Defining the functions of trans-SNARE pairs

Disassembly of membrane-associated NSF 20S complexes is slow relative to vesicle fusion and is Ca(2+)-independent

N-ethylmaleimide-sensitive fusion protein (NSF) and its co-factor soluble NSF attachment protein (alpha)-SNAP) are essential components of the synaptic vesicle fusion machinery and form part of a structurally-conserved 20S protein complex. However, their precise function, relative to fusion itself, is not clear. Using a UV-activated cross-linking approach, we have measured the rate at which a single round of NSF-driven ATP hydrolysis leads to 20S complex disassembly within synaptic membranes. Although this rate is substantially faster than previous estimates of NSF-dependent ATP hydrolysis, it remains much lower than published rates for fusion of synaptic vesicles. Furthermore, the stability of 20S complexes is unaffected by Ca(2+) at concentrations that elicit rapid membrane fusion. We conclude that the ATPase activity of NSF does not contribute directly to vesicle fusion, but more likely plays an earlier role in the synaptic vesicle cycle.

A 56-kDa selenium-binding protein participates in intra-Golgi protein transport

Transport of proteins between intracellular membrane compartments is a highly regulated process that depends on several cytosolic factors. By using the well characterized intra-Golgi cell-free transport assay, we purified from bovine brain cytosol a 56-kDa protein that shows a significant transport activity. Partial sequencing of four tryptic peptides obtained from the 56-kDa protein revealed its identity to a cytosolic protein previously characterized as a selenium-binding protein, SBP56. Recombinant SBP56 expressed in Escherichia coli exhibited transport activity when added to the cell-free intra-Golgi transport. Affinity purified anti-SBP56 polyclonal antibodies specifically inhibited intra-Golgi transport in vitro. Although SBP56 is predominantly localized in the cytosol, a significant amount is associated with membranes. Subcellular fractionation showed that this protein is peripherally associated with the Golgi membrane. The experiments presented in this study indicate that SBP56 participates in late stages of intra-Golgi protein transport.

Priming in exocytosis: attaining fusion-competence after vesicle docking

Membrane contact established by tethering or docking mechanisms is not a sufficient condition for membrane fusion. In neural and neuroendocrine cells, only a small fraction of secretory vesicles docked at the plasma membrane are fusion-competent and undergo rapid ATP-independent fusion in response to Ca(2+) elevations. Additional biochemical events termed 'priming' are essential to render vesicles competent for Ca(2+)-triggered fusion. The priming of vesicles is ATP-dependent and a number of ATP-dependent priming reactions have been characterized in permeable neuroendocrine cells. These involve NSF-mediated priming of SNARE protein complexes, the ATP-dependent synthesis of phosphoinositides, and protein kinase-mediated protein phosphorylation. In addition, munc13 is an important protein involved in priming synaptic vesicles. An emphasis in this review is on recent work indicating that priming events identified in the pathways of regulated exocytosis share many features with pre-fusion processes characterized in constitutive fusion pathways.

Lysosome-endosome fusion and lysosome biogenesis

Recent data both from cell-free experiments and from cultured cells have shown that lysosomes can fuse directly with late endosomes to form a hybrid organelle. This has a led to a hypothesis that dense core lysosomes are in essence storage granules for acid hydrolases and that, when the former fuse with late endosomes, a hybrid organelle for digestion of endocytosed macromolecules is created. Lysosomes are then re-formed from hybrid organelles by a process involving condensation of contents. In this Commentary we review the evidence for formation of the hybrid organelles and discuss the current status of our understanding of the mechanisms of fusion and lysosome re-formation. We also review lysosome biosynthesis, showing how recent studies of lysosome-like organelles including the yeast vacuole, Drosophila eye pigment granules and mammalian secretory lysosomes have identified novel proteins involved in this process.

Genetic modifiers of the Drosophila NSF mutant, comatose, include a temperature-sensitive paralytic allele of the calcium channel alpha1-subunit gene, cacophony

The N-ethylmaleimide-sensitive fusion protein (NSF) has been implicated in vesicle trafficking in perhaps all eukaryotic cells. The Drosophila comatose (comt) gene encodes an NSF homolog, dNSF1. Our previous work with temperature-sensitive (TS) paralytic alleles of comt has revealed a function for dNSF1 at synapses, where it appears to prime synaptic vesicles for neurotransmitter release. To further examine the molecular basis of dNSF1 function and to broaden our analysis of synaptic transmission to other gene products, we have performed a genetic screen for mutations that interact with comt. Here we report the isolation and analysis of four mutations that modify TS paralysis in comt, including two intragenic modifiers (one enhancer and one suppressor) and two extragenic modifiers (both enhancers). The intragenic mutations will contribute to structure-function analysis of dNSF1 and the extragenic mutations identify gene products with related functions in synaptic transmission. Both extragenic enhancers result in TS behavioral phenotypes when separated from comt, and both map to loci not previously identified in screens for TS mutants. One of these mutations is a TS paralytic allele of the calcium channel alpha1-subunit gene, cacophony (cac). Analysis of synaptic function in these mutants alone and in combination will further define the in vivo functions and interactions of specific gene products in synaptic transmission.

Measurement of exocytosis by amperometry in adrenal chromaffin cells: effects of clostridial neurotoxins and activation of protein kinase C on fusion pore kinetics

We have used carbon-fibre amperometry to examine the kinetics of individual secretory granule fusion/release events in bovine adrenal chromaffin cells. Transfection with plasmids encoding the light chains of botulinum neurotoxins (BoNTs) was used to investigate the effects of cleavage of syntaxin or SNAP-25 on exocytosis. Expression of BoNT/C1 or BoNT/E inhibited the extent of exocytosis that was evoked by application of digitonin/Ca(2+) to permeabilise and stimulate single chromaffin cells. Following neurotoxin expression, the residual release events were no different from those of control cells in their magnitude and kinetics from analysis of the amperometric spikes. In contrast, activation of protein kinase C (PKC) resulted in a modification of the kinetics of single granule release events. Following phorbol ester treatment, the amperometric spikes showed a significant decrease in their total charge due to a decrease in their mean half-width with increases in the rate of the initial rise and also the fall to baseline of the spikes. These changes were prevented by pre-treatment with the PKC inhibitor bisindolylmaleimide. These results suggest that PKC regulates the rate of fusion pore expansion and also subsequent pore closure or granule retrieval. A PKC-mediated regulation of kiss-and-run fusion may, therefore, control the extent of catecholamine release from single secretory granules. The experimental approach used here may provide further information on the protein constituents and regulation of the fusion pore machinery.

GATE-16, a membrane transport modulator, interacts with NSF and the Golgi v-SNARE GOS-28

Membrane proteins located on vesicles (v-SNAREs) and on the target membrane (t-SNAREs) mediate specific recognition and, possibly, fusion between a transport vesicle and its target membrane. The activity of SNARE molecules is regulated by several soluble cytosolic proteins. We have cloned a bovine brain cDNA encoding a conserved 117 amino acid polypeptide, denoted Golgi-associated ATPase Enhancer of 16 kDa (GATE-16), that functions as a soluble transport factor. GATE-16 interacts with N-ethylmaleimidesensitive factor (NSF) and significantly stimulates its ATPase activity. It also interacts with the Golgi v-SNARE GOS-28 in an NSF-dependent manner. We propose that GATE-16 modulates intra-Golgi transport through coupling between NSF activity and SNAREs activation.

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.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

The docking stage of yeast vacuole fusion requires the transfer of proteins from a cis-SNARE complex to a Rab/Ypt protein

The homotypic fusion of yeast vacuoles requires Sec18p (NSF)-driven priming to allow vacuole docking, but the mechanism that links priming and docking is unknown. We find that a large multisubunit protein called the Vam2/6p complex is bound to cis-paired SNAP receptors (SNAREs) on isolated vacuoles. This association of the Vam2/6p complex with the cis-SNARE complex is disrupted during priming. The Vam2/6p complex then binds to Ypt7p, a guanosine triphosphate binding protein of the Rab family, to initiate productive contact between vacuoles. Thus, cis-SNARE complexes can contain Rab/Ypt effectors, and these effectors can be mobilized by NSF/Sec18p-driven priming, allowing their direct association with a Rab/Ypt protein to activate docking.

Proteins needed for vesicle budding from the Golgi complex are also required for the docking step of homotypic vacuole fusion

Vam2p/Vps41p is known to be required for transport vesicles with vacuolar cargo to bud from the Golgi. Like other VAM-encoded proteins, which are needed for homotypic vacuole fusion, we now report that Vam2p and its associated protein Vam6p/Vps39p are needed on each vacuole partner for homotypic fusion. In vitro vacuole fusion occurs in successive steps of priming, docking, and membrane fusion. While priming does not require Vam2p or Vam6p, the functions of these two proteins cannot be fulfilled until priming has occurred, and each is required for the docking reaction which culminates in trans-SNARE pairing. Consistent with their dual function in Golgi vesicle budding and homotypic fusion of vacuoles, approximately half of the Vam2p and Vam6p of the cell are recovered from cell lysates with purified vacuoles.

The Golgi apparatus segregates from the lysosomal/acrosomal vesicle during rhesus spermiogenesis: structural alterations

The acrosome is an acidic secretory vesicle containing hydrolytic enzymes that are involved in the sperm's passage across the zona pellucida. Imaging of the acrosomal vesicle and the Golgi apparatus in live rhesus monkey spermatids was accomplished by using the vital fluorescent probe LysoTracker DND-26. Concurrently, the dynamics of living spermatid mitochondria was visualized using the specific probe MitoTracker CMTRos and LysoTracker DND-26 detected the acrosomal vesicle from its formation through spermatid differentiation. LysoTracker DND-26 also labeled the Golgi apparatus in spermatogenic cells. In spermatocytes the Golgi is spherical and, in round spermatids, it is localized over the acrosomal vesicle, as confirmed by using polyclonal antibodies against Golgin-95/GM130, Golgin-97, and Golgin-160. Using both live LysoTracker DND-26 imaging and Golgi antibodies, we found that the Golgi apparatus is cast off from the acrosomal vesicle and migrates toward the sperm tail in elongated spermatids. The Golgi is discarded in the cytoplasmic droplet and is undetectable in mature ejaculated spermatozoa. The combined utilization of three vital fluorescent probes (Hoechst 33342, LysoTracker DND-26, and MitoTracker CMTRos) permits the dynamic imaging of four organelles during primate spermiogenesis: the nucleus, the mitochondria, the acrosomal vesicle, and the Golgi apparatus.

Exocytotic mechanism studied by truncated and zero layer mutants of the C-terminus of SNAP-25

The highly conserved SNARE proteins, SNAP-25, syntaxin and synaptobrevin, form a tight ternary complex, which is essential for exocytosis. Crystallization of this complex revealed a four-helix bundle with an unusual hydrophilic layer (zero layer) in its center. In order to evaluate the role of this layer in different kinetic components of secretion, we used the Semliki Forest virus (SFV) system to infect adrenal chromaffin cells with SNAP-25 Q174L, a point mutant in the zero layer. Using combined flash photolysis of caged calcium and membrane capacitance measurements, we investigated its effect on the exocytotic burst and sustained phase of exocytosis with high time resolution. Cells expressing SNAP-25 Q174L displayed a selective reduction in the sustained phase, while the two components of the exocytotic burst remained unaffected. Furthermore, the exocytotic response to the second flash was significantly reduced, indicating a decrease in refilling kinetics. We therefore conclude that the zero layer is critical for the formation of SNARE complexes, but that it plays no role in the dynamic equilibrium between the two exocytosis-competent vesicle pools.

In vitro formation of the endoplasmic reticulum occurs independently of microtubules by a controlled fusion reaction

We have established an in vitro system for the formation of the endoplasmic reticulum (ER). Starting from small membrane vesicles prepared from Xenopus laevis eggs, an elaborate network of membrane tubules is formed in the presence of cytosol. In the absence of cytosol, the vesicles only fuse to form large spheres. Network formation requires a ubiquitous cytosolic protein and nucleoside triphosphates, is sensitive to N-ethylmaleimide and high cytosolic Ca(2+) concentrations, and proceeds via an intermediate stage in which vesicles appear to be clustered. Microtubules are not required for membrane tubule and network formation. Formation of the ER network shares significant similarities with formation of the nuclear envelope. Our results suggest that the ER network forms in a process in which cytosolic factors modify and regulate a basic reaction of membrane vesicle fusion.

Low levels of Ypt protein prenylation cause vesicle polarization defects and thermosensitive growth that can be suppressed by genes involved in cell wall maintenance

The Rab/Ypt small G proteins are essential for intracellular vesicle trafficking in mammals and yeast. The vesicle-docking process requires that Ypt proteins are located in the vesicle membrane. C-terminal geranylgeranyl anchors mediate the membrane attachment of these proteins. The Rab escort protein (REP) is essential for the recognition of Rab/Ypt small G proteins by geranylgeranyltransferase II (GGTase II) and for their delivery to acceptor membranes. What effect an alteration in the levels of prenylated Rab/Ypt proteins has on vesicle transport or other cellular processes is so far unknown. Here, we report the characterization of a yeast REP mutant, mrs6-2, in which reduced prenylation of Ypt proteins occurs even at the permissive temperature. A shift to the restrictive temperature does not alter exponential growth during the first 3 h. The amount of Sec4p, but not Ypt1p, bound to vesicle membranes is reduced 2.5 h after the shift compared with wild-type or mrs6-2 cells incubated at 25 degrees C. In addition, vesicles fail to be polarized towards the bud and small budded binucleate cells accumulate at this time point. Growth in 1 M sorbitol or overexpression of MLC1, encoding a myosin light chain able to bind the unconventional type V myosin Myo2, or of genes involved in cell wall maintenance, such as SLG1, GFA1 and LRE1, suppresses mrs6-2 thermosensitivity. Our data suggest that, at least at high temperature, a critical minimal level of Ypt protein prenylation is required for maintaining vesicle polarization.

TRAPP stably associates with the Golgi and is required for vesicle docking

Bet3p, a component of a large novel complex called TRAPP, acts upstream of endoplasmic reticulum (ER)-Golgi SNAREs. Unlike the SNAREs, which reside on multiple compartments, Bet3p is localized exclusively to Golgi membranes. While other proteins recycle from the Golgi to the ER, Bet3p and other TRAPP subunits remain associated with this membrane under conditions that block anterograde traffic. We propose that the persistent localization of TRAPP to the Golgi may be important for its role in docking vesicles to this membrane. Consistent with this proposal, we find that transport vesicles fail to bind to Golgi membranes in vitro in the absence of Bet3p. Binding is restored by the addition of cytosol containing Bet3p. These findings indicate that TRAPP stably associates with the Golgi and is required for vesicle docking.

Phosphatidylinositol 4,5-bisphosphate regulates two steps of homotypic vacuole fusion

Yeast vacuoles undergo cycles of fragmentation and fusion as part of their transmission to the daughter cell and in response to changes of nutrients and the environment. Vacuole fusion can be reconstituted in a cell free system. We now show that the vacuoles synthesize phosphoinositides during in vitro fusion. Of these phosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are important for fusion. Monoclonal antibodies to PI(4,5)P(2), neomycin (a phosphoinositide ligand), and phosphatidylinositol-specific phospholipase C interfere with the reaction. Readdition of PI(4, 5)P(2) restores fusion in each case. Phosphatidylinositol 3-phosphate and PI(3,5)P(2) synthesis are not required. PI(4,5)P(2) is necessary for priming, i.e., for the Sec18p (NSF)-driven release of Sec17p (alpha-SNAP), which activates the vacuoles for subsequent tethering and docking. Therefore, it represents the kinetically earliest requirement identified for vacuole fusion so far. Furthermore, PI(4,5)P(2) is required at a step that can only occur after docking but before the BAPTA sensitive step in the latest stage of the reaction. We hence propose that PI(4,5)P(2) controls two steps of vacuole fusion.

A new class of fusion-associated small transmembrane (FAST) proteins encoded by the non-enveloped fusogenic reoviruses

The non-enveloped fusogenic avian and Nelson Bay reoviruses encode homologous 10 kDa non-structural transmembrane proteins. The p10 proteins localize to the cell surface of transfected cells in a type I orientation and induce efficient cell-cell fusion. Mutagenic studies revealed the importance of conserved sequence-predicted structural motifs in the membrane association and fusogenic properties of p10. These motifs included a centrally located transmembrane domain, a conserved cytoplasmic basic region, a small hydrophobic motif in the N-terminal domain and four conserved cysteine residues. Functional analysis indicated that the extreme C-terminus of p10 functions in a sequence-independent manner to effect p10 membrane localization, while the N-terminal domain displays a sequence-dependent effect on the fusogenic property of p10. The small size, unusual arrangement of structural motifs and lack of any homologues in previously described membrane fusion proteins suggest that the fusion-associated small transmembrane (FAST) proteins of reovirus represent a new class of membrane fusion proteins.

The role of the tethering proteins p115 and GM130 in transport through the Golgi apparatus in vivo

Biochemical data have shown that COPI-coated vesicles are tethered to Golgi membranes by a complex of at least three proteins: p115, giantin, and GM130. p115 binds to giantin on the vesicles and to GM130 on the membrane. We now examine the function of this tethering complex in vivo. Microinjection of an N-terminal peptide of GM130 or overexpression of GM130 lacking this N-terminal peptide inhibits the binding of p115 to Golgi membranes. Electron microscopic analysis of single microinjected cells shows that the number of COP-sized transport vesicles in the Golgi region increases substantially, suggesting that transport vesicles continue to bud but are less able to fuse. This was corroborated by quantitative immunofluorescence analysis, which showed that the intracellular transport of the VSV-G protein was significantly inhibited. Together, these data suggest that this tethering complex increases the efficiency with which transport vesicles fuse with their target membrane. They also provide support for a model of mitotic Golgi fragmentation in which the tethering complex is disrupted by mitotic phosphorylation of GM130.

Identification and characterization of five new subunits of TRAPP

TRAPP (transport protein particle), a multiprotein complex containing ten subunits, plays a key role in the late stages of endoplasmic reticulum to Golgi traffic in the yeast Saccharomyces cerevisiae. We previously described the identification of five TRAPP subunits (Bet5p, Trs20p, Bet3p, Trs23p and Trs33p). Now we report the identification of the remaining five subunits (Trs31p, Trs65p, Trs85p, Trs120p and Trs130p) as well as an initial characterization of the yeast complex and its human homologue. We find that three of the subunits are dispensable for growth and a novel sequence motif is found in Bet3p, Trs31p and Trs33p. Furthermore, biochemical characterization of both yeast and human TRAPP suggests that this complex is anchored to a Triton X-100 resistant fraction of the Golgi. Differences between yeast and human TRAPP as well as the relationship of TRAPP subunits to other docking/tethering factors are discussed.

Mouse VAP33 is associated with the endoplasmic reticulum and microtubules

VAMP/synaptobrevin is a synaptic vesicle protein that is essential for neurotransmitter release. Intracellular injection of antisera against the Aplysia californica VAMP/synaptobrevin-binding protein ApVAP33 inhibited evoked excitatory postsynaptic potentials (EPSPs) in cultured cells, suggesting that this association may regulate the function of VAMP/synaptobrevin. We have identified and characterized a mouse homologue of ApVAP33, mVAP33. The overall domain structure of the proteins is conserved, and they have similar biochemical properties. mVAP33 mRNA is detectable in all mouse tissues examined, in contrast to the more restricted expression seen in A. californica. We analyzed the cellular distribution of mVAP33 protein in brain slices and cultured cortical cells by light and electron microscopy. Although present at higher levels in neurons, immunoreactivity was detected throughout both neurons and glia in a reticular pattern similar to that of endoplasmic reticulum-resident proteins. mVAP33 does not colocalize with VAMP/synaptobrevin at synaptic structures, but expression overlaps with lower levels of VAMP/synaptobrevin in the soma. Ultrastructural analysis revealed mVAP33 associated with microtubules and intracellular vesicles of heterogeneous size. In primary neuronal cultures, large aggregates of mVAP33 are also detected in short filamentous structures, which are occasionally associated with intracellular membranes. There is no evidence for accumulation of mVAP33 on synaptic vesicles or at the plasma membrane. These data suggest that mVAP33 is an endoplasmic-reticulum-resident protein that associates with components of the cytoskeleton. Any functional interaction between mVAP33 and VAMP/synaptobrevin, therefore, most likely involves the delivery of components to synaptic terminals rather than a direct participation in synaptic vesicle exocytosis.

Molecular mechanisms of platelet exocytosis: role of SNAP-23 and syntaxin 2 in dense core granule release

To characterize the molecular mechanisms of platelet secretion, we focused on the calcium-induced exocytosis of dense core granules. Platelets contain several known t-SNAREs (soluble N-ethylmaleimide sensitive factor [NSF] attachment protein receptors) such as syntaxins 2, 4, and 7 and SNAP-23 (synaptosomal associated protein 23). By using an in vitro exocytosis assay, we have been able to assign roles for some of these t-SNAREs in dense core granule release. This calcium-induced secretion relies on the SNARE proteins because it is stimulated by the addition of recombinant -SNAP and inhibited by a dominant negative -SNAP–L294A mutant or by anti–-SNAP and anti-NSF antibodies. SNAP-23 antibodies and an inhibitory C-terminal SNAP-23 peptide both blocked dense core granule release, demonstrating a role for SNAP-23. Unlike other cell types, platelets contain a significant pool of soluble SNAP-23, which does not partition into Triton X-114. Of the anti-syntaxin antibodies tested, only anti–syntaxin 2 antibody inhibited dense core granule release. Immunoprecipitation studies showed that the 2 t-SNAREs syntaxin 2 and SNAP-23 do form a complex in vivo. These data clearly show that SNAPs, NSF, and specific t-SNAREs are used for dense core granule release; these data provide a greater understanding of regulated exocytosis in platelets.

Rab GTPases coordinate endocytosis

Endocytosis is characterized by vesicular transport along numerous pathways. Common steps in each pathway include membrane budding to form vesicles, transport to a particular destination, and ultimately docking and fusion with the target membrane. Specificity of vesicle targeting is rendered in part by associated Rab GTPases. This review summarizes current knowledge about Rab GTPase functions in the endocytic pathways and provides insight into the regulation of Rab GTPase activity and mechanisms of Rab protein function. Functional assays have identified some Rab proteins that operate on individual pathways, but Rab proteins in several pathways remain controversial or have not been identified. Control of Rab GTPase activity is exerted through multiple levels of regulation. Significant new information pertaining to Rab protein function in regulating transport has emerged. Remarkably, Rab5 GTPase links budding, cytoskeletal transport and docking/fusion activities. This paradigm will most likely be generally applicable to other Rab GTPase pathways. Together with the cross-talk between different Rab proteins and their effectors, this may provide an integrated system for the general coordination of endocytic pathways to maintain organelle homeostasis.

SNIP, a novel SNAP-25-interacting protein implicated in regulated exocytosis

Synaptosome-associated protein of 25 kDa (SNAP-25) is a presynaptic membrane protein that has been clearly implicated in membrane fusion in both developing and mature neurons, although its mechanisms of action are unclear. We have now identified a novel SNAP-25-interacting protein named SNIP. SNIP is a hydrophilic, 145-kDa protein that comprises two predicted coiled-coil domains, two highly charged regions, and two proline-rich domains with multiple PPXY and PXXP motifs. SNIP is selectively expressed in brain where it co-distributes with SNAP-25 in most brain regions. Biochemical studies have revealed that SNIP is tightly associated with the brain cytoskeleton. Subcellular fractionation and immunofluorescence localization studies have demonstrated that SNIP co-localizes with SNAP-25 as well as the cortical actin cytoskeleton, suggesting that SNIP serves as a linker protein connecting SNAP-25 to the submembranous cytoskeleton. By using deletion analysis, we have mapped the binding domains of SNIP and SNAP-25, and we have demonstrated that the SNIP-SNAP-25 association is mediated via coiled-coil interactions. Moreover, we have shown that overexpression of SNIP or its SNAP-25-interacting domain inhibits Ca(2+)-dependent exocytosis from PC12 cells. These results indicate that SNIP is involved in regulation of neurosecretion, perhaps via its interaction with SNAP-25 and the cytoskeleton.

The yeast endosomal t-SNARE, Pep12p, functions in the absence of its transmembrane domain

Delivery of proteins to the vacuole of the yeast Saccharomyces cerevisiae requires the function of two distinct SNARE complexes. Pep12p and Vam3p are both t-SNAREs of the syntaxin family that are components of these SNARE complexes. We have used a genetic approach to address the role of Pep12p in vacuolar protein transport. Our screen for temperature-sensitive pep12 mutants yielded six alleles that were rapidly inactivated upon exposure to the non-permissive temperature. Surprisingly, the proteins encoded by these alleles were all truncated immediately prior to the transmembrane domain. Here we demonstrate that Pep12p requires its transmembrane domain for proper localization, but not for its role in vesicle fusion. In addition, we show that although Pep12p can replace Vam3p in the vacuolar SNARE complex, its transmembrane domain is required to function at this step. Therefore, the transmembrane domain of Pep12p performs different roles in the prevacuolar and vacuolar SNARE complexes.

Inhibition of SNARE complex assembly differentially affects kinetic components of exocytosis

In chromaffin cells, an increase in intracellular Ca2+ leads to an exocytotic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of two separate pools of vesicles. To investigate how these components relate to SNARE complex formation, we introduced an antibody that blocks SNARE assembly but not disassembly. In the presence of the antibody, the sustained component was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form before Ca(2+)-triggered membrane fusion and exist in a dynamic equilibrium between a loose and a tight state, both of which support exocytosis. Interaction of the antibody with preformed SNARE complexes favors the loose state.

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.

Structure of the small G protein Rap2 in a non-catalytic complex with GTP

We report a novel crystal form of the small G protein Rap2A in complex with GTP which has no GTPase activity in the crystal. The asymmetric unit contains two complexes which show that a conserved switch I residue, Tyr 32, contributes an extra hydrogen bond to the gamma-phosphate of GTP as compared to related structures with GTP analogs. Since GTP is not hydrolyzed in the crystal, this interaction is unlikely to contribute to the intrinsic GTPase activity. The comparison of other G protein structures to the Rap2-GTP complex suggests that an equivalent interaction is likely to exist in their GTP form, whether unbound or bound to an effector. This interaction has to be released to allow the GAP-activated GTPase, and presumably the intrinsic GTPase activity as well. We also discuss the definition of the flexible regions and their hinges in the light of this structure and the expanding database of G protein structures. We propose that the switch I and switch II undergo either partial or complete disorder-to-order transitions according to their cellular status, thus defining a complex energy landscape comprising more than two conformational states. We observe in addition that the region connecting the switch I and switch II is flexible in Rap2 and other G proteins. This region may be important for protein-protein interactions and possibly behave as a conformational lever arm, as characterized for Arf. Taken together, these observations suggest that the structural mechanisms of small G proteins are significantly driven by entropy-based free energy changes.

Inhibition of neurotransmitter release in the lamprey reticulospinal synapse by antibody-mediated disruption of SNAP-25 function

Exocytosis - syntaxin - synaptobrevin - SNARE synaptic vesicle The lamprey giant reticulospinal synapse can be used to manipulate the molecular machinery of synaptic vesicle exocytosis by presynaptic microinjection. Here we test the effect of disrupting the function of the SNARE protein SNAP-25. Polyclonal SNAP-25 antibodies were shown in an in vitro assay to inhibit the binding between syntaxin and SNAP-25. When microinjected presynaptically, these antibodies produced a potent inhibition of the synaptic response. Ba2+ spikes recorded in the presynaptic axon were not altered, indicating that the effect was not due to a reduced presynaptic Ca2+ entry. Electron microscopic analysis showed that synaptic vesicle clusters had a similar organization in synapses of antibody-injected axons as in control axons, and the number of synaptic vesicles in apparent contact with the presynaptic plasma membrane was also similar. Clathrin-coated pits, which normally occur at the plasma membrane around stimulated synapses, were not detected after injection of SNAP-25 antibodies, consistent with a blockade of vesicle cycling. Thus, SNAP-25 antibodies, which disrupt the interaction with syntaxin, inhibit neurotransmitter release without affecting the number of synaptic vesicles at the plasma membrane. These results provide further support to the view that the formation of SNARE complexes is critical for membrane fusion, but not for the targeting of synaptic vesicles to the presynaptic membrane.

Content mixing and membrane integrity during membrane fusion driven by pairing of isolated v-SNAREs and t-SNAREs

Membrane bilayer fusion has been shown to be mediated by v- and t-SNAREs initially present in separate populations of liposomes and to occur with high efficiency at a physiologically meaningful rate. Lipid mixing was demonstrated to involve both the inner and the outer leaflets of the membrane bilayer. Here, we use a fusion assay that relies on duplex formation of oligonucleotides introduced in separate liposome populations and report that SNARE proteins suffice to mediate complete membrane fusion accompanied by mixing of luminal content. We also find that SNARE-mediated membrane fusion does not compromise the integrity of liposomes.

VAMP (synaptobrevin) is present in the plasma membrane of nerve terminals

Synaptic vesicle docking and exocytosis require the specific interaction of synaptic vesicle proteins (such as VAMP/synaptobrevin) with presynaptic plasma membrane proteins (such as syntaxin and SNAP 25). These proteins form a stable, SDS-resistant, multimolecular complex, the SNARE complex. The subcellular distribution of VAMP and syntaxin within Torpedo electric organ nerve endings was studied by immunogoldlabeling of SDS-digested freeze-fracture replicas (Fujimoto, 1995). This technique allowed us to visualize large surface areas of the presynaptic plasma membrane and numerous synaptic vesicles from rapidly frozen nerve endings and synaptosomes. VAMP was found associated with synaptic vesicles, as also shown by conventional electron microscopy immunolabeling, and to the presynaptic plasma membrane (P leaflet). Syntaxin was also detected in the nerve ending plasma membrane, without gold labeling of synaptic vesicles. Comparison of gold particle densities suggests that the presynaptic plasma membrane contains 3 VAMP molecules per molecule of syntaxin. After biotinylation of intact synaptosomes, the synaptosomal plasma membrane was isolated on Streptavidin coated magnetic beads. Its antigenic content was compared to that of purified synaptic vesicles. VAMP was present in both membranes whereas syntaxin and SNAP 25 were highly enriched in the synaptosomal plasma membrane. This membrane has a low content of classical synaptic vesicle proteins (synaptophysin, SV2 and the vesicular acetylcholine transporter). The VAMP to syntaxin stoichiometry in the isolated synaptosomal membrane was estimated by comparison with purified antigens and close to 2, in accordance with morphological data. SDS-resistant SNARE complexes were detected in the isolated presynaptic membrane but absent in purified synaptic vesicles. Taken together, these results show that the presence of VAMP in the plasma membrane of nerve endings cannot result from exocytosis of synaptic vesicles, a process which could, as far as SNAREs are concerned, very much resemble homotypic fusion.

The Rab5 effector EEA1 interacts directly with syntaxin-6

The fusion of transport vesicles with their cognate target membranes, an essential event in intracellular membrane trafficking, is regulated by SNARE proteins and Rab GTPases. Rab GTPases are thought to act prior to SNAREs in vesicle docking, but the exact biochemical relationship between the two classes of molecules is not known. We recently identified the early endosomal autoantigen EEA1 as an effector of Rab5 in endocytic membrane fusion. Here we demonstrate that EEA1 interacts directly and specifically with syntaxin-6, a SNARE implicated in trans-Golgi network to early endosome trafficking. The binding site for syntaxin-6 overlaps with that of Rab5-GTP at the C terminus of EEA1. Syntaxin-6 and EEA1 were found to colocalize extensively on early endosomes, although syntaxin-6 is present in the trans-Golgi network as well. Our results indicate that SNAREs can interact directly with Rab effectors, and suggest that EEA1 may participate in trans-Golgi network to endosome as well as in endocytic membrane traffic.

Protein phosphorylation and the regulation of synaptic membrane traffic

It is well established that protein phosphorylation has an important role in synaptic plasticity. This is achieved, in part, via the presynaptic modulation of neurotransmitter release by protein kinases and protein phosphatases. In recent years, the increase in information available about proteins that are involved in synaptic exocytosis and endocytosis has been exploited in order to study the effects of protein phosphorylation on synaptic-vesicle cycling at the molecular level. The best-characterized protein in this respect is synapsin, whose function in the release of synaptic vesicles from the reserve pool is regulated by phosphorylation. More recently, it has emerged that proteins that function at other stages of the synaptic-vesicle cycle, which include priming of vesicles for docking-fusion and endocytic recycling, are also controlled by phosphorylation. Furthermore, recent work suggests that this regulation of membrane traffic by phosphorylation also occurs postsynaptically, where it contributes to synaptic plasticity.

An NSF function distinct from ATPase-dependent SNARE disassembly is essential for Golgi membrane fusion

The precise biochemical role of N-ethylmaleimide-sensitive factor (NSF) in membrane fusion mediated by SNARE proteins is unclear. To provide further insight into the function of NSF, we have introduced a mutation into mammalian NSF that, in Drosophila dNSF-1, leads to temperature-sensitive neuroparalysis. This mutation is like the comatose mutation and renders the mammalian NSF temperature sensitive for fusion of postmitotic Golgi vesicles and tubules into intact cisternae. Unexpectedly, at the temperature that is permissive for membrane fusion, this mutant NSF binds to, but cannot disassemble, SNARE complexes and exhibits almost no ATPase activity. A well-charaterized NSF mutant containing an inactivating point mutation in the catalytic site of its ATPase domain is equally active in the Golgi-reassembly assay. These data indicate that the need for NSF during postmitotic Golgi membrane fusion may be distinct from its ATPase-dependent ability to break up SNARE pairs.

Proteins involved in synaptic vesicle trafficking

Neurotransmitter release relies on a series of synaptic vesicle trafficking reactions. We have determined the molecular basis of these reactions by microinjecting, into 'giant' nerve terminals of squid, probes that interfere with presynaptic proteins. These probes affect neurotransmitter release and disrupt nerve terminal structure. From the nature of these lesions, it is possible to deduce the roles of individual proteins in specific vesicle trafficking reactions. This approach has revealed the function of more than a dozen presynaptic proteins and we hypothesize that neurotransmitter release requires the coordinated action of perhaps 50-100 proteins.

Sea urchin egg preparations as systems for the study of calcium-triggered exocytosis

This paper reviews recent work in our laboratory on the mechanism of calcium-triggered exocytosis. Upon echinoderm egg fertilization, cortical secretory vesicle exocytosis is massive and synchronous. By combining physiological and molecular analyses with a variety of purified membrane isolates containing secretory vesicles that fuse to the plasma membrane or each other, we have characterized the final steps of this calcium-triggered exocytosis. Our kinetic analysis led to a functional definition of the fusion complex whose activation by calcium follows Poisson statistics. The properties of this complex are compared with the properties of the heterotrimeric SNARE protein complex that is present in the cortical vesicle system. Our data do not support the hypothesis that this particular heterotrimeric complex is by itself the biological fusogen.

Vacuole acidification is required for trans-SNARE pairing, LMA1 release, and homotypic fusion

Vacuole fusion occurs in three stages: priming, in which Sec18p mediates Sec17p release, LMA1 (low M(r) activity 1) relocation, and cis-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex disassembly; docking, mediated by Ypt7p and trans-SNARE association; and fusion of docked vacuoles. Ca(2+) and calmodulin regulate late stages of the reaction. We now show that the vacuole proton gradient, generated by the vacuolar proton ATPase, is needed for trans-SNARE complex formation during docking and hence for the subsequent LMA1 release. Though neither the vacuolar Pmc1p Ca(2+)-ATPase nor the Vcx1p Ca(2+)/H(+) exchanger are needed for the fusion reaction, they participate in Ca(2+) and Delta mu(H)(+) homeostasis. Fusion itself does not require the maintenance of trans-SNARE pairs.

Fusion of endosomes involved in synaptic vesicle recycling

Recycling of vesicles of the regulated secretory pathway presumably involves passage through an early endosomal compartment as an intermediate step. To learn more about the involvement of endosomes in the recycling of synaptic and secretory vesicles we studied in vitro fusion of early endosomes derived from pheochromocytoma (PC12) cells. Fusion was not affected by cleavage of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins synaptobrevin and syntaxin 1 that operate at the exocytotic limb of the pathway. Furthermore, fusion was inhibited by the fast Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid but not by the slow Ca(2+) chelator EGTA. Endosome fusion was restored by the addition of Ca(2+) with an optimum at a free Ca(2+) concentration of 0.3 x 10(-6) M. Other divalent cations did not substitute for Ca(2+). A membrane-permeant EGTA derivative caused inhibition of fusion, which was reversed by addition of Ca(2+). We conclude that the fusion of early endosomes participating in the recycling of synaptic and neurosecretory vesicles is mediated by a set of SNAREs distinct from those involved in exocytosis and requires the local release of Ca(2+) from the endosomal interior.

Control of the terminal step of intracellular membrane fusion by protein phosphatase 1

Intracellular membrane fusion is crucial for the biogenesis and maintenance of cellular compartments, for vesicular traffic between them, and for exo- and endocytosis. Parts of the molecular machinery underlying this process have been identified, but most of these components operate in mutual recognition of the membranes. Here it is shown that protein phosphatase 1 (PP1) is essential for bilayer mixing, the last step of membrane fusion. PP1 was also identified in a complex that contained calmodulin, the second known factor implicated in the regulation of bilayer mixing. The PP1-calmodulin complex was required at multiple sites of intracellular trafficking; hence, PP1 may be a general factor controlling membrane bilayer mixing.

Oligomeric complexes link Rab5 effectors with NSF and drive membrane fusion via interactions between EEA1 and syntaxin 13

SNAREs and Rab GTPases cooperate in vesicle transport through a mechanism yet poorly understood. We now demonstrate that the Rab5 effectors EEA1 and Rabaptin-5/Rabex-5 exist on the membrane in high molecular weight oligomers, which also contain NSF. Oligomeric assembly is modulated by the ATPase activity of NSF. Syntaxin 13, the t-SNARE required for endosome fusion, is transiently incorporated into the large oligomers via direct interactions with EEA1. This interaction is required to drive fusion, since both dominant-negative EEA1 and synthetic peptides encoding the FYVE Zn2+ finger hinder the interaction and block fusion. We propose a novel mechanism whereby oligomeric EEA1 and NSF mediate the local activation of syntaxin 13 upon membrane tethering and, by analogy with viral fusion proteins, coordinate the assembly of a fusion pore.

Membrane tethering in intracellular transport

Studies of various membrane trafficking steps over the past year indicate that membranes are tethered together prior to the interaction of v-SNAREs and t-SNAREs across the membrane junction. The tethering proteins identified to date are quite large, being either fibrous proteins or multimeric protein complexes. The tethering factors employed at different steps are evolutionarily unrelated, yet their function seems to be closely tied to the more highly conserved Rab GTPases. Tethering factors may collaborate with Rabs and SNAREs to generate targeting specificity in the secretory pathway.

The role of ARF and Rab GTPases in membrane transport

Two key events of intracellular transport and membrane trafficking in eukaryotic cells, the formation of transport vesicles and their specific delivery to target membranes, are controlled by small GTPases of the ADP-ribosylation factor (ARF) and Rab families, respectively. The past 18 months have seen the identification of proteins that regulate ARF and Rab GDP/GTP cycle, as well as the characterization of their effectors, shedding light on the molecular mechanisms of ARF and Rab function.

Intracellular membrane fusion: SNAREs only?

The past two years have seen vigorous attempts to elucidate the mechanism driving intracellular membrane fusion. Much attention was focused on the role of SNARE complexes. Their crystal structure was solved and fusion was reconstituted using proteoliposomes with purified SNAREs suggesting them to be the minimal machinery for fusion. Work on physiological membranes, however, points in another direction and has spurred a hot debate on the function of SNAREs.

Sec1p binds to SNARE complexes and concentrates at sites of secretion

Proteins of the Sec1 family have been shown to interact with target-membrane t-SNAREs that are homologous to the neuronal protein syntaxin. We demonstrate that yeast Sec1p coprecipitates not only the syntaxin homologue Ssop, but also the other two exocytic SNAREs (Sec9p and Sncp) in amounts and in proportions characteristic of SNARE complexes in yeast lysates. The interaction between Sec1p and Ssop is limited by the abundance of SNARE complexes present in sec mutants that are defective in either SNARE complex assembly or disassembly. Furthermore, the localization of green fluorescent protein (GFP)-tagged Sec1p coincides with sites of vesicle docking and fusion where SNARE complexes are believed to assemble and function. The proposal that SNARE complexes act as receptors for Sec1p is supported by the mislocalization of GFP-Sec1p in a mutant defective for SNARE complex assembly and by the robust localization of GFP-Sec1p in a mutant that fails to disassemble SNARE complexes. The results presented here place yeast Sec1p at the core of the exocytic fusion machinery, bound to SNARE complexes and localized to sites of secretion.

ERG30, a VAP-33-related protein, functions in protein transport mediated by COPI vesicles

Intracellular transport of newly synthesized and mature proteins via vesicles is controlled by a large group of proteins. Here we describe a ubiquitous rat protein-endoplasmic reticulum (ER) and Golgi 30-kD protein (ERG30)-which shares structural characteristics with VAP-33, a 33-kD protein from Aplysia californica which was shown to interact with the synaptic protein VAMP. The transmembrane topology of the 30-kD ERG30 corresponds to a type II integral membrane protein, whose cytoplasmic NH(2) terminus contains a predicted coiled-coil motif. We localized ERG30 to the ER and to pre-Golgi intermediates by biochemical and immunocytochemical methods. Consistent with a role in vesicular transport, anti-ERG30 antibodies specifically inhibit intra-Golgi transport in vitro, leading to significant accumulation of COPI-coated vesicles. It appears that ERG30 functions early in the secretory pathway, probably within the Golgi and between the Golgi and the ER.