT reinhardtii IFT27 None Inactive Inactive Inactive BB IFT-B Rt5 Rabaptin 5 1 – 6 IFT steps †Caenorhabditis dynein kinesin-II OSM-3 IFT20 Y110A7A.20 elegans

Coordination of two anterograde IFT motors in worm cilia

Wild type Kinesin-II mutant osm-3 mutant bbs mutant dyf-1 mutant

0.7 μm/s 1.3 μm/s 0.5 μm/s 0.5 μm/s μ 1.3 m/s 1.3 μm/s 0.5 μm/s 1.3 μm/s BBS BBS BBS 1.3 μm/s BBS BBS BBS A B A B A B A B A B A B A B B Inactive Kinesin-II OSM-3 OSM-3 –+–+–+–+–+ Middle-segment MT Distal-segment MT Middle-segment MT Distal-segment MTMiddle-segment MT Middle-segment MT Distal-segment MT Middle-segment MT

Abbreviations: BBSome, BBS protein complex; CEP290, centrosomal protein of 290 kDa; CHE, chemotaxis; DAF, dauer formation; DYF, dye-filling defective; IFT, intraflagellar transport; IFT-A, IFT-particle subcomplex A; IFT-B, IFT-particle subcomplex B; MT, ; OSM, osmotic-avoidance defective; PCM1, pericentriolar material 1; PtdIns, phosphoinositide. © Journal of Cell Science 2009 (122, pp. 889-892)

(See poster insert) 890 Journal of Cell Science 122 (7)

Model for the mechanism of IFT An isolated IFT-B subcomplex dissociates promoter region that are regulated by the IFT is proposed to occur in six phases under high-salt conditions to yield the transcription factor daf-19, similar to other (Pedersen et al., 2008). The first step is to IFT-B core – which contains IFT81, IFT74 IFT components. They are conserved in all recruit and assemble the IFT machinery and and IFT72 (IFT74/72), IFT88, IFT52, ciliated, but not non-ciliated, organisms, its cargo at the transition fiber, which links IFT46 and IFT27 – and four dissociated supporting the idea that they are IFT- the basal body and the membrane components – IFT172, IFT80, IFT57 and particle components, although biochemical around the neck of the cilium (Deane et al., IFT20. Within the core, IFT74 and IFT72 data are needed to confirm this (Omori 2001). The second step is anterograde are protein variants that are encoded by the et al., 2008). transport of IFT particles; these particles same and can interact with the IFT81 carry cargo and inactive retrograde IFT- dimer to form a tetramer (Lucker et al., Most protein components of IFT particles dynein motor protein from the basal body 2005). C. elegans IFT81 and IFT74 single contain sequence motifs or repeats (WD40 to the tip of cilia. The third and fourth steps mutants display very similar phenotypes or TPR, respectively) that are associated involve a poorly understood series of events (Kobayashi et al., 2007). IFT88 and IFT52 with protein-protein interactions (Cole, termed ‘turnaround’. Step 3 includes have been characterized in a few 2003; Jekely and Arendt, 2006). On the inactivation of the anterograde motors organisms, establishing their essential roles basis of the homology between IFT- (kinesin-II or OSM-3), cargo unloading, in ciliogenesis (Absalon et al., 2008; complex proteins and components of coat and dissociation of IFT-A and IFT-B, Rosenbaum and Witman, 2002; Scholey, protein I (COPI) and clathrin-coated whereas step 4 involves the activation of 2003). Interestingly, IFT52 has been found vesicles, it has been proposed that IFT the retrograde dynein motor, the assembly to move separately from all other IFT-B evolved as a specialized form of coated- of the retrograde IFT machinery and the proteins in nephrocystin mutants (Jauregui vesicle transport from a protocoatomer uploading of retrograde cargo. A structure et al., 2008). IFT46 is responsible for the complex. IFT thus shares common termed the ‘flagellar tip complex’, which transport of outer-arm dynein along ancestry with all protocoatomer contains the MT-plus-end-binding protein the in C. reinhardtii (Hou et al., derivatives, including all vesicle coats and (EB1) is proposed to facilitate turnaround 2007), whereas IFT27 is a Rab-like small the nuclear pore complex (NPC) (Jekely (Sloboda, 2005). The fifth step is the G protein that is required for the assembly and Arendt, 2006). retrograde IFT of ciliary turnover proteins of C. reinhardtii flagella. There are no and inactive anterograde motors back from orthologs of IFT27 in C. elegans or The BBSome the distal tip to the base of the cilium. Drosophila melanogaster, however, The BBS1, 2, 4, 5, 7, 8 and 9 proteins form Finally, in step 6, the IFT machinery is implying that it has roles specifically in a complex termed the BBSome and are disassembled for possible re-use. motile cilia (Blacque et al., 2008). encoded by that are associated with Mutations in the dissociated component the ciliary disease BBS (Nachury et al., The molecular composition of the IFT80 lead to defects in cilia formation 2007). This complex is thought to be IFT particle (Qin et al., 2001). IFT20 is found at the conserved among ciliated organisms, in IFT particles were isolated and Golgi complex and can interact with IFT57 which it promotes ciliary membrane

Journal of Cell Science characterized first from the flagellar matrix (Follit et al., 2006). IFT20 might also biogenesis by increasing the level of Rab8- of C. reinhardtii, and they can be resolved interact with one of the kinesin-II subunits, GTP (Nachury et al., 2007). BBS4 is into two subcomplexes – IFT-A and IFT-B but this interaction is less certain in vivo distributed between two pools inside the (Cole et al., 1998; Piperno and Mead, (Baker et al., 2003; Follit et al., 2006). cell: one at centriolar satellites and the other 1997). In vivo transport and genetic data Recently, IFT20 has been found to interact inside the cilium (Kim et al., 2004; from C. elegans also support such a ‘two with the zebrafish Rabaptin5- and Rab8- Nachury et al., 2007). BBS5 contains two subcomplex’ model (Ou et al., 2005a). The interacting protein Elipsa (known as pleckstrin homology (PH)-like domains IFT-A subcomplex in C. reinhardtii DYF-11 in C. elegans), thus providing a and binds to phosphoinositides; inhibition comprises six components, namely potential bridging mechanism between IFT of phosphoinositide production prevents IFT144, IFT140, IFT139, IFT122A, transport and membrane-associated protein ciliogenesis (Nachury et al., 2007), which IFT122B and IFT43. Four of these complexes (Omori et al., 2008). IFT172 provides a possible mechanism for components (IFT144, IFT140, IFT122A interacts with EB1, indicating a possible tethering the IFT machinery to the cilium and IFT122B) have been characterized in role in ‘turnaround’ (Sloboda, 2005), a membrane. In C. elegans, the loss of C. elegans (Blacque et al., 2006; Efimenko proposal that is supported by the analysis BBSome components, such as BBS-7 or et al., 2006; Qin et al., 2001), Trypanosoma of IFT172 in Tetrahymena thermophila and BBS-8, leads to the dissociation of the brucei (Absalon et al., 2008) and other T. brucei (Absalon et al., 2008; Tsao IFT-A and IFT-B subcomplexes (Ou et al., organisms (Adams et al., 2008; Tsao and and Gorovsky, 2008a). 2005a). Gorovsky, 2008b). It is possible that IFT-A plays a role in returning IFT proteins from Four additional putative IFT-B proteins in IFT motors the ciliary tip to the cell body, as the C. elegans (DYF-1, DYF-3, DYF-13 and Anterograde and retrograde IFT are knockout or knockdown of IFT-A DYF-11) appear to be associated with the powered by two members of the kinesin-2 components results in the accumulation of IFT-B subcomplex and undergo IFT family (heterotrimeric kinesin-II and IFT particles at the ciliary tip (Absalon et al., (Blacque et al., 2005; Omori et al., 2008; homodimeric OSM-3) and by the IFT- 2008; Blacque et al., 2006; Efimenko Ou et al., 2005a; Ou et al., 2005b). dynein motor, respectively, as discussed in et al., 2006; Iomini et al., 2001; Depletion of these proteins causes defects a very recent review (Scholey, 2008). In Piperno et al., 1998; Tsao and Gorovsky, in cilium formation. All these genes some cilia, it is thought that the 2008b). contain X-box sequences in their putative heterotrimeric kinesin-2 motor (kinesin-II Journal of Cell Science 122 (7) 891

conserved in motile flagella and sensory cilia. J. Biol. or KIF3) acts alone to drive anterograde outer segment in the mouse photoreceptor Chem. 278, 34211-34218. transport, but in other types of cilia, such (Marszalek et al., 2000). The motility of Bisgrove, B. W. and Yost, H. J. (2006). The roles of cilia as in C. elegans sensory neurons and two transient receptor potential vanilloid in developmental disorders and disease. Development 133, 4131-4143. zebrafish photoreceptors, its activity is (TRPV) channels, OSM-9 and OCR-2, in Blacque, O. E. and Leroux, M. R. (2006). Bardet-Biedl augmented by a homodimeric kinesin-2 C. elegans ciliary membranes was directly syndrome: an emerging pathomechanism of intracellular motor [OSM-3 or KIF17 (Insinna et al., transport. Cell Mol. Life Sci. 63, 2145-2161. observed to occur at rates that are Blacque, O. E., Perens, E. A., Boroevich, K. A., Inglis, 2008)]. In this case, kinesin-II (or KIF3) comparable to that of the IFT machinery P. N., Li, C., Warner, A., Khattra, J., Holt, R. A., Ou, builds the core of the axoneme, which (Qin et al., 2005). G., Mah, A. K. et al. (2005). Functional genomics of the cilium, a sensory organelle. Curr. Biol. 15, 935-941. consists of nine MT doublets called the Blacque, O. E., Li, C., Inglis, P. N., Esmail, M. A., Ou, ‘middle segment’, and dissociates from Perspectives G., Mah, A. K., Baillie, D. L., Scholey, J. M. and the IFT machinery at the tip of this domain, A framework for the entire IFT process has Leroux, M. R. (2006). The WD repeat-containing protein IFTA-1 is required for retrograde intraflagellar transport. whereas OSM-3 (or KIF17) continues been established, but important questions Mol. Biol. Cell 17, 5053-5062. moving tip-wards and specifically remain. For example, how do the Blacque, O. E., Cevik, S. and Kaplan, O. I. (2008). assembles nine distal singlets, which are Intraflagellar transport: from molecular characterisation to components assemble to form a functional mechanism. Front. Biosci. 13, 2633-2652. thought to be required for cilium-based complex? What is the three-dimensional Briggs, L. J., Davidge, J. A., Wickstead, B., Ginger, M. signaling (Scholey and Anderson, 2006). structure of the IFT machinery? What is L. and Gull, K. (2004). More than one way to build a The recently identified mitogen-activated flagellum: comparative genomics of parasitic protozoa. the role of each component in the Curr. Biol. 14, R611-R612. protein (MAP) kinase DYF-5 is proposed complex? And how do the IFT particles Burghoorn, J., Dekkers, M. P., Rademakers, S., de to modulate the processivity of OSM-3 Jong, T., Willemsen, R. and Jansen, G. (2007). Mutation interact with the IFT motors? All the of the MAP kinase DYF-5 affects docking and undocking and to mediate the dissociation of kinesin-II motors that are directly involved in IFT- of kinesin-2 motors and reduces their speed in the cilia of at the middle-segment tips (Burghoorn particle transport appear to have been Caenorhabditis elegans. Proc. Natl. Acad. Sci. USA 104, et al., 2007). The two anterograde motors 7157-7162. identified and characterized, but how they Cole, D. G. (2003). The intraflagellar transport machinery are proposed to be coordinated by the are activated and deactivated in the right of Chlamydomonas reinhardtii. Traffic 4, 435-442. BBSome (Ou et al., 2005a). Cole, D. G., Diener, D. R., Himelblau, A. L., Beech, P. place and right time is not known. Other L., Fuster, J. C. and Rosenbaum, J. L. (1998). questions, such as how the IFT machinery Chlamydomonas kinesin-II-dependent intraflagellar The retrograde motor, IFT-dynein, is turns around at the tip of cilia, how the transport (IFT): IFT particles contain proteins required for ciliary assembly in Caenorhabditis elegans sensory formed by two identical cytoplasmic components are reorganized at the base and dynein heavy chain 1b (Dhc1b) heavy neurons. J. Cell Biol. 141, 993-1008. tip of cilia, and how the cargo is uploaded Deane, J. A., Cole, D. G., Seeley, E. S., Diener, D. R. chains, together with several intermediate, and downloaded, need to be elucidated. To and Rosenbaum, J. L. (2001). Localization of light-intermediate and light chains. How intraflagellar transport protein IFT52 identifies basal body answer these questions, a combination of the IFT motors interact with the IFT transitional fibers as the docking site for IFT particles. approaches, including biochemistry, Curr. Biol. 11, 1586-1590. particles is not known, although in Efimenko, E., Blacque, O. E., Ou, G., Haycraft, C. J., genetics, cell biology, electron microscopy C. elegans bbs mutants, in which IFT-A Yoder, B. K., Scholey, J. M., Leroux, M. R. and and X-ray diffraction, is necessary. Swoboda, P. (2006). Caenorhabditis elegans DYF-2, an and IFT-B are dissociated and move orthologue of human WDR19, is a component of the We apologize to the authors whose original work is intraflagellar transport machinery in sensory cilia. Mol. Journal of Cell Science separately, kinesin-II moves together with IFT-A and OSM-3 moves with IFT-B. not cited owing to space restriction. We are indebted Biol. Cell 17, 4801-4811. to Joel Rosenbaum, George Witman, Frank McNally, Eggenschwiler, J. T. and Anderson, K. V. (2007). Cilia IFT-dynein is proposed to associate with Guangshuo Ou and Seyda Acar for critical review of and developmental signaling. Annu. Rev. Cell Dev. Biol. 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