Ykt6p, a prenylated SNARE essential for endoplasmic reticulum-Golgi transport

Vesicular transport between secretory compartments requires specific recognition molecules called SNAREs. Here we report the identification of three putative SNAREs, p14 (Sft1p), p28 (Gos1p), and a detailed characterization of p26 (Ykt6p). All three were originally isolated as interacting partners of the cis Golgi target membrane-associated SNARE Sed5p, when Sec18p (yeast NSF) was inactivated. YKT6 is an essential gene that codes for a novel vesicle-associated SNARE functioning at the endoplasmic reticulum-Golgi transport step in the yeast secretory pathway. Depletion of Ykt6p results in the accumulation of the p1 precursor (endoplasmic reticulum form) of the vacuolar enzyme carboxypeptidase Y and morphological abnormalities consistent with a defect in secretion. Membrane localization of Ykt6p is essential for protein function and is normally mediated by isoprenylation. However, replacement of the isoprenylation motif with a bona fide transmembrane anchor results in a functional protein confirming that membrane localization, but not isoprenylation per se, is required for function. Ykt6p and its homologues are highly conserved from yeast to human as demonstrated by the functional complementation of the loss of Ykt6p by its human counterpart. This is the first example of a human SNARE protein functionally replacing a yeast SNARE. This observation implies that the specific details of the vesicle targeting code, like the genetic code, are conserved in evolution.

ERS-24, a mammalian v-SNARE implicated in vesicle traffic between the ER and the Golgi

We report the identification and characterization of ERS-24 (Endoplasmic Reticulum SNARE of 24 kD), a new mammalian v-SNARE implicated in vesicular transport between the ER and the Golgi. ERS24 is incorporated into 20S docking and fusion particles and disassembles from this complex in an ATP-dependent manner. ERS-24 has significant sequence homology to Sec22p, a v-SNARE in Saccharomyces cerevisiae required for transport between the ER and the Golgi. ERS-24 is localized to the ER and to the Golgi, and it is enriched in transport vesicles associated with these organelles.

Patterns of synaptic activity in neural networks recorded by light emission from synaptolucins

The emission of light, coupled to exocytosis, can in principle be utilized to monitor the activity of a large number of individual synapses simultaneously. To illustrate this concept, fusion proteins of Cypridina luciferase and synaptotagmin-I or VAMP-2/synaptobrevin (which we term "synaptolucins") were expressed in cultured hippocampal neurons with the help of viral vectors. Synaptolucins were targeted to synaptic vesicles and, upon exocytosis, formed light-emitting complexes with their cognate luciferin, which was added to the extracellular medium. Photon emissions required a depolarizing stimulus, occurred from regions with high synaptic density as ascertained by vital staining of recycling synaptic vesicles, and were sensitive to Ca2+ depletion and clostridial neurotoxins. The method can currently detect exocytosis of the readily releasable pool of synaptic vesicles at a hippocampal synapse, corresponding to about two dozen quanta, but has the potential for greater sensitivity.

Binding of the synaptic vesicle v-SNARE, synaptotagmin, to the plasma membrane t-SNARE, SNAP-25, can explain docked vesicles at neurotoxin-treated synapses

Neurotransmitter release requires the specific docking of synaptic vesicles to the presynaptic plasma membrane followed by a calcium-triggered fusion event. Herein we report a previously unsuspected interaction of the synaptic vesicle protein and likely calcium sensor synaptotagmin with the plasma membrane t-SNARE SNAP-25. This interaction appears to resolve the apparent paradox that synaptic vesicles are capable of docking even when VAMP (vesicle-associated membrane protein) or syntaxin is cleaved or deleted and suggests that two species of v-SNAREs (VAMP and synaptotagmin) and two species of t-SNAREs (SNAP-25 and syntaxin) interact to functionally dock synaptic vesicles.

Molecular machinery mediating vesicle budding, docking and fusion

A general machinery buds and fuses transport vesicles which connect intracellular compartments with each other and allow communication with the extracellular environment. Cytoplasmic coat proteins deform membranes to bud vesicles and interact directly or indirectly with cargo molecules. Compartment-specific SNAREs (SNAP receptors) on vesicles and target membranes dock vesicles and provide a scaffolding for the general fusion machinery to initiate lipid bilayer fusion.

Calcium-dependent switching of the specificity of phosphoinositide binding to synaptotagmin

The synaptic vesicle membrane protein synaptotagmin (tagmin) is essential for fast, calcium-dependent, neurotransmitter release and is likely to be the calcium sensor for exocytosis, because of its many calcium-dependent properties. Polyphosphoinositides are needed for exocytosis, but it has not been known why. We now provide a possible connection between these observations with the finding that the C2B domain of tagmin I binds phosphatidylinositol-4,5-bisphosphate (PIns-4,5-P2), its isomer phosphatidylinositol-3,4-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate (PIns-3,4,5-P3). Calcium ions switch the specificity of this binding from PIns-3,4,5-P3 (at calcium concentrations found in resting nerve terminals) to PIns-4,5-P2 (at concentration of calcium required for transmitter release). Inositol polyphosphates, known blockers of neurotransmitter release, inhibit the binding of both PIns-4,5-P2 and PIns-3,4,5-P3 to tagmin. Our findings imply that tagmin may operate as a bimodal calcium sensor, switching bound lipids during exocytosis. This connection to polyphosphoinositides, compounds whose levels are physiologically regulated, could be important for long-term memory and learning.

Molecular machinery mediating vesicle budding, docking and fusion

A general machinery buds and fuses transport vesicles which connect intracellular compartments with each other and allow communication with the extracellular environment. Cytoplasmic coat proteins deform membranes to bud vesicles and interact directly or indirectly with cargo molecules. Compartment-specific SNAREs on vesicles and target membranes dock vesicles and provide a scaffolding for the general fusion machinery to initiate lipid bilayer fusion.

Bimodal interaction of coatomer with the p24 family of putative cargo receptors

Cytoplasmic domains of members of the p24 family of putative cargo receptors were shown to bind to coatomer, the coat protein of COPI-coated transport vesicles. Domains that contained dilysine endoplasmic reticulum retrieval signals bound the alpha-, beta'-, and epsilon-COP subunits of coatomer, whereas other p24 domains bound the beta-, gamma-, and zeta-COP subunits and required a phenylalanine-containing motif. Transit of a CD8-p24 chimera from the endoplasmic reticulum through the Golgi complex was slowed when the phenylalanine motif was mutated, suggesting that this motif may function as an anterograde transport signal. The either-or bimodal binding of coatomer to p24 tails suggests models for how coatomer can potentially package retrograde-directed and anterograde-directed cargo into distinct COPI-coated vesicles.

Felix Hoppe-Seyler Lecture 1996. Mechanisms of intracellular protein transport

Discovery of the protein machinery of vesicle transport has followed the reconstitution of transport in a cell-free system, and established general mechanisms that underlie a broad variety of physiological processes, including cell surface growth, the biogenesis of distinct intracellular organelles, endocytosis, and the controlled release of hormones and neurotransmitters. Transport vesicles are formed as coat proteins assemble on membranes, are targetted by SNARE proteins, and fuse using a general mechanism involving SNAP proteins and the ATPase NSF.

A v-SNARE implicated in intra-Golgi transport

We report the identification of a putative v-SNARE (GOS-28), localized primarily to transport vesicles at the terminal rims of Golgi stacks. In vitro, GOS-28, A Golgi SNARE of 28 kD, is efficiently packaged into Golgi-derived vesicles, which are most likely COPI coated. Antibodies directed against GOS-28 block its ability to bind alpha-SNAP, partially inhibit transport from the cis to the medial cisternae, and do not inhibit budding of COP-coated vesicles, but do accumulate docked uncoated vesicles.

Multiple palmitoylation of synaptotagmin and the t-SNARE SNAP-25

Synaptotagmin, a likely calcium sensor for synaptic transmission, and SNAP-25, a t-SNARE of the presynaptic plasma membrane, are key proteins for the docking and fusion of synaptic and other vesicles. We report that both synaptotagmin and SNAP-25 are palmitoylated with their fatty acids attached in a labile thioester-type bond. A SNAP-25 mutant with deleted palmitoylation sites is found exclusively in the cytosol after cell fractionation, whereas the palmitoylated form of SNAP-25 is membrane-bound, establishing that SNAP-25 is membrane-anchored via covalently linked palmitate.

Protein sorting by transport vesicles

Eukaryotic life depends on the spatial and temporal organization of cellular membrane systems. Recent advances in understanding the machinery of vesicle transport have established general principles that underlie a broad variety of physiological processes, including cell surface growth, the biogenesis of distinct intracellular organelles, endocytosis, and the controlled release of hormones and neurotransmitters.

The protein machinery of vesicle budding and fusion

A general protein machinery that buds and fuses transport vesicles is harnessed to generate the complex web of intracellular transport pathways critical for such diverse processes as cell growth, endocytosis, hormone release, and neurotransmission. With this appreciation, the challenge of understanding the precise molecular mechanisms of these many facets of cell biology has been reduced to a series of problems in protein structure and chemistry.

A possible docking and fusion particle for synaptic transmission

Several proteins have been implicated in the rapid (millisecond) calcium-controlled release of transmitters at nerve endings, including soluble N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (alpha-SNAP), the synaptic SNAP receptor (SNARE) and the calcium-binding protein synaptotagmin, which may function as a calcium sensor in exocytosis. A second SNAP isoform (beta-SNAP), which is 83% identical to alpha-SNAP, is highly expressed in brain, but its role is still unclear. Here we show that these proteins assemble cooperatively to form a docking and fusion complex. beta-SNAP (but not alpha-SNAP) binds synaptotagmin and recruits NSF, indicating that the complex may link the process of membrane fusion to calcium entry by attaching a specialized fusion protein (beta-SNAP) to a calcium sensor (synaptotagmin). Polyphosphoinositols that block transmitter release, inositol 1,3,4,5-tetrakisphosphate (InsP4), inositol 1,3,4,5,6-pentakisphosphate (InsP5) and inositol 1,2,3,4,5,6-hexakisphosphate (InsP6), also block the assembly of the particle by preventing beta-SNAP from binding to synaptotagmin.

An integral membrane component of coatomer-coated transport vesicles defines a family of proteins involved in budding

We have isolated a major integral membrane protein from Golgi-derived coatomer-coated vesicles. This 24-kDa protein, p24, defines a family of integral membrane proteins with homologs present in yeast and humans. In addition to sequence similarity, all p24 family members contain a motif with the characteristic heptad repeats found in coiled coils. When the yeast p24 isoform, yp24A, is knocked out in a strain defective for vesicle fusion, a dramatic reduction in the accumulation of transport vesicles is observed. Together, these results indicate a role for this protein family in the budding of coatamer-coated and other species of coated vesicles.

The capacity to retrieve escaped ER proteins extends to the trans-most cisterna of the Golgi stack

To explore how far into the Golgi stack the capacity to retrieve KDEL proteins extends, we have introduced an exogenous probe (the peptide YHPNSTCSEKDEL) into the TGN of living cells. For this purpose, a CHO cell line expressing a c-myc-tagged version of the transmembrane protein TGN38--which cycles between the TGN and the cell surface--was generated. The cells internalized peptides that were disulfide bonded to anti-myc antibodies and accumulated the peptide-antibody complexes in the TGN. Peptides released from these complexes underwent retrograde transport to the ER, as evidenced by the transfer of N-linked carbohydrate to their acceptor site. The KDEL-tagged glycopeptides (approximately 10% of the endocytosed load) behaved like endogenous ER residents: they stayed intracellular, and their oligosaccharide side chains remained sensitive to endoglycosidase H. An option thus exists to extract ER residents even at the most distant pole of the Golgi stack, suggesting that sorting of resident from exported ER proteins may occur in a multistage process akin to fractional distillation.

SNAP-mediated protein-protein interactions essential for neurotransmitter release

The constitutive fusion of transport vesicles with intracellular membranes requires soluble proteins called SNAPs. Certain presynaptic proteins implicated in synaptic vesicle exocytosis also bind SNAPs, suggesting that SNAPs participate in the calcium-regulated membrane fusion events mediating neurotransmitter release. Here we show that injection of recombinant SNAPs into the giant synapse of squid enhances transmitter release. Conversely, injection of peptides designed to mimic the sites at which SNAP interacts with its binding partners inhibits transmitter release downstream of synaptic vesicle docking. A SNAP-dependent protein complex must therefore mediate transmitter release, showing that transmitter release shares a common molecular mechanism with constitutive membrane fusion.

Coatomer-rich endoplasmic reticulum

We identify in normal cells the existence of two distinct sites of the transitional endoplasmic reticulum (ER), one housing the Sec23p protein complex (the classical transitional element), the other the coatomer protein complex (the coatomer-rich ER). Experimental conditions that reduce transport from the ER to the Golgi complex lead to the overexpression of this newly defined coatomer-rich ER.

Mechanisms of intracellular protein transport

Recent advances have uncovered the general protein apparatus used by all eukaryotes for intracellular transport, including secretion and endocytosis, and for triggered exocytosis of hormones and neurotransmitters. Membranes are shaped into vesicles by cytoplasmic coats which then dissociate upon GTP hydrolysis. Both vesicles and their acceptor membranes carry targeting proteins which interact specifically to initiate docking. A general apparatus then assembles at the docking site and fuses the vesicle with its target.

A rab protein is required for the assembly of SNARE complexes in the docking of transport vesicles

Rab proteins are generally required for transport vesicle docking. We have exploited yeast secretion mutants to demonstrate that a rab protein is required for v-SNAREs and t-SNAREs to assemble. The absence of the rab protein in the docking complex suggests that, in a broad sense, rab proteins participate in a reaction catalyzing SNARE complex assembly. In so doing, rab proteins could help impart an additional layer of specificity to vesicle docking. This mechanism likely involves the Sec1 homolog Sly1, which we identified in isolated docking complexes. We also report the identification of a novel v-SNARE (Ykt6p) component of the yeast ER-Golgi docking complex that has a CAAX box and is predicted to be lipid anchored. The surprising finding that docking complexes can contain many distinct species of SNAREs (Sed5p, Bos1p, Sec22p, Ykt6p, and likely Bet1p, p28, and p14) suggests that multimeric interactions are features of the fusion machinery, and may also improve the fidelity of vesicle targeting.

N-ethylmaleimide-sensitive fusion protein: a trimeric ATPase whose hydrolysis of ATP is required for membrane fusion

The NEM-sensitive fusion protein, NSF, together with SNAPs (soluble NSF attachment proteins) and the SNAREs (SNAP receptors), is thought to be generally used for the fusion of transport vesicles to their target membranes. NSF is a homotrimer whose polypeptide subunits are made up of three distinct domains: an amino-terminal domain (N) and two homologous ATP-binding domains (D1 and D2). Mutants of NSF were produced in which either the order or composition of the three domains were altered. These mutants could not support intra-Golgi transport, but they indicated that the D2 domain was required for trimerization of the NSF subunits. Mutations of the first ATP-binding site that affected either the binding (K266A) or hydrolysis (E329Q) of ATP completely eliminated NSF activity. The hydrolysis mutant was an effective, reversible inhibitor of Golgi transport with an IC50 of 125 ng/50 microliters assay. Mutants in the second ATP-binding site (binding, K549A; hydrolysis, D604Q) had either 14 or 42% the specific activity of the wild-type protein, respectively. Using coexpression of an inactive mutant with wild-type subunits, it was possible to produce a recombinant form of trimeric NSF that contained a mixture of subunits. The mixed NSF trimers were inactive, even when only one mutant subunit was present, suggesting that NSF action requires each of the three subunits in a concerted mechanism. These studies demonstrate that the ability of the D1 domain to hydrolyze ATP is required for NSF activity and, therefore is required for membrane fusion. The D2 domain is required for trimerization, but its ability to hydrolyze ATP is not absolutely required for NSF function.

Neurotransmission: harnessing fusion machinery at the synapse

Neurotransmission requires the docking of synaptic vesicles to the presynaptic plasma membrane, and their signal-dependent fusion. These processes use a general 'machinery' operating at several intracellular vesicular transport steps and, in addition, use a set of unique components that characterizes this specific form of regulated secretion. This review summarizes recent progress that has significantly increased our understanding of how intracellular transport vesicles dock and fuse with their target membrane, both in the synapse and elsewhere.

Sar1 promotes vesicle budding from the endoplasmic reticulum but not Golgi compartments

Two new members (Sar1a and Sar1b) of the SAR1 gene family have been identified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein was enriched 20-40-fold in vesicular carriers mediating ER to Golgi traffic. Biochemical analysis revealed that Sar1 was essential for an early step in vesicle budding. A Sar1-specific antibody potently inhibited export of vesicular stomatitis virus glycoprotein (VSV-G) from the ER in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferential affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport of VSV-G between sequential Golgi compartments, suggesting that components active in formation of vesicular carriers mediating ER to Golgi traffic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages of the secretory pathway may reflect the recently recognized differences in protein transport between the Golgi stacks as opposed to the selective sorting and concentration of protein during export from the ER.