Molecular Membrane Biology, 2010; Early Online, 1–14 Bidirectional transport between the trans-Golgi network and the endosomal system MIHAELA ANITEI1*, THOMAS WASSMER1,2*, CHRISTOPH STANGE1 & BERNARD HOFLACK 1 1Biotechnology Center, Dresden University of Technology, Tatzberg, Dresden, Germany, and 2School of Life and Health Sciences, Aston University, Birmingham, UK (Received 2 June 2010; and in revised form 6 September 2010) Abstract The exchange of proteins and lipids between the trans-Golgi network (TGN) and the endosomal system requires multiple cellular machines, whose activities are coordinated in space and time to generate pleomorphic, tubulo-vesicular carriers that deliver their content to their target compartments. These machines and their associated protein networks are recruited and/ or activated on specific membrane domains where they select proteins and lipids into carriers, contribute to deform/ elongate and partition membrane domains using the mechanical forces generated by actin polymerization or movement along microtubules. The coordinated action of these protein networks contributes to regulate the dynamic state of multiple receptors recycling between the cell surface, endosomes and the TGN, to maintain cell homeostasis as exemplified by the biogenesis of lysosomes and related organelles, and to establish/maintain cell polarity. The dynamic assembly and disassembly of these protein networks mediating the exchange of membrane domains between the TGN and endosomes regulates cell- cell signalling and thus the development of multi-cellular organisms. Somatic mutations in single network components lead to changes in transport dynamics that may contribute to pathological modifications underlying several human diseases such as mental retardation. Keywords: Assembly proteins, retromer, clathrin, trans-Golgi network, endosome, membrane traffic Introduction the cell surface or to the TGN, a retrieval pathway also For personal use only. used by several endocytosed molecules. In the TGN, The trans-Golgi network (TGN) is the last sorting VPS10 family receptors (e.g., sortilin or neurotensin station of the secretory pathway from which a plethora receptor 3) or LIMP-2 (Reczek et al. 2007) may also of soluble or membrane proteins and lipids are sorted account for the M6P-independent delivery of some into distinct domains for subsequent transport to dif- soluble lysosomal enzymes to the endosomal system. ferent destinations: the cell surface (apical and baso- The second class includes transmembrane proteins lateral in polarized cells), the endosomal system, destined to reside in lysosomes or in lysosome- secretory granules in endocrine cells, synaptic domains related organelles in pigmented cells, such as lyso- in neurons. somal membrane glycoproteins (LAMPs and LIMPs) Two major classes of transmembrane proteins are and melanosomal proteins. The cytoplasmic domains Mol Membr Biol Downloaded from informahealthcare.com by Dr George Diallinas on 11/08/10 sorted in the TGN for delivery to the endosomal of these transmembrane proteins contain single or system. The first class comprises transmembrane multiple sorting motifs of different types: tyrosine proteins cycling between the TGN and endosomes, based motifs (YxxØ) and variations of acidic clusters such as the two mannose-6-phosphate receptors combined with di-Leucine motifs ([D/E]xxxL[L/I] or (MPRs), which deliver their bound lysosomal DxxLL) (Bonifacino and Traub 2003), (Braulke and enzymes to endosomes in a mannose 6-phosphate Bonifacino 2009). Here, we will give a general (M6P) dependent manner (Munier-Lehmann et al. overview of the machineries and mechanisms regu- 1996), (Ghosh et al. 2003). After unloading their lating the sorting and the exchange of these cargos bound ligands in endosomes, they return either to between the TGN and endosomes. Correspondence: Prof. Bernard Hoflack, Biotechnology Center TU Dresden, Dresden, Germany. Tel: +49 351 463 40235. Fax: +49 351 463 40244. E-mail: bernard.hofl[email protected] *Authors Mihaela Anitei and Thomas Wassmer contributed equally to the paper. ISSN 0968-7688 print/ISSN 1464-5203 online Ó 2010 Informa UK, Ltd. DOI: 10.3109/09687688.2010.522601 2 M. Anitei et al. Sorting machineries mediating post-TGN AP-1 (Peden et al. 2004), thereby suggesting that transport to endosomes AP-3 binds the clathrin heavy chains with a lower affinity. Therefore, AP-3 would not drastically Heterotetrameric coat protein complexes known as compete with the ESCRT component hepatocyte adaptor proteins (AP-1, AP-2, AP-3 and AP-4) decode growth-factor-regulated tyrosine kinase substrate Hrs, the sorting motifs in cargo tails, thereby segregating which binds clathrin efficiently (Raiborg et al. 2001) and cargos into transport carriers. Similar to other APs, could more easily segregate cargos into endosomal the ubiquitous AP-1A adaptor protein complex consists membrane domains distinct from those containing ubi- of four subunits (b1, g, m1a and s1); m1 recognizes quitinated proteins destined to be degraded in multi- tyrosine-based-motifs whereas a g/s1 hemicomplex vesicular bodies (Raiborg et al. 2002) (Figure 1). binds[D/E]xxxL[L/I]signatures(BonifacinoandTraub In specialized cells, AP-1 and AP-3 isoforms have 2003), (Janvier et al. 2003). A class of monomeric slightly different functions. In epithelial cells, the clathrin adaptors termed GGAs [Golgi-localized, AP-1B complex (containing a m1B subunit) targets gamma-ear containing, ARF (ADP ribosylation fac- specific cargos from recycling endosomes to the baso- tor)-binding proteins], including GGA1, GGA2 and lateral plasma membrane (Folsch et al. 2003), as does GGA3, with similar sorting functions, bind acidic AP-4 (Bonifacino and Traub 2003). Neuronal cluster-dileucine motifs through a VHS domain (Boni- AP-3 isoforms and AP-1 s1B regulate the synaptic facino 2004). Although AP-1A and GGAs cooperate in vesicle cycle (Newell-Litwa et al. 2007), (Glyvuk et al. cargo selection (Doray et al. 2002), their relative func- 2010). tions are not yet fully understood. GGAs have been detected both on the same or different clathrin/ AP-1-coated domains of the TGN but have not been Interaction of coat components with detected in purified clathrin-coated carriers, possibly membrane microdomains due to their unstable association with TGN mem- branes (Hirst et al. 2001), (2009), (Doray et al. 2002), The accuracy of membrane traffic depends upon orga- (Mardones et al. 2007). Thus, it has been suggested that nelles being correctly recognized by coat components. GGAs function prior to AP-1 in protein sorting The recruitment of adaptor proteins onto specificmem- (Hirst et al. 2001), (Doray et al. 2002) or that they brane microdomains relies on the combinatorial use of function in parallel pathways (Mardones et al. 2007), multiplelow-affinity,sometimesshort-lived,membrane (Hirst et al. 2009). The VHS and GAT domains of at components, which comprise an active, GTP- least GGA3 bind ubiquitin (Puertollano and Bonifacino bound ARF-1 GTPase, phosphatidylinositide (PIs) 2004), (Scott et al. 2004). Therefore, GGAs may have a and sorting motifs in cargo tails (Baust et al. 2006). For personal use only. more specific role in sorting ubiquitinated cargos at ARF-1, which cycles between an inactive, the TGN for subsequent transport to endosomes, GDP-bound, cytosolic and an active, GTP-bound, where the endosomal sorting complexes required for membrane-associated form, functions as a general reg- transport (ESCRT complexes), which also bind ubiqui- ulator of AP-1, GGAs and AP-3 coat association tinated cargos, could sort them into nascent multivesi- (D’Souza-Schorey and Chavrier 2006). Therefore, cular bodies for subsequent degradation (David 2007). pathway-specific local nucleotide exchange and hydro- A second coat mediating transport of cargos to lyso- lysis on ARF-1 must rely on the recruitment of specific some and lysosome-related organelles (e.g., melano- guanine nucleotide exchange factors(GEFs)(Casanova somes in pigmented cells) is AP-3 (Le Borgne and 2007) and GTPase-activating proteins (GAPs) to these Hoflack 1998), (Bonifacino and Traub 2003), which, membrane subdomains (Donaldson and Honda 2005), Mol Membr Biol Downloaded from informahealthcare.com by Dr George Diallinas on 11/08/10 like AP-1A, binds tyrosine motifs via m3 and acidic (Spang et al. 2010). Thus, an ARF-1 GEF, BIG2, is di-leucine motifs via d/s3(Dell’Angelica et al. 1997), implicated in AP-1 and GGA1 membrane association (Janvier et al. 2003). It is not yet clear where (Shinotsuka et al. 2002), (Baust et al. 2006), AP-3 functions. Although most of AP-3 is found on (Ishizaki et al. 2008), whereas BIG1 is linked to endosomal tubular profiles(Pedenetal. 2004),itcannot AP-3-dependent transport (Baust et al. 2008). be excluded that AP-3 also functions at the TGN or that AGAP1 and AGAP2 associate with AP-3 and AP-1, AP-1 and AP-3 function sequentially. In yeast, respectively (Nie et al. 2003), and the clathrin heavy AP-3 does not appear to require clathrin association chain (CHC)-interacting SMAP-type GAP (SMAP2) for its function (Vowels and Payne 1998), (Peden et al. with AP-1 (Natsume et al. 2006). ARF GAPs may 2002).Inmammalshowever,AP-3can bind theclathrin exhibit a dual function; they favour vesicle uncoating heavy chain via its b3 subunit (Dell’Angelica et al. 1998) via ARF-1 hydrolysis in a manner analogous to Sec23 in and ultrastructural analyses have shown that AP-3 co- COP-II vesicle-mediated ER-to-Golgi transport localizes with clathrin, but to a much lower extent than (Bi et al. 2002) or, alternatively, control ARF-1 Transport between TGN and endosomes 3 A B Plasma membrane Plasma membrane (AP1-B) RE RE AP-2 AP-2 (AP1-B) TGN TGN EE EE GGA/Clathrin AP-1A/Clathrin AP-3 AP-1A/Clathrin Retromer ESCRT AP-3 TIP47, Rab9 Golgi LE Golgi LE Lysosome Lysosome Figure 1. Model of (A) anterograde and (B) retrograde trafficking pathways between the secretory and endocytic pathway. Clathrin/ AP-2-coated endocytic vesicles traffick between the plasma membrane and early endosomes (EE). At the trans-Golgi network (TGN), proteins are incorporated into distinct carriers coated with either clathrin/AP-1A or clathrin/GGAs or AP-3 to be transported to the endosomal system.