Vps51p links the VFT complex to the SNARE Tlg1p

Intracellular membrane fusion requires the complex coordination of SNARE, rab/ypt, and rab effector function. In the yeast Saccharomyces cerevisiae, fusion of endosome-derived vesicles with the late Golgi depends on a cascade of protein-protein interactions that results in the recruitment to Golgi membranes of a conserved docking complex, VFT. This complex binds to Ypt6-GTP, which is necessary for its localization to the Golgi, and also to the SNARE Tlg1p. We show here that the VFT complex contains a fourth, previously uncharacterized, subunit, Vps51p (Ykr020w). Yeast cells lacking VPS51 have defects in vacuole morphology and recycling of the SNARE Snc1p to the plasma membrane, but still assemble a core VFT complex consisting of Vps52p, Vps53p, and Vps54p that localizes properly to the Golgi. Binding to Ypt6-GTP is a property of Vps52p. In contrast, binding to Tlg1p is mediated by a short sequence at the N terminus of Vps51p. Recent evidence suggests that components of a number of rab/ypt effector complexes share a common, distantly related helical coiled-coil motif. We show that each VFT subunit requires this coiled-coil motif for assembly into the complex.     

Characterization of the human GARP (Golgi associated retrograde protein) complex

The Golgi associated retrograde protein complex (GARP) or Vps fifty-three (VFT) complex is part of cellular inter-compartmental transport systems. Here we report the identification of the VFT tethering factor complex and its interactions in mammalian cells. Subcellular fractionation shows that human Vps proteins are found in the smooth membrane/Golgi fraction but not in the cytosol. Immunostaining of human Vps proteins displays a vesicular distribution most concentrated at the perinuclear envelope. Co-staining experiments with endosomal markers imply an endosomal origin of these vesicles. Significant accumulation of VFT complex positive endosomes is found in the vicinity of the Trans Golgi Network area. This is in accordance with a putative role in Golgi associated transport processes. In Saccharomyces cerevisiae, GARP is the main effector of the small GTPase Ypt6p and interacts with the SNARE Tlg1p to facilitate membrane fusion. Accordingly, the human homologue of Ypt6p, Rab6, specifically binds hVps52. In human cells, the "orphan" SNARE Syntaxin 10 is the genuine binding partner of GARP mediated by hVps52. This reveals a previously unknown function of human Syntaxin 10 in membrane docking and fusion events at the Golgi. Taken together, GARP shows significant conservation between various species but diversification and specialization result in important differences in human cells.     

The GTPase-activating enzyme Gyp1p is required for recycling of internalized membrane material by inactivation of the Rab/Ypt GTPase Ypt1p

Rab/Ypt GTPases are key regulators of membrane trafficking and together with SNARE proteins mediate selective fusion of vesicles with target compartments. A family of GTPase-activating enzymes (GAPs) specific for Rab/Ypt GTPases has been discovered, but little is known about their function and substrate specificity in vivo. Here we show that the GAP activity of Gyp1p, a yeast member of this family, is specifically required for recycling of the SNARE Snc1p and the membrane dye FM4-64, implying that inactivation of a Rab/Ypt GTPase may be necessary for recycling of membrane material. Interestingly, recycling of GFP-Snc1p in gyp1 Delta cells is partially restored by reducing the activity of Ypt1p. Moreover, GFP-Snc1p accumulated intracellularly in wild-type cells expressing a GTP-locked, mutant form of Ypt1p (Ypt1p-Q67L), suggesting that GTP hydrolysis of Ypt1p is essential for recycling. Ypt6p is known to be required for the fusion of recycling vesicles to the late Golgi compartment. Interestingly, the deletions of GYP1 and YPT6 were synthetic lethal, raising the possibility that at least two distinct pathways are involved in recycling of membrane material.     

Promiscuity in Rab-SNARE interactions

Fusion of post-Golgi secretory vesicles with the plasma membrane in yeast requires the function of a Rab protein, Sec4p, and a set of v- and t-SNAREs, the Snc, Sso, and Sec9 proteins. We have tested the hypothesis that a selective interaction between Sec4p and the exocytic SNAREs is responsible for ensuring that secretory vesicles fuse with the plasma membrane but not with intracellular organelles. Assembly of Sncp and Ssop into a SNARE complex is defective in a sec4-8 mutant strain. However, Snc2p binds in vivo to many other syntaxin-like t-SNAREs, and binding of Sncp to the endosomal/Golgi t-SNARE Tlg2p is also reduced in sec4-8 cells. In addition, binding of Sncp to Ssop is reduced by mutations in two other Rab genes and four non-Rab genes that block the secretory pathway before the formation of secretory vesicles. In an alternate approach to look for selective Rab-SNARE interactions, we report that the nucleotide-free form of Sec4p coimmunoprecipitates with Ssop. However, Rab-SNARE binding is nonselective, because the nucleotide-free forms of six Rab proteins bind with similar low efficiency to three SNARE proteins, Ssop, Pep12p, and Sncp. We conclude that Rabs and SNAREs do not cooperate to specify the target membrane.     

Purification of active HOPS complex reveals its affinities for phosphoinositides and the SNARE Vam7p

Coupling of Rab GTPase activation and SNARE complex assembly during membrane fusion is poorly understood. The homotypic fusion and vacuole protein sorting (HOPS) complex links these two processes: it is an effector for the vacuolar Rab GTPase Ypt7p and is required for vacuolar SNARE complex assembly. We now report that pure, active HOPS complex binds phosphoinositides and the PX domain of the vacuolar SNARE protein Vam7p. These binding interactions support HOPS complex association with the vacuole and explain its enrichment at the same microdomains on docked vacuoles as phosphoinositides, Ypt7p, Vam7p, and the other SNARE proteins. Concentration of the HOPS complex at these microdomains may be a key factor for coupling Rab GTPase activation to SNARE complex assembly.     

Vps51p mediates the association of the GARP (Vps52/53/54) complex with the late Golgi t-SNARE Tlg1p

Multisubunit tethering complexes may contribute to the specificity of membrane fusion events by linking transport vesicles to their target membrane in an initial recognition event that promotes SNARE assembly. However, the interactions that link tethering factors to the other components of the vesicle fusion machinery are still largely unknown. We have previously identified three subunits of a Golgi-localized complex (the Vps52/53/54 complex) that is required for retrograde transport to the late Golgi. This complex interacts with a Rab and a SNARE protein found at the late Golgi and is related to two other multisubunit tethering complexes: the COG complex and the exocyst. Here we show that the Vps52/53/54 complex has an additional subunit, Vps51p. All four members of this tetrameric GARP (Golgi-associated retrograde protein) complex are required for two distinct retrograde transport pathways, from both early and late endosomes, back to the TGN. vps51 mutants exhibit a distinct phenotype suggestive of a regulatory role. Indeed, we find that Vps51p mediates the interaction between Vps52/53/54 and the t-SNARE Tlg1p. The binding of this small, coiled-coil protein to the conserved N-terminal domain of the t-SNARE therefore provides a crucial link between components of the tethering and the fusion machinery.     

Specific retrieval of the exocytic SNARE Snc1p from early yeast endosomes

Many endocytosed proteins in yeast travel to the vacuole, but some are recycled to the plasma membrane. We have investigated the recycling of chimeras containing green fluorescent protein (GFP) and the exocytic SNARE Snc1p. GFP-Snc1p moves from the cell surface to internal structures when Golgi function or exocytosis is blocked, suggesting continuous recycling via the Golgi. Internalization is mediated by a conserved cytoplasmic signal, whereas diversion from the vacuolar pathway requires sequences within and adjacent to the transmembrane domain. Delivery from the Golgi to the surface is also influenced by the transmembrane domain, but the requirements are much less specific. Recycling requires the syntaxins Tlg1p and Tlg2p but not Pep12p or proteins such as Vps4p and Vps5p that have been implicated in late endosome-Golgi traffic. Subtle changes to the recycling signal cause GFP-Snc1p to accumulate preferentially in punctate internal structures, although it continues to recycle to the surface. The internal GFP-Snc1p colocalizes with Tlg1p, and immunofluorescence and immunoelectron microscopy reveal structures that contain Tlg1p, Tlg2p, and Kex2p but lack Pep12p and Sec7p. We propose that these represent early endosomes in which sorting of Snc1p and late Golgi proteins occurs, and that transport can occur directly from them to the Golgi apparatus.     

Asymmetric requirements for a Rab GTPase and SNARE proteins in fusion of COPII vesicles with acceptor membranes

Soluble NSF attachment protein receptor (SNARE) proteins are essential for membrane fusion in transport between the yeast ER and Golgi compartments. Subcellular fractionation experiments demonstrate that the ER/Golgi SNAREs Bos1p, Sec22p, Bet1p, Sed5p, and the Rab protein, Ypt1p, are distributed similarly but localize primarily with Golgi membranes. All of these SNARE proteins are efficiently packaged into COPII vesicles and suggest a dynamic cycling of SNARE machinery between ER and Golgi compartments. Ypt1p is not efficiently packaged into vesicles under these conditions. To determine in which membranes protein function is required, temperature-sensitive alleles of BOS1, BET1, SED5, SLY1, and YPT1 that prevent ER/Golgi transport in vitro at restrictive temperatures were used to selectively inactivate these gene products on vesicles or on Golgi membranes. Vesicles bearing mutations in Bet1p or Bos1p inhibit fusion with wild-type acceptor membranes, but acceptor membranes containing these mutations are fully functional. In contrast, vesicles bearing mutations in Sed5p, Sly1p, or Ypt1p are functional, whereas acceptor membranes containing these mutations block fusion. Thus, this set of SNARE proteins is symmetrically distributed between vesicle and acceptor compartments, but they function asymmetrically such that Bet1p and Bos1p are required on vesicles and Sed5p activity is required on acceptor membranes. We propose the asymmetry in SNARE protein function is maintained by an asymmetric distribution and requirement for the Ypt1p GTPase in this fusion event. When a transmembrane-anchored form of Ypt1p is used to restrict this GTPase to the acceptor compartment, vesicles depleted of Ypt1p remain competent for fusion.     

Ric1p and Rgp1p form a complex that catalyses nucleotide exchange on Ypt6p

Cells lacking the GTPase Ypt6p have defects in intracellular traffic and are temperature sensitive. Their growth is severely impaired by additional mutation of IMH1, which encodes a non-essential Golgi-associated coiled-coil protein. A screen for mutants that, like ypt6, specifically impair the growth of imh1 cells led to the identification of RIC1. Ric1p forms a tight complex with a previously uncharacterized protein, Rgp1p. The Ric1p-Rgp1p complex binds Ypt6p in a nucleotide-dependent manner, and purified Ric1p-Rgp1 stimulates guanine nucleotide exchange on Ypt6p in vitro. Deletion of RIC1 or RGP1, like that of YPT6, blocks the recycling of the exocytic SNARE Snc1p from early endosomes to the Golgi and causes temperature-sensitive growth, but this defect can be relieved by overexpression of YPT6. Ric1p largely colocalizes with the late Golgi marker Sec7p. Ypt6p shows a similar distribution, but this is altered when RIC1 or RGP1 is mutated. We infer that the Ric1p-Rgp1p complex serves to activate Ypt6p on Golgi membranes by nucleotide exchange, and that this is required for efficient fusion of endosome-derived vesicles with the Golgi.     

Ypt32p regulates the translocation of Chs3p from an internal pool to the plasma membrane

The transport of the chitin synthase III, Chs3p, to the plasma membrane is temporally and spatially regulated. Chs3p is delivered to the plasma membrane at the beginning of the cell cycle, forming chitin rings, and at the end of the cell cycle, forming the primary septum. During the rest of the cell cycle, it is maintained in intracellular compartments, termed chitosomes that share characteristics with the late Golgi and the early endosomes. Chs5p and Chs6p are required for the cell cycle-dependent delivery of Chs3p to the cell surface, but the mechanisms underlying the temporal regulation are still unknown. The Rab proteins, Ypt31/32p, are required for exit of secretory vesicles from the late Golgi and for recycling of proteins between the late Golgi and early endosomes. Either gain of Ypt32p function, by overexpression, or loss-of-function mutations alter the localization of Chs3p-GFP. Moreover, cells overexpressing Ypt32p accumulate chitin at the cell surface independent of Chs5p. Overexpression of Ypt32p also disrupts the localization of the late Golgi protein Sec7. We propose that Ypt31/32p have a role in regulating the delivery of Chs3p to the plasma membrane and deposition of chitin at the cell surface.     

The Sec34/Sec35p complex,a Ypt1p effector required for retrograde intra-Golgi trafficking,interacts with Golgi SNAREs and COPI vesicle coat proteins

The Sec34/35 complex was identified as one of the evolutionarily conserved protein complexes that regulates a cis-Golgi step in intracellular vesicular transport. We have identified three new proteins that associate with Sec35p and Sec34p in yeast cytosol. Mutations in these Sec34/35 complex subunits result in defects in basic Golgi functions, including glycosylation of secretory proteins, protein sorting, and retention of Golgi resident proteins. Furthermore, the Sec34/35 complex interacts genetically and physically with the Rab protein Ypt1p, intra-Golgi SNARE molecules, as well as with Golgi vesicle coat complex COPI. We propose that the Sec34/35 protein complex acts as a tether that connects cis-Golgi membranes and COPI-coated, retrogradely targeted intra-Golgi vesicles.     

The Yip1p.Yif1p complex is required for the fusion competence of endoplasmic reticulum-derived vesicles

Here we report that Yip1p and Yif1p, two members of an integral membrane protein complex that bind to the Rab Ypt1p, are required for membrane fusion with the Golgi in vitro. To block fusion, anti-Yip1p or anti-Yif1p antibodies must be added before vesicles bud from the endoplasmic reticulum (ER). These antibodies do not block the packaging of Yip1p, Yif1p, or the soluble NSF attachment protein receptor (SNAREs) into vesicles. We propose that Yip1p and Yif1p perform a critical role in establishing the fusion competence of ER to Golgi vesicles at the time of budding. Consistent with this proposal, we observe that the Yip1p.Yif1p complex binds to the ER to Golgi SNAREs Bos1p and Sec22p, two components of the membrane fusion machinery.     

A Ypt32p exchange factor is a putative effector of Ypt1p

Ypt1p regulates vesicle tethering and fusion events from the ER to the Golgi and through the early Golgi. Genetic studies have suggested a functional relationship between Ypt1p and Ypt31p/Ypt32p. Ypt31p and Ypt32p are a pair of functionally redundant GTPases that act after Ypt1p to mediate intra-Golgi traffic or the budding of post-Golgi vesicles from the trans-Golgi. Here we report that a novel Ypt32p exchange factor is a putative effector of Ypt1p. These findings implicate small GTP-binding proteins of the Ypt/Rab family in a signal cascade that directs membrane traffic through the secretory pathway.     

How Tlg2p/syntaxin 16 'snares' Vps45

Soluble N-ethylmaleimide sensitive factor-attachment protein receptors (SNAREs) and Sec1p/Munc18-homologs (SM proteins) play key roles in intracellular membrane fusion. The SNAREs form tight four-helix bundles (core complexes) that bring the membranes together, but it is unclear how this activity is coupled to SM protein function. Studies of the yeast trans-Golgi network (TGN)/endosomal SNARE complex, which includes the syntaxin-like SNARE Tlg2p, have suggested that its assembly requires activation by binding of the SM protein Vps45p to the cytoplasmic region of Tlg2p folded into a closed conformation. Nuclear magnetic resonance and biochemical experiments now show that Tlg2p and Pep12p, a late- endosomal syntaxin that interacts functionally but not directly with Vps45p, have a domain structure characteristic of syntaxins but do not adopt a closed conformation. Tlg2p binds tightly to Vps45p via a short N-terminal peptide motif that is absent in Pep12p. The Tlg2p/Vps45p binding mode is shared by the mammalian syntaxin 16, confirming that it is a Tlg2p homolog, and resembles the mode of interaction between the SM protein Sly1p and the syntaxins Ufe1p and Sed5p. Thus, this mechanism represents the most widespread mode of coupling between syntaxins and SM proteins.     

Arf1 orchestrates Rab GTPase conversion at the trans-Golgi network

Rab family GTPases are key organizers of membrane trafficking and function as markers of organelle identity. Accordingly, Rab GTPases often occupy specific membrane domains, and mechanisms exist to prevent the inappropriate mixing of distinct Rab domains. The yeast Golgi complex can be divided into two broad Rab domains: Ypt1 (Rab1) and Ypt6 (Rab6) are present at the early/medial Golgi and sharply transition to Ypt31/32 (Rab11) at the late Golgi/trans-Golgi network (TGN). This Rab conversion has been attributed to GTPase-activating protein (GAP) cascades in which Ypt31/32 recruits the Rab-GAPs Gyp1 and Gyp6 to inactivate Ypt1 and Ypt6, respectively. Here we report that Rab transition at the TGN involves additional layers of regulation. We provide new evidence confirming the TRAPPII complex as an important regulator of Ypt6 inactivation and uncover an unexpected role of the Arf1 GTPase in recruiting Gyp1 to drive Ypt1 inactivation at the TGN. Given its established role in directly recruiting TRAPPII to the TGN, Arf1 is therefore a master regulator of Rab conversion on maturing Golgi compartments.     

p115–SNARE Interactions: A Dynamic Cycle of p115 Binding Monomeric SNARE Motifs and Releasing Assembled Bundles

Tethering factors regulate the targeting of membrane‐enclosed vesicles under the control of Rab GTPases. p115, a golgin family tether, has been shown to participate in multiple stages of ER/Golgi transport. Despite extensive study, the mechanism of action of p115 is poorly understood. SNARE proteins make up the machinery for membrane fusion, and strong evidence shows that function of p115 is directly linked to its interaction with SNAREs. Using a gel filtration binding assay, we have demonstrated that in solution p115 stably interacts with ER/Golgi SNAREs rbet1 and sec22b, but not membrin and syntaxin 5. These binding preferences stemmed from selectivity of p115 for monomeric SNARE motifs as opposed to SNARE oligomers. Soluble monomeric rbet1 can compete off p115 from coat protein II (COPII) vesicles. Furthermore, excess p115 inhibits p115 function in trafficking. We conclude that monomeric SNAREs are a major binding site for p115 on COPII vesicles, and that p115 dissociates from its SNARE partners upon SNAREpin assembly. Our results suggest a model in which p115 forms a mixed p115/SNARE helix bundle with a monomeric SNARE, facilitates the binding activity and/or concentration of the SNARE at prefusion sites and is subsequently ejected as SNARE complex formation and fusion proceed.      

Distinct targeting and fusion functions of the PX and SNARE domains of yeast vacuolar Vam7p

Regulated membrane fusion requires organelle tethering, enrichment of selected proteins and lipids at the fusion site, bilayer distortion, and lipid rearrangement. Yeast vacuole homotypic fusion requires regulatory lipids (ergosterol, diacylglycerol, and phosphoinositides), the Rab family GTPase Ypt7p, the multisubunit Ypt7p-effector complex HOPS (homotypic fusion and vacuole protein sorting), and four SNAREs. One SNARE, Vam7p, has an N-terminal PX domain which binds to phosphatidylinositol 3-phosphate (PI(3)P) and to HOPS and a C-terminal SNARE domain but no apolar membrane anchor. We have exploited an in vitro reaction of vacuole fusion to analyze the functions of each domain, removing the PX domain or mutating it to abolish its PI(3)P affinity. Lowering the PI(3)P affinity of the PX domain, or even deleting the PX domain, affects the fusion K(m) for Vam7p but not the maximal fusion rate. Fusion driven by the SNARE domain alone is strikingly enhanced by the PLC inhibitor U73122 through enhanced binding of Vam7p SNARE domain to vacuoles, and the further addition of Plc1p blocks this U73122 effect. The PX domain, through its affinities for phosphoinositides and HOPS, is thus exclusively required for enhancing the targeting of Vam7p rather than for execution of the Vam7p functions in HOPS.SNARE complex assembly and fusion.     

A Rab requirement is not bypassed in SLY1-20 suppression

The Rab GTPase Ypt1p and the large homodimer Uso1p are both required for tethering endoplasmic reticulum-derived vesicles to early Golgi compartments in yeast. Loss-of-function ypt1 and uso1 mutations are suppressed by SLY1-20, a dominant allele that encodes the Sed5p-associated protein, Sly1p. Here, we investigate the mechanism of SLY1-20 suppression. In wild-type strains, Ypt1p can be coimmunoprecipitated with Uso1p; however, in a ypt1Delta/SLY1-20 strain, which lacks this complex, membrane binding of Uso1p was reduced. In spite of Ypt1p depletion, Uso1p-dependent vesicle tethering was not bypassed under the ypt1Delta/SLY1-20 condition. Moreover, tethering and fusion assays with ypt1Delta/SLY1-20 membranes remained sensitive to Rab GDP dissociation inhibitor. These results indicate that an alternative Rab protein satisfies the Ypt1p requirement in Uso1p-dependent tethering when SLY1-20 is expressed. Further genetic and biochemical tests revealed that a related Rab protein, Ypt6, might substitute for Ypt1p in ypt1Delta/SLY1-20 cells. Additional experimentation to address the mechanism of SLY1-20 suppression in a cog2Delta [sec35Delta] strain indicated that the Cog2p subunit of the conserved oligomeric Golgi complex is either functionally redundant or is not directly required for anterograde transport to the Golgi complex.     

mTrs130 Is a Component of a Mammalian TRAPPII Complex,a Rab1 GEF That Binds to COPI-coated Vesicles

The GTPase Rab1 regulates endoplasmic reticulum-Golgi and early Golgi traffic. The guanine nucleotide exchange factor (GEF) or factors that activate Rab1 at these stages of the secretory pathway are currently unknown. Trs130p is a subunit of the yeast TRAPPII (transport protein particle II) complex, a multisubunit tethering complex that is a GEF for the Rab1 homologue Ypt1p. Here, we show that mammalian Trs130 (mTrs130) is a component of an analogous TRAPP complex in mammalian cells, and we describe for the first time the role that this complex plays in membrane traffic. mTRAPPII is enriched on COPI (Coat Protein I)-coated vesicles and buds, but not Golgi cisternae, and it specifically activates Rab1. In addition, we find that mTRAPPII binds to γ1COP, a COPI coat adaptor subunit. The depletion of mTrs130 by short hairpin RNA leads to an increase of vesicles in the vicinity of the Golgi and the accumulation of cargo in an early Golgi compartment. We propose that mTRAPPII is a Rab1 GEF that tethers COPI-coated vesicles to early Golgi membranes.     

Structural analysis of the interaction between the SNARE Tlg1 and Vps51

Membrane fusion in cells involves the interaction of SNARE proteins on apposing membranes. Formation of SNARE complexes is preceded by tethering events, and a number of protein complexes that are thought to mediate this have been identified. The VFT or GARP complex is required for endosome-Golgi traffic in yeast. It consists of four subunits, one of which, Vps51, has been shown to bind specifically to the SNARE Tlg1, which participates in the same fusion event. We have determined the structure of the N-terminal domain of Tlg1 bound to a peptide from the N terminus of Vps51. Binding depends mainly on residues 18-30 of Vps51. These form a short helix which lies in a conserved groove in the three-helix bundle formed by Tlg1. Surprisingly, although both Vps51 and Tlg1 are required for transport to the late Golgi from endosomes, removal of the Tlg1-binding sequences from Vps51 does not block such traffic in vivo. Thus, this particular interaction cannot be crucial to the process of vesicle docking or fusion.