The Tlg SNARE complex is required for TGN homotypic fusion

Distinct functional domains of the epsin-related Ent5p, a cargo adaptor for the SNARE Tlg2p in transport between endosomes and Golgi

The epsin-related adaptor proteins Ent3p and Ent5p participate in budding of clathrin coated vesicles in transport between trans-Golgi network and endosomes in yeast. Transport of the arginine permease Can1p was analyzed, which recycles between plasma membrane and endosomes and can be targeted to the vacuole for degradation. ent3∆ cells accumulate Can1p-GFP in endosomes. Can1p-GFP is transported faster to the vacuole upon induction of degradation in ent5∆ cells than in wild type cells. The C-terminal domain of Ent5p was sufficient to restore recycling of the secretory SNARE GFP-Snc1p between plasma membrane and TGN in ent3∆ ent5∆ cells. The SNARE Tlg2p was identified as interaction partner of the Ent5p ENTH domain by in vitro binding assays and the interaction site on Ent5p was mapped. Tlg2p functions in transport from early endosomes to the trans-Golgi network and in homotypic fusion of these organelles. Tlg2p is partially shifted to denser fractions in sucrose density gradients of organelles from ent5∆ cells while distribution of Kex2p is unaffected demonstrating that Ent5p acts as cargo adaptor for Tlg2p in vivo. Taken together we show that Ent3p and Ent5p have different roles in transport and function as cargo adaptors for distinct SNAREs.

Developmental program-independent secretory granule degradation in larval salivary gland cells of Drosophila

Both constitutive and regulated secretion require cell organelles that are able to store and release the secretory cargo. During development, the larval salivary gland of Drosophila initially produces high amount of glue-containing small immature secretory granules, which then fuse with each other and reach their normal 3-3.5 μm in size. Following the burst of secretion, obsolete glue granules directly fuse with late endosomes or lysosomes by a process called crinophagy, which leads to fast degradation and recycling of the secretory cargo. However, hindering of endosome-to-TGN retrograde transport in these cells causes abnormally small glue granules which are not able to fuse with each other. Here, we show that loss of function of the SNARE genes Syntaxin 16 (Syx16) and Synaptobrevin (Syb), the small GTPase Rab6 and the GARP tethering complex members Vps53 and Scattered (Vps54) all involved in retrograde transport cause intense early degradation of immature glue granules via crinophagy independently of the developmental program. Moreover, silencing of these genes also provokes secretory failure and accelerated crinophagy during larval development. Our results provide a better understanding of the relations among secretion, secretory granule maturation and degradation and paves the way for further investigation of these connections in other metazoans.

Vps13 and Cdc31/centrin: Puzzling partners in membrane traffic

Yeast Vps13 is a member of a conserved protein family that includes human homologues associated with neurodegenerative and developmental disorders. In this issue, De et al. (2017. J. Cell Biol. https://doi.org/10.1083/jcb.201606078) establish direct roles for Vps13 and its surprising binding partner, the calcium-binding centrin Cdc31, in trans-Golgi network (TGN) to endosome traffic and TGN homotypic fusion.

The Vps13p-Cdc31p complex is directly required for TGN late endosome transport and TGN homotypic fusion

VPS13 proteins are widely conserved in eukaryotes and associated with human neurodegenerative and neurodevelopmental diseases. De et al. describe the lipid specificity and structure of yeast Vps13p, providing insight into its role in both TGN late endosome transport and TGN homotypic fusion.

Yeast VPS13 is the founding member of a eukaryotic gene family of growing interest in cell biology and medicine. Mutations in three of four human VPS13 genes cause autosomal recessive neurodegenerative or neurodevelopmental disease, making yeast Vps13p an important structural and functional model. Using cell-free reconstitution with purified Vps13p, we show that Vps13p is directly required both for transport from the trans-Golgi network (TGN) to the late endosome/prevacuolar compartment (PVC) and for TGN homotypic fusion. Vps13p must be in complex with the small calcium-binding protein Cdc31p to be active. Single-particle electron microscopic analysis of negatively stained Vps13p indicates that this 358-kD protein is folded into a compact rod-shaped density (20 × 4 nm) with a loop structure at one end with a circular opening ∼6 nm in diameter. Vps13p exhibits ATP-stimulated binding to yeast membranes and specific interactions with phosphatidic acid and phosphorylated forms of phosphatidyl inositol at least in part through the binding affinities of conserved N- and C-terminal domains.

Direct binding of the Kex2p cytosolic tail to the VHS domain of yeast Gga2p facilitates TGN to prevacuolar compartment transport and is regulated by phosphorylation

The VHS domains of yeast Gga1p and Gga2p bind sites (GBSs) in the Kex2p and Vps10p cytosolic tails. Phosphorylation of Ser-780 in the Kex2p GBS enhances Kex2p transport from the TGN to the PVC and is induced by cell wall damage. Kex2p GBS function is shown by direct binding, cell-free transport, and in vivo assays for Kex2 localization.

Human Golgi-localized, γ-ear–containing, ADP-ribosylation factor–binding proteins (Ggas) bind directly to acidic dileucine sorting motifs in the cytosolic tails (C-tails) of intracellular receptors. Despite evidence for a role in recruiting ubiquitinated cargo, it remains unclear whether yeast Ggas also function by binding peptide-sorting signals directly. Two-hybrid analysis shows that the Gga1p and Gga2p Vps27, Hrs, Stam (VHS) domains both bind a site in the Kex2p C-tail and that the Gga2p VHS domain binds a site in the Vps10p C-tail. Binding requires deletion of an apparently autoinhibitory sequence in the Gga2p hinge. Ser₇₈₀ in the Kex2p C-tail is crucial for binding: an Ala substitution blocks but an Asp substitution permits binding. Biochemical assays using purified Gga2p VHS–GGA and TOM1 (GAT) and glutathione S-transferase–Kex2p C-tail fusions show that Gga2p binds directly to the Kex2p C-tail, with relative affinities Asp₇₈₀ > Ser₇₈₀ > Ala₇₈₀. Affinity-purified antibody against a peptide containing phospho-Ser­₇₈₀ recognizes wild-type Kex2p but not S₇₈₀A Kex2p, showing that Ser₇₈₀ is phosphorylated in vivo; phosphorylation of Ser₇₈₀ is up-regulated by cell wall–damaging drugs. Finally, mutation of Ser₇₈₀ alters trafficking of Kex2p both in vivo and in cell-free trans-Golgi network (TGN)–prevacuolar compartment (PVC) transport. Thus yeast Gga adaptors facilitate TGN–PVC transport by direct binding of noncanonical phosphoregulated Gga-binding sites in cargo molecules.

Importance of the N-terminal domain of the Qb-SNARE Vti1p for different membrane transport steps in the yeast endosomal system

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) on transport vesicles and target membranes are crucial for vesicle targeting and fusion. They form SNARE complexes, which contain four α-helical SNARE motifs contributed by three or four different SNAREs. Most SNAREs function only in a single transport step. The yeast SNARE Vti1p participates in four distinct SNARE complexes in transport from the trans Golgi network to late endosomes, in transport to the vacuole, in retrograde transport from endosomes to the trans Golgi network and in retrograde transport within the Golgi. So far, all vti1 mutants investigated had mutations within the SNARE motif. Little is known about the function of the N-terminal domain of Vti1p, which forms a three helix bundle called H_abc domain. Here we generated a temperature-sensitive mutant of this domain to study the effects on different transport steps. The secondary structure of wild type and vti1-3 H_abc domain was analyzed by circular dichroism spectroscopy. The amino acid exchanges identified in the temperature-sensitive vti1-3 mutant caused unfolding of the H_abc domain. Transport pathways were investigated by immunoprecipitation of newly synthesized proteins after pulse-chase labeling and by fluorescence microscopy of a GFP-tagged protein cycling between plasma membrane, early endosomes and Golgi. In vti1-3 cells transport to the late endosome and assembly of the late endosomal SNARE complex was blocked at 37°C. Retrograde transport to the trans Golgi network was affected while fusion with the vacuole was possible but slower. Steady state levels of SNARE complexes mediating these steps were less affected than that of the late endosomal SNARE complex. As different transport steps were affected our data demonstrate the importance of a folded Vti1p H_abc domain for transport.

Identification of SHRUBBY, a SHORT-ROOT and SCARECROW interacting protein that controls root growth and radial patterning

The timing and extent of cell division is particularly important for the growth and development of multicellular organisms. Roots of the model organism Arabidopsis thaliana have been widely studied as a paradigm for organ development in plants. In the Arabidopsis root, the plant-specific GRAS family transcription factors SHORT-ROOT (SHR) and SCARECROW (SCR) are key regulators of root growth and of the asymmetric cell divisions that separate the ground tissue into two separate layers: the endodermis and cortex. To elucidate the role of SHR in root development, we identified 17 SHR-interacting proteins. Among those isolated was At5g24740, which we named SHRUBBY (SHBY). SHBY is a vacuolar sorting protein with similarity to the gene responsible for Cohen syndrome in humans. Hypomorphic alleles of shby caused poor root growth, decreased meristematic activity and defects in radial patterning that are characterized by an increase in the number of cell divisions in the ground tissue that lead to extra cells in the cortex and endodermis, as well as additional cell layers. Analysis of genetic and molecular markers indicates that SHBY acts in a pathway that partially overlaps with SHR, SCR, PLETHORA1 and PLETHORA2 (PLT1 and PLT2). The shby-1 root phenotype was partially phenocopied by treatment of wild-type roots with the proteosome inhibitor MG132 or the gibberellic acid (GA) synthesis inhibitor paclobutrazol (PAC). Our results indicate that SHBY controls root growth downstream of GA in part through the regulation of SHR and SCR.

Direct binding of the Kex2p cytosolic tail to the VHS domain of yeast Gga2p facilitates TGN to prevacuolar compartment transport and is regulated by phosphorylation

Human Golgi-localized, γ-ear–containing, ADP-ribosylation factor–binding proteins (Ggas) bind directly to acidic dileucine sorting motifs in the cytosolic tails (C-tails) of intracellular receptors. Despite evidence for a role in recruiting ubiquitinated cargo, it remains unclear whether yeast Ggas also function by binding peptide-sorting signals directly. Two-hybrid analysis shows that the Gga1p and Gga2p Vps27, Hrs, Stam (VHS) domains both bind a site in the Kex2p C-tail and that the Gga2p VHS domain binds a site in the Vps10p C-tail. Binding requires deletion of an apparently autoinhibitory sequence in the Gga2p hinge. Ser780in the Kex2p C-tail is crucial for binding: an Ala substitution blocks but an Asp substitution permits binding. Biochemical assays using purified Gga2p VHS–GGA and TOM1 (GAT) and glutathione S-transferase–Kex2p C-tail fusions show that Gga2p binds directly to the Kex2p C-tail, with relative affinities Asp780> Ser780> Ala780. Affinity-purified antibody against a peptide containing phospho-Ser­780recognizes wild-type Kex2p but not S780A Kex2p, showing that Ser780is phosphorylated in vivo; phosphorylation of Ser780is up-regulated by cell wall–damaging drugs. Finally, mutation of Ser780alters trafficking of Kex2p both in vivo and in cell-free trans-Golgi network (TGN)–prevacuolar compartment (PVC) transport. Thus yeast Gga adaptors facilitate TGN–PVC transport by direct binding of noncanonical phosphoregulated Gga-binding sites in cargo molecules.

Transcription factor binding to a DNA zip code controls interchromosomal clustering at the nuclear periphery

Active genes in yeast can be targeted to the nuclear periphery through interaction of cis-acting "DNA zip codes" with the nuclear pore complex. We find that genes with identical zip codes cluster together. This clustering was specific; pairs of genes that were targeted to the nuclear periphery by different zip codes did not cluster together. Insertion of two different zip codes (GRS I or GRS III) at an ectopic site induced clustering with endogenous genes that have that zip code. Targeting to the nuclear periphery and interaction with the nuclear pore is a prerequisite for gene clustering, but clustering can be maintained in the nucleoplasm. Finally, we find that the Put3 transcription factor recognizes the GRS I zip code to mediate both targeting to the NPC and interchromosomal clustering. These results suggest that zip-code-mediated clustering of genes at the nuclear periphery influences the three-dimensional arrangement of the yeast genome.

The yeast GRASP Grh1 colocalizes with COPII and is dispensable for organizing the secretory pathway

In mammalian cells, the 'Golgi reassembly and stacking protein' (GRASP) family has been implicated in Golgi stacking, but the broader functions of GRASP proteins are still unclear. The yeast Saccharomyces cerevisiae contains a single non-essential GRASP homolog called Grh1. However, Golgi cisternae in S. cerevisiae are not organized into stacks, so a possible structural role for Grh1 has been difficult to test. Here, we examined the localization and function of Grh1 in S. cerevisiae and in the related yeast Pichia pastoris, which has stacked Golgi cisternae. In agreement with earlier studies indicating that Grh1 interacts with coat protein II (COPII) vesicle coat proteins, we find that Grh1 colocalizes with COPII at transitional endoplasmic reticulum (tER) sites in both yeasts. Deletion of P. pastoris Grh1 had no obvious effect on the structure of tER-Golgi units. To test the role of S. cerevisiae Grh1, we exploited the observation that inhibiting ER export in S. cerevisiae generates enlarged tER sites that are often associated with the cis Golgi. This tER-Golgi association was preserved in the absence of Grh1. The combined data suggest that Grh1 acts early in the secretory pathway, but is dispensable for the organization of secretory compartments.

Endocytic and secretory traffic in Arabidopsis merge in the trans-Golgi network/early endosome, an independent and highly dynamic organelle

Plants constantly adjust their repertoire of plasma membrane proteins that mediates transduction of environmental and developmental signals as well as transport of ions, nutrients, and hormones. The importance of regulated secretory and endocytic trafficking is becoming increasingly clear; however, our knowledge of the compartments and molecular machinery involved is still fragmentary. We used immunogold electron microscopy and confocal laser scanning microscopy to trace the route of cargo molecules, including the BRASSINOSTEROID INSENSITIVE1 receptor and the REQUIRES HIGH BORON1 boron exporter, throughout the plant endomembrane system. Our results provide evidence that both endocytic and secretory cargo pass through the trans-Golgi network/early endosome (TGN/EE) and demonstrate that cargo in late endosomes/multivesicular bodies is destined for vacuolar degradation. Moreover, using spinning disc microscopy, we show that TGN/EEs move independently and are only transiently associated with an individual Golgi stack.

TVP23 interacts genetically with the yeast SNARE VTI1 and functions in retrograde transport from the early endosome to the late Golgi

SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins contribute to specific recognition between transport vesicles and target membranes and are required for fusion of membranes. The SNARE Vti1p is required for several transport steps between late Golgi, endosomes and the vacuole in the yeast Saccharomyces cerevisiae. Here, we identified the late Golgi membrane protein TVP23 as a multicopy suppressor of the growth defect in vti1-2 cells. By contrast, the growth defect in vti1-11 cells was not suppressed by TVP23 overexpression. Deletion of TVP23 aggravated the growth defect in vti1-2 cells. Genetic interactions between TVP23 and vti1-2 were not found in transport from the late Golgi via the late endosome to the vacuole or in transport from the Golgi directly to the vacuole. These results suggest that Tvp23p is not involved in forward transport from the late Golgi. Therefore retrograde traffic to the late Golgi was analysed. vti1-2 cells accumulated GFP (green fluorescent protein)–Snc1p within the cell, indicating that retrograde transport from the early endosome to the late Golgi was defective in these cells. Deletion of TVP23 in vti1-2 cells resulted in a synthetic defect in GFP–Snc1p recycling, whereas tvp23Δ cells had a slight defect. These results indicate that Tvp23p performs a partially redundant function in retrograde transport from the early endosome to the late Golgi. This transport step was unaffected in vti1-11 cells, providing an explanation for the allele-specific multicopy suppression by TVP23.

Yeast Golgi-localized, gamma-Ear-containing, ADP-ribosylation factor-binding proteins are but adaptor protein-1 is not required for cell-free transport of membrane proteins from the trans-Golgi network to the prevacuolar compartment

Golgi-localized, gamma-Ear-containing, ADP-ribosylation factor-binding proteins (GGAs) and adaptor protein-1 (AP-1) mediate clathrin-dependent trafficking of transmembrane proteins between the trans-Golgi network (TGN) and endosomes. In yeast, the vacuolar sorting receptor Vps10p follows a direct pathway from the TGN to the late endosome/prevacuolar compartment (PVC), whereas, the processing protease Kex2p partitions between the direct pathway and an indirect pathway through the early endosome. To examine the roles of the Ggas and AP-1 in TGN-PVC transport, we used a cell-free assay that measures delivery to the PVC of either Kex2p or a chimeric protein (K-V), in which the Vps10p cytosolic tail replaces the Kex2p tail. Either antibody inhibition or dominant-negative Gga2p completely blocked K-V transport but only partially blocked Kex2p transport. Deletion of APL2, encoding the beta subunit of AP-1, did not affect K-V transport but partially blocked Kex2p transport. Residual Kex2p transport seen with apl2Delta membranes was insensitive to dominant-negative Gga2p, suggesting that the apl2Delta mutation causes Kex2p to localize to a compartment that precludes Gga-dependent trafficking. These results suggest that yeast Ggas facilitate the specific and direct delivery of Vps10p and Kex2p from the TGN to the PVC and that AP-1 modulates Kex2p trafficking through a distinct pathway, presumably involving the early endosome.

Trans-Golgi network and endosome dynamics connect ceramide homeostasis with regulation of the unfolded protein response and TOR signaling in yeast

Synthetic genetic array analyses identify powerful genetic interactions between a thermosensitive allele (sec14-1(ts)) of the structural gene for the major yeast phosphatidylinositol transfer protein (SEC14) and a structural gene deletion allele (tlg2Delta) for the Tlg2 target membrane-soluble N-ethylmaleimide-sensitive factor attachment protein receptor. The data further demonstrate Sec14 is required for proper trans-Golgi network (TGN)/endosomal dynamics in yeast. Paradoxically, combinatorial depletion of Sec14 and Tlg2 activities elicits trafficking defects from the endoplasmic reticulum, and these defects are accompanied by compromise of the unfolded protein response (UPR). UPR failure occurs downstream of Hac1 mRNA splicing, and it is further accompanied by defects in TOR signaling. The data link TGN/endosomal dynamics with ceramide homeostasis, UPR activity, and TOR signaling in yeast, and they identify the Sit4 protein phosphatase as a primary conduit through which ceramides link to the UPR. We suggest combinatorial Sec14/Tlg2 dysfunction evokes inappropriate turnover of complex sphingolipids in endosomes. One result of this turnover is potentiation of ceramide-activated phosphatase-mediated down-regulation of the UPR. These results provide new insight into Sec14 function, and they emphasize the TGN/endosomal system as a central hub for homeostatic regulation in eukaryotes.

ENTH domain proteins are cargo adaptors for multiple SNARE proteins at the TGN endosome

ENTH and ANTH domain proteins are involved in budding of clathrin-coated vesicles. SNAREs are fusogenic proteins that function in the targeting and fusion of transport vesicles. In mammalian and yeast cells, ENTH domain proteins (epsinR and Ent3p) interact with SNAREs of the vti1 family (Vti1b or Vti1p). This interaction indicates that ENTH proteins could function in cargo sorting, which prompted us to search for additional SNAREs as potential cargo for Ent3p and epsinR. We carried out specific yeast two-hybrid assays, which identified interactions between epsinR and the mammalian late endosomal SNAREs syntaxin 7 and syntaxin 8 as well as between Ent3p and the endosomal SNAREs Pep12p and Syn8p from yeast. Lack of Ent3p affected the trafficking of Pep12p. Ent3p binding to Pep12p required the FSD late endosomal sorting signal in Pep12p. Inactivation of the sorting signal had a similar effect to removal of Ent3p on Pep12p stability indicating that Ent3p acts as a cargo adaptor for Pep12p by binding to the sorting signal. As Vti1p, Pep12p and Syn8p participate in a SNARE complex whereas Vti1b, syntaxin 7 and syntaxin 8 are mammalian SNARE partners, we propose that ENTH domain proteins at the TGN-endosome are cargo adaptors for these endosomal SNAREs.

Cog1p plays a central role in the organization of the yeast conserved oligomeric Golgi complex

The conserved oligomeric Golgi (COG) complex is an evolutionarily conserved peripheral membrane oligomeric protein complex that is involved in intra-Golgi protein trafficking. The COG complex is composed of eight subunits that are located in two lobes; Lobe A contains COG1-4, and Lobe B is composed of COG5-8. Both in vivo and in vitro protein-protein interaction techniques were applied to characterize interactions between individual COG subunits. In vitro assays revealed binary interactions between Cog2p and Cog3p, Cog2p and Cog4p, and Cog6p and Cog8p and a strong interaction between Cog5p and Cog7p. The two-hybrid assay confirmed these findings and revealed that Cog1p interacted with subunits from both lobes of the complex. Antibodies to COG subunits were utilized to determine the protein levels and membrane association of COG subunits in yeast delta cog1-8 mutants. As a result, we created a model of the protein-protein interactions within the yeast COG complex and proposed that Cog1p is a bridging subunit between the two COG lobes. In support of this hypothesis, we have demonstrated that Cog1p is required for the stable association between two COG subcomplexes.

Cell-free transport from the trans-golgi network to late endosome requires factors involved in formation and consumption of clathrin-coated vesicles

Transport between the trans-Golgi network (TGN) and late endosome represents a conserved, clathrin-dependent sorting event that separates lysosomal from secretory cargo molecules and is also required for localization of integral membrane proteins to the TGN. Previously, we reported a cell-free reaction that reconstitutes transport from the yeast TGN to the late endosome/prevacuolar compartment (PVC) and requires the PVC t-SNARE Pep12p. Here, we report that factors required both for formation of clathrin-coated vesicles at the TGN (the Chc1p clathrin heavy chain and the Vps1p dynamin homolog) and for vesicle fusion at the PVC (the Vps21p rab protein and Vps45p SM (Sec1/Munc18) protein) are required for cell-free transport. The marker for TGN-PVC transport, Kex2p, is initially present in a clathrin-containing membrane compartment that is competent for delivery of Kex2p to the PVC. A Kex2p chimera containing the cytosolic tail (C-tail) of the vacuolar protein sorting receptor, Vps10p, is also efficiently transported to the PVC. Antibodies against the Kex2p and Vps10p C-tails selectively block transport of Kex2p and the Kex2-Vps10p chimera. The requirements for factors involved in vesicle formation and fusion, the identification of the donor compartment as a clathrin-containing membrane, and the need for accessibility of C-tail sequences argue that the TGN-PVC transport reaction involves selective incorporation of TGN cargo molecules into clathrin-coated vesicle intermediates. Further biochemical dissection of this reaction should help elucidate the molecular requirements and hierarchy of events in TGN-to-PVC sorting and transport.

Cell-free reconstitution of transport from the trans-golgi network to the late endosome/prevacuolar compartment

Vesicle-mediated transport between the trans-Golgi network (TGN) and the late endosome/prevacuolar compartment (PVC) is an essential step in lysosomal/vacuolar biogenesis. In addition, localization of integral membrane proteins to the TGN requires continual cycles of vesicular transport between the TGN and endosomal compartments. Genetic and biochemical analyses in yeast have identified a variety of proteins required for TGN-to-PVC transport. However, the precise mechanisms of vesicle formation, transport, and fusion have not been fully elucidated. To study the steps of TGN-to-PVC transport in mechanistic detail, we have developed a cell-free assay to monitor delivery of the processing protease Kex2p from the TGN to PVC compartments containing a Kex2p substrate. Transport is time-, temperature-, and ATP-dependent and requires the t-SNARE Pep12p. Moreover, cell-free delivery of Kex2p to the PVC results in the co-integration of Kex2p into PVC membranes containing the Kex2p substrate as determined by co-immunoisolation of Kex2p and the substrate using antibody against the Kex2p cytosolic tail. This work represents the first cell-free reconstitution and biochemical analysis of the essential vacuolar/lysosomal sorting step TGN to late endosome transport.

Soi3p/Rav1p functions at the early endosome to regulate endocytic trafficking to the vacuole and localization of trans-Golgi network transmembrane proteins

SOI3 was identified by a mutation, soi3-1, that suppressed a mutant trans-Golgi network (TGN) localization signal in the Kex2p cytosolic tail. SOI3, identical to RAV1, encodes a protein important for regulated assembly of vacuolar ATPase. Here, we show that Soi3/Rav1p is required for transport between the early endosome and the late endosome/prevacuolar compartment (PVC). By electron microscopy, soi3-1 mutants massively accumulated structures that resembled early endosomes. soi3Delta mutants exhibited a kinetic delay in transfer of the endocytic tracer dye FM4-64, from the 14 degrees C endocytic intermediate to the vacuole. The soi3Delta mutation delayed vacuolar degradation but not internalization of the a-factor receptor Ste3p. By density gradient fractionation, Soi3/Rav1p associated as a peripheral protein with membranes of a density characteristic of early endosomes. The soi3 null mutation markedly reduced the rate of Kex2p transport from the TGN to the PVC but had no effect on vacuolar protein sorting or cycling of Vps10p. These results suggest that assembly of vacuolar ATPase at the early endosome is required for transport of both Ste3p and Kex2p from the early endosome to the PVC and support a model in which cycling through the early endosome is part of the normal itinerary of Kex2p and other TGN-resident proteins.

SNAP-23 functions in docking/fusion of granules at low Ca2+

Ca(2+)-triggered exocytosis of secretory granules mediates the release of hormones from endocrine cells and neurons. The plasma membrane protein synaptosome-associated protein of 25 kDa (SNAP-25) is thought to be a key component of the membrane fusion apparatus that mediates exocytosis in neurons. Recently, homologues of SNAP-25 have been identified, including SNAP-23, which is expressed in many tissues, albeit at different levels. At present, little is known concerning functional differences among members of this family of proteins. Using an in vitro assay, we show here that SNAP-25 and SNAP-23 mediate the docking of secretory granules with the plasma membrane at high (1 microM) and low (100 nM) Ca(2+) levels, respectively, by interacting with different members of the synaptotagmin family. In intact endocrine cells, expression of exogenous SNAP-23 leads to high levels of hormone secretion under basal conditions. Thus, the relative expression levels of SNAP-25 and SNAP-23 might control the mode (regulated vs. basal) of granule release by forming docking complexes at different Ca(2+) thresholds.

Specific Interaction between SNAREs and Epsin N-terminal Homology (ENTH) Domains of Epsin-related Proteins in trans-Golgi Network to Endosome Transport

SNARE proteins on transport vesicles and target membranes have important roles in vesicle targeting and fusion. Therefore, localization and activity of SNAREs have to be tightly controlled. Regulatory proteins bind to N-terminal domains of some SNAREs. vti1b is a mammalian SNARE that functions in late endosomal fusion. To investigate the role of the N terminus of vti1b we performed a yeast two-hybrid screen. The N terminus of vti1b interacted specifically with the epsin N-terminal homology (ENTH) domain of enthoprotin/CLINT/epsinR. The interaction was confirmed using in vitro binding assays. This complex formation between a SNARE and an ENTH domain was conserved between mammals and yeast. Yeast Vti1p interacted with the ENTH domain of Ent3p. ENTH proteins are involved in the formation of clathrin-coated vesicles. Both epsinR and Ent3p bind adaptor proteins at the trans-Golgi network. Vti1p is required for multiple transport steps in the endosomal system. Genetic interactions between VTI1 and ENT3 were investigated. Synthetic defects suggested that Vti1p and Ent3p cooperate in transport from the trans-Golgi network to the prevacuolar endosome. Our experiments identified the first cytoplasmic protein binding to specific ENTH domains. These results point toward a novel function of the ENTH domain and a connection between proteins that function either in vesicle formation or in vesicle fusion.

i-SNAREs: inhibitory SNAREs that fine-tune the specificity of membrane fusion

A new functional class of SNAREs, designated inhibitory SNAREs (i-SNAREs), is described here. An i-SNARE inhibits fusion by substituting for or binding to a subunit of a fusogenic SNAREpin to form a nonfusogenic complex. Golgi-localized SNAREs were tested for i-SNARE activity by adding them as a fifth SNARE together with four other SNAREs that mediate Golgi fusion reactions. A striking pattern emerges in which certain subunits of the cis-Golgi SNAREpin function as i-SNAREs that inhibit fusion mediated by the trans-Golgi SNAREpin, and vice versa. Although the opposing distributions of the cis- and trans-Golgi SNAREs themselves could provide for a countercurrent fusion pattern in the Golgi stack, the gradients involved would be strongly sharpened by the complementary countercurrent distributions of the i-SNAREs.

Deletion of the SNARE vti1b in mice results in the loss of a single SNARE partner, syntaxin 8

SNARE proteins participate in recognition and fusion of membranes. A SNARE complex consisting of vti1b, syntaxin 8, syntaxin 7, and endobrevin/VAMP-8 which is required for fusion of late endosomes in vitro has been identified recently. Here, we generated mice deficient in vti1b to study the function of this protein in vivo. vti1b-deficient mice had reduced amounts of syntaxin 8 due to degradation of the syntaxin 8 protein, while the amounts of syntaxin 7 and endobrevin did not change. These data indicate that vti1b is specifically required for the stability of a single SNARE partner. vti1b-deficient mice were viable and fertile. Most vti1b-deficient mice were indistinguishable from wild-type mice and did not display defects in transport to the lysosome. However, 20% of the vti1b-deficient mice were smaller. Lysosomal degradation of an endocytosed protein was slightly delayed in hepatocytes derived from these mice. Multivesicular bodies and autophagic vacuoles accumulated in hepatocytes of some smaller vti1b-deficient mice. This suggests that other SNAREs can compensate for the reduction in syntaxin 8 and for the loss of vti1b in most mice even though vti1b shows only 30% amino acid identity with its closest relative.

Golgi architecture and inheritance

Golgi inheritance proceeds via sequential biogenesis and partitioning phases. Although little is known about Golgi growth and replication (biogenesis), ultrastructural and fluorescence analyses have provided a detailed, though still controversial, perspective of Golgi partitioning during mitosis in mammalian cells. Partitioning requires the fragmentation of the juxtanuclear ribbon of interconnected Golgi stacks into a multitude of tubulovesicular clusters. This process is choreographed by a cohort of mitotic kinases and an inhibition of heterotypic and homotypic Golgi membrane-fusion events. Our model posits that accurate partitioning occurs early in mitosis by the equilibration of Golgi components on either side of the metaphase plate. Disseminated Golgi components then coalesce to regenerate Golgi stacks during telophase. Semi-intact cell and cell-free assays have accurately recreated these processes and allowed their molecular dissection. This review attempts to integrate recent findings to depict a more coherent, synthetic molecular picture of mitotic Golgi fragmentation and reassembly. Of particular importance is the emerging concept of a highly regulated and dynamic Golgi structural matrix or template that interfaces with cargo receptors, Golgi enzymes, Rab-GTPases, and SNAREs to tightly couple biosynthetic transport to Golgi architecture. This structural framework may be instructive for Golgi biogenesis and may encode sufficient information to ensure accurate Golgi inheritance, thereby helping to resolve some of the current discrepancies between different workers.

Membrane fusion

Membrane fusion, one of the most fundamental processes in life, occurs when two separate lipid membranes merge into a single continuous bilayer. Fusion reactions share common features, but are catalyzed by diverse proteins. These proteins mediate the initial recognition of the membranes that are destined for fusion and pull the membranes close together to destabilize the lipid/water interface and to initiate mixing of the lipids. A single fusion protein may do everything or assemblies of protein complexes may be required for intracellular fusion reactions to guarantee rigorous regulation in space and time. Cellular fusion machines are adapted to fit the needs of different reactions but operate by similar principles in order to achieve merging of the bilayers.

Mammalian ykt6 is a neuronal SNARE targeted to a specialized compartment by its profilin-like amino terminal domain

SNAREs are required for specific membrane fusion throughout the endomembrane system. Here we report the characterization of rat ykt6, a prenylated SNARE selectively expressed in brain neurons. Immunofluorescence microscopy in neuronal and neuroendocrine cell lines revealed that membrane-associated ykt6 did not colocalize significantly with any conventional markers of endosomes, lysosomes, or the secretory pathway. However, ykt6-containing membranes displayed very minor overlaps with lysosomes and dense-core secretory granules and were similar to lysosomes in buoyant density. Thus, ykt6 appears to be specialized for the trafficking of a unique membrane compartment, perhaps related to lysosomes, involved in aspects of neuronal function. Targeting of this SNARE to the ykt6 compartment was mediated by its profilin-like amino-terminal domain, even in the absence of protein prenylation. Although several other R-SNAREs contain related amino-terminal domains, only the ykt6 version was able to confer the specialized localization. Rat ykt6, which contains an arginine in its SNARE motif zero-layer, was found to behave like other R-SNAREs in its SNARE assembly properties. Interestingly, cytosolic ykt6, constituting more than half of the total cellular pool, appeared to be conformationally inactive for SNARE complex assembly, perhaps indicative of a regulatory mechanism that prevents promiscuous and potentially deleterious SNARE interactions.

Insights from yeast endosomes

The endosomal system of yeast is simpler than that of animal cells, but as it is mapped more similarities are emerging. A key role for ubiquitin in sorting proteins to and into multivesicular bodies has been demonstrated. The finding that Phox homology domains recognise phosphatidylinositol 3-phosphate explains how sorting nexins are recruited to endosomes, where they mediate the retrieval of membrane proteins from the endocytic 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.

t-SNARE phosphorylation regulates endocytosis in yeast

Earlier we demonstrated that activation of a ceramide-activated protein phosphatase (CAPP) conferred normal growth and secretion to yeast lacking their complement of exocytic v-SNAREs (Snc1,2) or bearing a temperature-sensitive mutation in an exocytic t-SNARE (Sso2). CAPP activation led to Sso dephosphorylation and enhanced the assembly of t-SNAREs into functional complexes. Thus, exocytosis in yeast is modulated by t-SNARE phosphorylation. Here, we show that endocytic defects in cells lacking the v- and t-SNAREs involved in endocytosis are also rescued by CAPP activation. Yeast lacking the Tlg1 or Tlg2 t-SNAREs, the Snc v-SNAREs, or both, undergo endocytosis after phosphatase activation. CAPP activation correlated with restored uptake of FM4-64 to the vacuole, the uptake and degradation of the Ste2 receptor after mating factor treatment, and the dephosphorylation and assembly of Tlg1,2 into SNARE complexes. Activation of the phosphatase by treatment with C(2)-ceramide, VBM/ELO gene inactivation, or by the overexpression of SIT4 was sufficient to confer rescue. Finally, we found that mutation of single PKA sites in Tlg1 (Ser31 to Ala31) or Tlg2 (Ser90 to Ala90) was sufficient to restore endocytosis, but not exocytosis, to snc cells. These results suggest that endocytosis is also modulated by t-SNARE phosphorylation in vivo.

A t-SNARE of the endocytic pathway must be activated for fusion

The t-SNARE in a late Golgi compartment (Tlg2p) syntaxin is required for endocytosis and localization of cycling proteins to the late Golgi compartment in yeast. We show here that Tlg2p assembles with two light chains, Tlg1p and Vti1p, to form a functional t-SNARE that mediates fusion, specifically with the v-SNAREs Snc1p and Snc2p. In vitro, this t-SNARE is inert, locked in a nonfunctional state, unless it is activated for fusion. Activation can be mediated by a peptide derived from the v-SNARE, which likely bypasses additional regulatory proteins in the cell. Locking t-SNAREs creates the potential for spatial and temporal regulation of fusion by signaling processes that unleash their fusion capacity.