rsly1 binding to syntaxin 5 is required for endoplasmic reticulum-to-Golgi transport but does not promote SNARE motif accessibility

An imaging-based RNA interference screen for modulators of the Rab6-mediated Golgi-to-ER pathway in mammalian cells

In mammalian cells, membrane traffic pathways play a critical role in connecting the various compartments of the endomembrane system. Each of these pathways is highly regulated, requiring specific machinery to ensure their fidelity. In the early secretory pathway, transport between the endoplasmic reticulum (ER) and Golgi apparatus is largely regulated via cytoplasmic coat protein complexes that play a role in identifying cargo and forming the transport carriers. The secretory pathway is counterbalanced by the retrograde pathway, which is essential for the recycling of molecules from the Golgi back to the ER. It is believed that there are at least two mechanisms to achieve this - one using the cytoplasmic COPI coat complex, and another, poorly characterised pathway, regulated by the small GTPase Rab6. In this work, we describe a systematic RNA interference screen targeting proteins associated with membrane fusion, in order to identify the machinery responsible for the fusion of Golgi-derived Rab6 carriers at the ER. We not only assess the delivery of Rab6 to the ER, but also one of its cargo molecules, the Shiga-like toxin B-chain. These screens reveal that three proteins, VAMP4, STX5, and SCFD1/SLY1, are all important for the fusion of Rab6 carriers at the ER. Live cell imaging experiments also show that the depletion of SCFD1/SLY1 prevents the membrane fusion event, suggesting that this molecule is an essential regulator of this pathway.

ER-PM Contact Sites - SNARING Actors in Emerging Functions

The compartmentalisation achieved by confining cytoplasm into membrane-enclosed organelles in eukaryotic cells is essential for maintaining vital functions including ATP production, synthetic and degradative pathways. While intracellular organelles are highly specialised in these functions, the restricting membranes also impede exchange of molecules responsible for the synchronised and responsive cellular activities. The initial identification of contact sites between the ER and plasma membrane (PM) provided a potential candidate structure for communication between organelles without mixing by fusion. Over the past decades, research has revealed a far broader picture of the events. Membrane contact sites (MCSs) have been recognized as increasingly important actors in cell differentiation, plasticity and maintenance, and, upon dysfunction, responsible for pathological conditions such as cancer and neurodegenerative diseases. Present in multiple organelles and cell types, MCSs promote transport of lipids and Ca²⁺ homoeostasis, with a range of associated protein families. Interestingly, each MCS displays a unique molecular signature, adapted to organelle functions. This review will explore the literature describing the molecular components and interactions taking place at ER-PM contact sites, their functions, and implications in eukaryotic cells, particularly neurons, with emphasis on lipid transfer proteins and emerging function of SNAREs.

Stx5-Mediated ER-Golgi Transport in Mammals and Yeast

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxin 5 (Stx5) in mammals and its ortholog Sed5p in Saccharomyces cerevisiae mediate anterograde and retrograde endoplasmic reticulum (ER)-Golgi trafficking. Stx5 and Sed5p are structurally highly conserved and are both regulated by interactions with other ER-Golgi SNARE proteins, the Sec1/Munc18-like protein Scfd1/Sly1p and the membrane tethering complexes COG, p115, and GM130. Despite these similarities, yeast Sed5p and mammalian Stx5 are differently recruited to COPII-coated vesicles, and Stx5 interacts with the microtubular cytoskeleton, whereas Sed5p does not. In this review, we argue that these different Stx5 interactions contribute to structural differences in ER-Golgi transport between mammalian and yeast cells. Insight into the function of Stx5 is important given its essential role in the secretory pathway of eukaryotic cells and its involvement in infections and neurodegenerative diseases.

Sec24C/D-isoform-specific sorting of the preassembled ER-Golgi Q-SNARE complex

SNAREs are incorporated into COPII vesicles by direct interaction with Sec24. In mammals, Sec24 isoforms recruit either Sec22b or the Q-SNARE complex comprising Syntaxin5, GS27, and Bet1. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicates that all ER-Golgi SNAREs are present on the same vesicles.

Secretory proteins are exported from the endoplasmic reticulum in COPII vesicles. SNARE proteins—core machinery for membrane fusion—are incorporated into COPII vesicles by direct interaction with Sec24. Here we report a novel mechanism for sorting of the ER–Golgi Q-SNAREs into COPII vesicles. Different mammalian Sec24 isoforms recruit either the R-SNARE Sec22b or the Q-SNAREs Syntaxin5, GS27, and Bet1. Syntaxin5 is the only Q-SNARE that directly interacts with Sec24C, requiring its “open” conformation. Mutation within the IxM cargo-binding site of Sec24C led to a drastic reduction in sorting of all three Q-SNAREs into COPII vesicles, implying their ER export as a preassembled complex. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicate that all ER–Golgi SNAREs are present on the same vesicle. Combined with existing data, our findings yield a general concept of how Sec24 isoforms can recruit fusogenic SNARE subunits to keep them functionally apart and thus prime mammalian COPII vesicles for homotypic fusion.

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.

The SM protein Sly1 accelerates assembly of the ER-Golgi SNARE complex

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins constitute the core of an ancient vesicle fusion machine that diversified into distinct sets that now function in different trafficking steps in eukaryotic cells. Deciphering their precise mode of action has proved challenging. SM proteins are thought to act primarily through one type of SNARE protein, the syntaxins. Despite high structural similarity, however, contrasting binding modes have been found for different SM proteins and syntaxins. Whereas the secretory SM protein Munc18 binds to the ‟closed conformation" of syntaxin 1, the ER-Golgi SM protein Sly1 interacts only with the N-peptide of Sed5. Recent findings, however, indicate that SM proteins might interact simultaneously with both syntaxin regions. In search for a common mechanism, we now reinvestigated the Sly1/Sed5 interaction. We found that individual Sed5 adopts a tight closed conformation. Sly1 binds to both the closed conformation and the N-peptide of Sed5, suggesting that this is the original binding mode of SM proteins and syntaxins. In contrast to Munc18, however, Sly1 facilitates SNARE complex formation by loosening the closed conformation of Sed5.

GPR107, a G-protein-coupled receptor essential for intoxication by Pseudomonas aeruginosa exotoxin A, localizes to the Golgi and is cleaved by furin

A number of toxins, including exotoxin A (PE) of Pseudomonas aeruginosa, kill cells by inhibiting protein synthesis. PE kills by ADP-ribosylation of the translation elongation factor 2, but many of the host factors required for entry, membrane translocation, and intracellular transport remain to be elucidated. A genome-wide genetic screen in human KBM7 cells was performed to uncover host factors used by PE, several of which were confirmed by CRISPR/Cas9-gene editing in a different cell type. Several proteins not previously implicated in the PE intoxication pathway were identified, including GPR107, an orphan G-protein-coupled receptor. GPR107 localizes to the trans-Golgi network and is essential for retrograde transport. It is cleaved by the endoprotease furin, and a disulfide bond connects the two cleaved fragments. Compromising this association affects the function of GPR107. The N-terminal region of GPR107 is critical for its biological function. GPR107 might be one of the long-sought receptors that associates with G-proteins to regulate intracellular vesicular transport.

The highly conserved COPII coat complex sorts cargo from the endoplasmic reticulum and targets it to the golgi

Protein egress from the endoplasmic reticulum (ER) is driven by a conserved cytoplasmic coat complex called the COPII coat. The COPII coat complex contains an inner shell (Sec23/Sec24) that sorts cargo into ER-derived vesicles and an outer cage (Sec13/Sec31) that leads to coat polymerization. Once released from the ER, vesicles must tether to and fuse with the target membrane to deliver their protein and lipid contents. This delivery step also depends on the COPII coat, with coat proteins binding directly to tethering and regulatory factors. Recent findings have yielded new insight into how COPII-mediated vesicle traffic is regulated. Here we discuss the molecular basis of COPII-mediated ER-Golgi traffic, focusing on the surprising complexity of how ER-derived vesicles form, package diverse cargoes, and correctly target these cargoes to their destination.

Munc18-1 mutations that strongly impair SNARE-complex binding support normal synaptic transmission

Synaptic transmission depends critically on the Sec1p/Munc18 protein Munc18-1, but it is unclear whether Munc18-1 primarily operates as a integral part of the fusion machinery or has a more upstream role in fusion complex assembly. Here, we show that point mutations in Munc18-1 that interfere with binding to the free Syntaxin1a N-terminus and strongly impair binding to assembled SNARE complexes all support normal docking, priming and fusion of synaptic vesicles, and normal synaptic plasticity in munc18-1 null mutant neurons. These data support a prevailing role of Munc18-1 before/during SNARE-complex assembly, while its continued association to assembled SNARE complexes is dispensable for synaptic transmission.

Myosin VI and its binding partner optineurin are involved in secretory vesicle fusion at the plasma membrane

During constitutive secretion, proteins synthesized at the endoplasmic reticulum (ER) are transported to the Golgi complex for processing and then to the plasma membrane for incorporation or extracellular release. This study uses a unique live-cell constitutive secretion assay to establish roles for the molecular motor myosin VI and its binding partner optineurin in discrete stages of secretion. Small interfering RNA-based knockdown of myosin VI causes an ER-to-Golgi transport delay, suggesting an unexpected function for myosin VI in the early secretory pathway. Depletion of myosin VI or optineurin does not affect the number of vesicles leaving the trans-Golgi network (TGN), indicating that these proteins do not function in TGN vesicle formation. However, myosin VI and optineurin colocalize with secretory vesicles at the plasma membrane. Furthermore, live-cell total internal reflection fluorescence microscopy demonstrates that myosin VI or optineurin depletion reduces the total number of vesicle fusion events at the plasma membrane and increases both the proportion of incomplete fusion events and the number of docked vesicles in this region. These results suggest a novel role for myosin VI and optineurin in regulation of fusion pores formed between secretory vesicles and the plasma membrane during the final stages of secretion.

A targeted siRNA screen to identify SNAREs required for constitutive secretion in mammalian cells

The role of SNAREs in mammalian constitutive secretion remains poorly defined. To address this, we have developed a novel flow cytometry-based assay for measuring constitutive secretion and have performed a targeted SNARE and Sec1/Munc18 (SM) protein-specific siRNA screen (38 SNAREs, 4 SNARE-like proteins and 7 SM proteins). We have identified the endoplasmic reticulum (ER)/Golgi SNAREs syntaxin 5, syntaxin 17, syntaxin 18, GS27, SLT1, Sec20, Sec22b, Ykt6 and the SM protein Sly1, along with the post-Golgi SNAREs SNAP-29 and syntaxin 19, as being required for constitutive secretion. Depletion of SNAP-29 or syntaxin 19 causes a decrease in the number of fusion events at the cell surface and in SNAP-29-depleted cells causes an increase in the number of docked vesicles at the plasma membrane as determined by total internal reflection fluorescence (TIRF) microscopy. Analysis of syntaxin 19-interacting partners by mass spectrometry indicates that syntaxin 19 can form SNARE complexes with SNAP-23, SNAP-25, SNAP-29, VAMP3 and VAMP8, supporting its role in Golgi to plasma membrane transport or fusion. Surprisingly, we have failed to detect any requirement for a post-Golgi-specific R-SNARE in this process.

Vesicular calcium regulates coat retention, fusogenicity, and size of pre-Golgi intermediates

This study establishes a role for luminal Ca²⁺ in ER/Golgi transport organelles and elucidates an effector mechanism involving the EF-hand protein ALG-2 and regulation of COPII coat retention.

The significance and extent of Ca²⁺ regulation of the biosynthetic secretory pathway have been difficult to establish, and our knowledge of regulatory relationships integrating Ca²⁺ with vesicle coats and function is rudimentary. Here, we investigated potential roles and mechanisms of luminal Ca²⁺ in the early secretory pathway. Specific depletion of luminal Ca²⁺ in living normal rat kidney cells using cyclopiazonic acid (CPA) resulted in the extreme expansion of vesicular tubular cluster (VTC) elements. Consistent with this, a suppressive role for vesicle-associated Ca²⁺ in COPII vesicle homotypic fusion was demonstrated in vitro using Ca²⁺ chelators. The EF-hand–containing protein apoptosis-linked gene 2 (ALG-2), previously implicated in the stabilization of sec31 at endoplasmic reticulum exit sites, inhibited COPII vesicle fusion in a Ca²⁺-requiring manner, suggesting that ALG-2 may be a sensor for the effects of vesicular Ca²⁺ on homotypic fusion. Immunoisolation established that Ca²⁺ chelation inhibits and ALG-2 specifically favors residual retention of the COPII outer shell protein sec31 on pre-Golgi fusion intermediates. We conclude that vesicle-associated Ca²⁺, acting through ALG-2, favors the retention of residual coat molecules that seem to suppress membrane fusion. We propose that in cells, these Ca²⁺-dependent mechanisms temporally regulate COPII vesicle interactions, VTC biogenesis, cargo sorting, and VTC maturation.

Direct interaction between the COG complex and the SM protein, Sly1, is required for Golgi SNARE pairing

The crucial roles of Sec1/Munc18 (SM)-like proteins in membrane fusion have been evidenced in genetic and biochemical studies. SM proteins interact directly with SNAREs and contribute to SNARE pairing by a yet unclear mechanism. Here, we show that the SM protein, Sly1, interacts directly with the conserved oligomeric Golgi (COG) tethering complex. The Sly1-COG interaction is mediated by the Cog4 subunit, which also interacts with Syntaxin 5 through a different binding site. We provide evidence that disruption of Cog4-Sly1 interaction impairs pairing of SNAREs involved in intra-Golgi transport thereby markedly attenuating Golgi-to-ER retrograde transport. These results highlight the mechanism by which SM proteins link tethering to SNAREpin assembly.

UNC-18 promotes both the anterograde trafficking and synaptic function of syntaxin

The SM protein UNC-18 has been proposed to regulate several aspects of secretion, including synaptic vesicle docking, priming, and fusion. Here, we show that UNC-18 has a chaperone function in neurons, promoting anterograde transport of the plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein Syntaxin-1. In unc-18 mutants, UNC-64 (Caenorhabditis elegans Syntaxin-1) accumulates in neuronal cell bodies. Colocalization studies and analysis of carbohydrate modifications both suggest that this accumulation occurs in the endoplasmic reticulum. This trafficking defect is specific for UNC-64 Syntaxin-1, because 14 other SNARE proteins and two active zone markers were unaffected. UNC-18 binds to Syntaxin through at least two mechanisms: binding to closed Syntaxin, or to the N terminus of Syntaxin. It is unclear which of these binding modes mediates UNC-18 function in neurons. The chaperone function of UNC-18 was eliminated in double mutants predicted to disrupt both modes of Syntaxin binding, but it was unaffected in single mutants. By contrast, mutations predicted to disrupt UNC-18 binding to the N terminus of Syntaxin caused significant defects in locomotion behavior and responsiveness to cholinesterase inhibitors. Collectively, these results demonstrate the UNC-18 acts as a molecular chaperone for Syntaxin transport in neurons and that the two modes of UNC-18 binding to Syntaxin are involved in different aspects of UNC-18 function.

Interaction of the conserved oligomeric Golgi complex with t-SNARE Syntaxin5a/Sed5 enhances intra-Golgi SNARE complex stability

Tethering factors mediate initial interaction of transport vesicles with target membranes. Soluble N-ethylmaleimide–sensitive fusion protein attachment protein receptors (SNAREs) enable consequent docking and membrane fusion. We demonstrate that the vesicle tether conserved oligomeric Golgi (COG) complex colocalizes and coimmunoprecipitates with intra-Golgi SNARE molecules. In yeast cells, the COG complex preferentially interacts with the SNARE complexes containing yeast Golgi target (t)-SNARE Sed5p. In mammalian cells, hCog4p and hCog6p interact with Syntaxin5a, the mammalian homologue of Sed5p. Moreover, fluorescence resonance energy transfer reveals an in vivo interaction between Syntaxin5a and the COG complex. Knockdown of the mammalian COG complex decreases Golgi SNARE mobility, produces an accumulation of free Syntaxin5, and decreases the steady-state levels of the intra-Golgi SNARE complex. Finally, overexpression of the hCog4p N-terminal Syntaxin5a-binding domain destabilizes intra-Golgi SNARE complexes, disrupting the Golgi. These data suggest that the COG complex orchestrates vesicular trafficking similarly in yeast and mammalian cells by binding to the t-SNARE Syntaxin5a/Sed5p and enhancing the stability of intra-Golgi SNARE complexes.

Cellular levels of the syntaxin Tlg2p are regulated by a single mode of binding to Vps45p

Sec1p/Munc18 (SM) proteins play a key role in the regulation of soluble N-ethylmaleimide-sensitive fusion (NSF)-attachment protein receptor (SNARE)-mediated intracellular membrane trafficking events in all eukaryotic cells. Understanding the molecular mechanisms by which SM proteins function has not been straight forward as SM proteins bind to their cognate SNARE proteins by at least two distinct mechanisms, suggesting that they provide more than one function. We have previously characterised two binding modes used by the yeast SM protein Vps45p to interact with its SNARE proteins. In one of these modes, the N terminus of the syntaxin Tlg2p inserts into a hydrophobic pocket in the SM protein. We now report that disruption of this high-affinity binding between Vps45p and Tlg2p leads to downregulation of Tlg2p, and propose that this pocket-mode of binding of SM proteins to their cognate syntaxins serves to regulate cellular levels of the syntaxin.

The SM protein VPS-45 is required for RAB-5-dependent endocytic transport in Caenorhabditis elegans

Rab5, a small guanosine triphosphatase, is known to regulate the tethering and docking reaction leading to SNARE (soluble NSF attachment protein receptors)-mediated fusion between endosomes. However, it is uncertain how the signal of the activated Rab5 protein is transduced by its downstream effectors during endosome fusion. Here, we show that the Sec1/Munc18 gene vps-45 is essential for not only viability and development but also receptor-mediated and fluid-phase endocytosis pathways in Caenorhabditis elegans. We found that VPS-45 interacts with a Rab5 effector, Rabenosyn-5 (RABS-5), and the mutants of both vps-45 and rabs-5 show similar endocytic phenotypes. In the macrophage-like cells of vps-45 and rabs-5 mutants, aberrantly small endosomes were accumulated, and the endosome fusion stimulated by the mutant RAB-5 (Q78L) is suppressed by these mutations. Our results indicate that VPS-45 is a key molecule that functions downstream from RAB-5, cooperating with RABS-5, to regulate the dynamics of the endocytic system in multicellular organisms.

Selective Activation of Cognate SNAREpins by Sec1/Munc18 Proteins

Sec1/Munc18 (SM) proteins are required for every step of intracellular membrane fusion, but their molecular mechanism of action has been unclear. In this work, we demonstrate a fundamental role of the SM protein: to act as a stimulatory subunit of its cognate SNARE fusion machinery. In a reconstituted system, mammalian SNARE pairs assemble between bilayers to drive a basal fusion reaction. Munc18-1/nSec1, a synaptic SM protein required for neurotransmitter release, strongly accelerates this reaction through direct contact with both t- and v-SNAREs. Munc18-1 accelerates fusion only for the cognate SNAREs for exocytosis, therefore enhancing fusion specificity.

SNARE status regulates tether recruitment and function in homotypic COPII vesicle fusion

In mammals, coat complex II (COPII)-coated transport vesicles deliver secretory cargo to vesicular tubular clusters (VTCs) that facilitate cargo sorting and transport to the Golgi. We documented in vitro tethering and SNARE-dependent homotypic fusion of endoplasmic reticulum-derived COPII transport vesicles to form larger cargo containers characteristic of VTCs ( Xu, D., and Hay, J. C. (2004) J. Cell Biol. 167, 997-1003). COPII vesicles thus appear to contain all necessary components for homotypic tethering and fusion, providing a pathway for de novo VTC biogenesis. Here we demonstrate that antibodies against the endoplasmic reticulum/Golgi SNARE Syntaxin 5 inhibit COPII vesicle homotypic tethering as well as fusion, implying an unanticipated role for SNAREs upstream of fusion. Inhibition of SNARE complex access and/or disassembly with dominant-negative alpha-soluble NSF attachment protein (SNAP) also inhibited tethering, implicating SNARE status as a critical determinant in COPII vesicle tethering. The tethering-defective vesicles generated in the presence of dominant-negative alpha-SNAP specifically lacked the Rab1 effectors p115 and GM130 but not other peripheral membrane proteins. Furthermore, Rab effectors, including p115, were shown to be required for homotypic COPII vesicle tethering. Thus, our results demonstrate a requirement for SNARE-dependent tether recruitment and function in COPII vesicle fusion. We anticipate that recruitment of tether molecules by an upstream SNARE signal ensures that tethering events are initiated only at focal sites containing appropriately poised fusion machinery.

The Sec1p/Munc18 protein Vps45p binds its cognate SNARE proteins via two distinct modes

Sec1p/Munc18 (SM) proteins are essential for SNARE-mediated membrane trafficking. The formulation of unifying hypotheses for the function of the SM protein family has been hampered by the observation that two of its members bind their cognate syntaxins (Sxs) in strikingly different ways. The SM protein Vps45p binds its Sx Tlg2p in a manner analogous to that captured by the Sly1p–Sed5p crystal structure, whereby the NH₂-terminal peptide of the Sx inserts into a hydrophobic pocket on the outer face of domain I of the SM protein. In this study, we report that although this mode of interaction is critical for the binding of Vps45p to Tlg2p, the SM protein also binds Tlg2p-containing SNARE complexes via a second mode that involves neither the NH₂ terminus of Tlg2p nor the region of Vps45p that facilitates this interaction. Our findings point to the possibility that SM proteins interact with their cognate SNARE proteins through distinct mechanisms at different stages in the SNARE assembly/disassembly cycle.

Identification of new Golgi complex specific proteins by direct organelle proteomic analysis

The Golgi complex is in the crossroad of the endocytic and secretory pathways. Its function is to post-translationally modify and sort proteins and lipids, and regulate the membrane balance in the cell. To understand the structure-function relationship of the Golgi complex the Golgi proteome has to be identified first. We have used a direct organelle proteomic analysis to identify new Golgi complex proteins. Enriched stacked Golgi membrane fractions from rat livers were isolated, and the proteins from these membranes were subsequently digested into peptides. The peptides were fractionated by cation-exchange chromatography followed by protein identification by automated capillary-LC/ESI-MS/MS analysis and database searches. Two different search programs, ProID and MASCOT were used. This resulted in a total of 1125 protein identifications in two experiments. In addition to the known Golgi resident proteins, a significant number of unknown proteins were identified. Some of these were further characterized in silico using different programs to provide insight into their structure, intracellular localization and biological functions. The Golgi localization of two of these newly identified proteins was also confirmed by indirect immunofluorescence.

Evidence for regulation of ER/Golgi SNARE complex formation by hsc70 chaperones

SNARE proteins control intracellular membrane fusion through formation of membrane-bridging helix bundles of amphipathic SNARE motifs. Repetitive cycles of membrane fusion likely involve repetitive folding/unfolding of the SNARE motif helical structure. Despite these conformational demands, little is known about conformational regulation of SNAREs by other proteins. Here we demonstrate that hsc70 chaperones stimulate in vitro SNARE complex formation among the ER/Golgi SNAREs syntaxin 5, membrin, rbetl and sec22b, under conditions in which assembly is normally inhibited. Thus, molecular chaperones can render the SNARE motif more competent for assembly. Partially purified hsc70 fractions from brain cytosol had higher specific activities than fully purified hsc70, suggesting the involvement of unidentified cofactors. Using chemical crosslinking of cells followed by immunoprecipitation, we found that hsc70 was associated with ER/Golgi SNAREs in vivo. Consistent with a modulatory role for hsc70 in transport, we found that excess hsc70 specifically inhibited ER-to-Golgi transport in permeabilized cells.

Syntaxin 5 interacts specifically with presenilin holoproteins and affects processing of betaAPP in neuronal cells

The specific roles of syntaxin 5 (Syx 5) in the interaction with presenilin (PS) and the accumulation of beta-amyloid precursor protein (betaAPP), as well as the secretion of beta-amyloid peptide (Abeta peptide) were examined in NG108-15 cells. Syx 5, which localizes from the endoplasmic reticulum (ER) to the Golgi, bound to PS holoproteins, while the other Syxs studied did not. Among familial Alzheimer's disease (FAD)-linked PS mutants, PS1deltaE9, which lacks the endoproteolytic cleavage site, showed markedly decreased binding to Syx 5. The interaction domains in Syx 5 were mapped to the transmembrane region and to the cytoplasmic region containing the alpha-helical domains, which are distinct from the H3 (SNARE motif). Among all of the Syxs examined, only overexpression of Syx 5 resulted in the accumulation of betaAPP in the ER to cis-Golgi compartment, an attenuation of the amount of the C-terminal fragment (APP-CTF) of betaAPP, and a reduction in the secretion of Abeta peptides. Furthermore, co-expression of Syx 5 with C99 resulted in an increase in APP-CTF and suppressed Abeta secretion. Taken together, these results indicate that Syx 5 may play a specific role in the modulation of processing and/or trafficking of FAD-related proteins in neuronal cells by interaction with PS holoproteins in the early secretory compartment of neuronal cells.

RNA interference-mediated silencing of the syntaxin 5 gene induces Golgi fragmentation but capable of transporting vesicles

It has been suggested that syntaxin 5 (Syx5) participates in vesicular transport. We examined the effects of Syx5 down-regulation on the morphology of the Golgi apparatus and the transport of vesicles in mammalian cells. Knockdown of the Syx5 gene resulted in Golgi fragmentation without changing the level of endoplasmic reticulum (ER)-resident proteins, other Golgi-SNAREs (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors), and coatmer proteins. Strikingly, a major decrease in Syx5 expression barely affected the anterograde transport of vesicular stomatitis virus G (VSVG) protein to the plasma membrane. These results suggest that Syx5 is required for the maintenance of the Golgi structures but may not play a major role in the transport of vesicles carrying VSVG between the ER and the Golgi compartment.

The subcellular localization of syntaxin 17 varies among different cell types and is altered in some malignant cells

Syntaxin 17 (STX17) is a divergent member of the syntaxin family of proteins first discovered by Scheller and colleagues in a yeast two-hybrid screen designed to identify novel mammalian SNAREs (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors). We recently independently identified STX17 as a novel Ras-interacting protein, but immunohistochemical studies suggested that STX17 is localized to the nucleus in normal pancreatic ductal epithelial, acinar, and islet cells in contrast to previous reports of cytoplasmic localization, albeit in other cell types. Therefore, we have conducted a more thorough survey of various human and mouse tissues to better establish the expression pattern of STX17 in different tissues and cell types. Although RT-PCR experiments demonstrate ubiquitous expression of STX17, closer examination by immunohistochemistry reveal that STX17 expression is limited to certain cell types. Furthermore, in contrast to the cytoplasmic localization previously reported in a limited number of cell types, we find that in many other cell types, syntaxin 17 can be found in the nucleus. Finally, we demonstrate that in human hepatocellular carcinoma cell lines, STX17 localization is altered relative to normal hepatocytes, although the localization of STX17 differs even between these established human cancer cell lines and fresh human hepatocellular carcinoma cells, emphasizing the caution that must be exercised in drawing conclusions from data gathered in cell lines. The sequence divergence of STX17, the unexpected nuclear localization of STX17 in many cell types, and the altered localization of STX17 in malignant cells argue for a novel function of syntaxin 17 distinct from its hypothesized role in mediating membrane fusion events.

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.

Reconstitution of COPII vesicle fusion to generate a pre-Golgi intermediate compartment

What is the first membrane fusion step in the secretory pathway? In mammals, transport vesicles coated with coat complex (COP) II deliver secretory cargo to vesicular tubular clusters (VTCs) that ferry cargo from endoplasmic reticulum exit sites to the Golgi stack. However, the precise origin of VTCs and the membrane fusion step(s) involved have remained experimentally intractable. Here, we document in vitro direct tethering and SNARE-dependent fusion of endoplasmic reticulum–derived COPII transport vesicles to form larger cargo containers. The assembly did not require detectable Golgi membranes, preexisting VTCs, or COPI function. Therefore, COPII vesicles appear to contain all of the machinery to initiate VTC biogenesis via homotypic fusion. However, COPI function enhanced VTC assembly, and early VTCs acquired specific Golgi components by heterotypic fusion with Golgi-derived COPI vesicles.

Three-dimensional Structure of the rSly1 N-terminal Domain Reveals a Conformational Change Induced by Binding to Syntaxin 5

Sec1/Mun18-like (SM) proteins and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) play central roles in intracellular membrane fusion. Diverse modes of interaction between SM proteins and SNAREs from the syntaxin family have been described. However, the observation that the N-terminal domains of Sly1 and Vps45, the SM proteins involved in traffic at the endoplasmic reticulum, the Golgi, the trans-Golgi network and the endosomes, bind to similar N-terminal sequences of their cognate syntaxins suggested a unifying theme for SM protein/SNARE interactions in most internal membrane compartments. To further understand this mechanism of SM protein/SNARE coupling, we have elucidated the structure in solution of the isolated N-terminal domain of rat Sly1 (rSly1N) and analyzed its complex with an N-terminal peptide of rat syntaxin 5 by NMR spectroscopy. Comparison with the crystal structure of a complex between Sly1p and Sed5p, their yeast homologues, shows that syntaxin 5 binding requires a striking conformational change involving a two-residue shift in the register of the C-terminal beta-strand of rSly1N. This conformational change is likely to induce a significant alteration in the overall shape of full-length rSly1 and may be critical for its function. Sequence analyses indicate that this conformational change is conserved in the Sly1 family but not in other SM proteins, and that the four families represented by the four SM proteins found in yeast (Sec1p, Sly1p, Vps45p and Vps33p) diverged early in evolution. These results suggest that there are marked distinctions between the mechanisms of action of each of the four families of SM proteins, which may have arisen from different regulatory requirements of traffic in their corresponding membrane compartments.

Multiple SNARE interactions of an SM protein: Sed5p/Sly1p binding is dispensable for transport

Sec1/Munc18 (SM) proteins are central to intracellular transport and neurotransmitter release but their exact role is still elusive. Several SM proteins, like the neuronal N-Sec1 and the yeast Sly1 protein, bind their cognate t-SNAREs with high affinity. This has been thought to be critical for their function. Here, we show that various mutant forms of Sly1p and the Golgi-localized syntaxin Sed5p, which abolish their high-affinity interaction, are fully functional in vivo, indicating that the tight interaction of the two molecules per se is not relevant for proper function. Mutant Sly1p unable to bind Sed5p is excluded from core SNARE complexes, also demonstrating that Sly1p function is not directly coupled to assembled SNARE complexes thought to execute membrane fusion. We also find that wild-type Sly1p and mutant Sly1p unable to bind Sed5p directly interact with selected ER-to-Golgi and intra-Golgi nonsyntaxin SNAREs. The newly identified, direct interactions of the SM protein with nonsytaxin SNAREs might provide a molecular mechanism by which SNAREs can be activated to engage in pairing and assemble into fusogenic SNARE complexes.