Topological restriction of SNARE-dependent membrane fusion

CAPS and Munc13: CATCHRs that SNARE Vesicles

CAPS (Calcium-dependent Activator Protein for Secretion, aka CADPS) and Munc13 (Mammalian Unc-13) proteins function to prime vesicles for Ca(2+)-triggered exocytosis in neurons and neuroendocrine cells. CAPS and Munc13 proteins contain conserved C-terminal domains that promote the assembly of SNARE complexes for vesicle priming. Similarities of the C-terminal domains of CAPS/Munc13 proteins with Complex Associated with Tethering Containing Helical Rods domains in multi-subunit tethering complexes (MTCs) have been reported. MTCs coordinate multiple interactions for SNARE complex assembly at constitutive membrane fusion steps. We review aspects of these diverse tethering and priming factors to identify common operating principles.

ER-associated SNAREs and Sey1p mediate nuclear fusion at two distinct steps during yeast mating

During yeast mating, two haploid nuclei fuse membranes to form a single diploid nucleus. However, the known proteins required for nuclear fusion are unlikely to function as direct fusogens (i.e., they are unlikely to directly catalyze lipid bilayer fusion) based on their predicted structure and localization. Therefore we screened known fusogens from vesicle trafficking (soluble N-ethylmaleimide-sensitive factor attachment protein receptors [SNAREs]) and homotypic endoplasmic reticulum (ER) fusion (Sey1p) for additional roles in nuclear fusion. Here we demonstrate that the ER-localized SNAREs Sec20p, Ufe1p, Use1p, and Bos1p are required for efficient nuclear fusion. In contrast, Sey1p is required indirectly for nuclear fusion; sey1Δ zygotes accumulate ER at the zone of cell fusion, causing a block in nuclear congression. However, double mutants of Sey1p and Sec20p, Ufe1p, or Use1p, but not Bos1p, display extreme ER morphology defects, worse than either single mutant, suggesting that retrograde SNAREs fuse ER in the absence of Sey1p. Together these data demonstrate that SNAREs mediate nuclear fusion, ER fusion after cell fusion is necessary to complete nuclear congression, and there exists a SNARE-mediated, Sey1p-independent ER fusion pathway.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Organization of the ER-Golgi interface for membrane traffic control

Coat protein complex I (COPI) and COPII are required for bidirectional membrane trafficking between the endoplasmic reticulum (ER) and the Golgi. While these core coat machineries and other transport factors are highly conserved across species, high-resolution imaging studies indicate that the organization of the ER-Golgi interface is varied in eukaryotic cells. Regulation of COPII assembly, in some cases to manage distinct cellular cargo, is emerging as one important component in determining this structure. Comparison of the ER-Golgi interface across different systems, particularly mammalian and plant cells, reveals fundamental elements and distinct organization of this interface. A better understanding of how these interfaces are regulated to meet varying cellular secretory demands should provide key insights into the mechanisms that control efficient trafficking of proteins and lipids through the secretory pathway.

Two coiled-coil domains of Chlamydia trachomatis IncA affect membrane fusion events during infection

Chlamydia trachomatis replicates in a parasitophorous membrane-bound compartment called an inclusion. The inclusions corrupt host vesicle trafficking networks to avoid the degradative endolysosomal pathway but promote fusion with each other in order to sustain higher bacterial loads in a process known as homotypic fusion. The Chlamydia protein IncA (Inclusion protein A) appears to play central roles in both these processes as it participates to homotypic fusion and inhibits endocytic SNARE-mediated membrane fusion. How IncA selectively inhibits or activates membrane fusion remains poorly understood. In this study, we analyzed the spatial and molecular determinants of IncA’s fusogenic and inhibitory functions. Using a cell-free membrane fusion assay, we found that inhibition of SNARE-mediated fusion requires IncA to be on the same membrane as the endocytic SNARE proteins. IncA displays two coiled-coil domains showing high homology with SNARE proteins. Domain swap and deletion experiments revealed that although both these domains are capable of independently inhibiting SNARE-mediated fusion, these two coiled-coil domains cooperate in mediating IncA multimerization and homotypic membrane interaction. Our results support the hypothesis that Chlamydia employs SNARE-like virulence factors that positively and negatively affect membrane fusion and promote infection.

Secretory protein biogenesis and traffic in the early secretory pathway

The secretory pathway is responsible for the synthesis, folding, and delivery of a diverse array of cellular proteins. Secretory protein synthesis begins in the endoplasmic reticulum (ER), which is charged with the tasks of correctly integrating nascent proteins and ensuring correct post-translational modification and folding. Once ready for forward traffic, proteins are captured into ER-derived transport vesicles that form through the action of the COPII coat. COPII-coated vesicles are delivered to the early Golgi via distinct tethering and fusion machineries. Escaped ER residents and other cycling transport machinery components are returned to the ER via COPI-coated vesicles, which undergo similar tethering and fusion reactions. Ultimately, organelle structure, function, and cell homeostasis are maintained by modulating protein and lipid flux through the early secretory pathway. In the last decade, structural and mechanistic studies have added greatly to the strong foundation of yeast genetics on which this field was built. Here we discuss the key players that mediate secretory protein biogenesis and trafficking, highlighting recent advances that have deepened our understanding of the complexity of this conserved and essential process.

The Golgi puppet master: COG complex at center stage of membrane trafficking interactions

The central organelle within the secretory pathway is the Golgi apparatus, a collection of flattened membranes organized into stacks. The cisternal maturation model of intra-Golgi transport depicts Golgi cisternae that mature from cis to medial to trans by receiving resident proteins, such as glycosylation enzymes via retrograde vesicle-mediated recycling. The conserved oligomeric Golgi (COG) complex, a multi-subunit tethering complex of the complexes associated with tethering containing helical rods family, organizes vesicle targeting during intra-Golgi retrograde transport. The COG complex, both physically and functionally, interacts with all classes of molecules maintaining intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, vesicular coats, and molecular motors. In this report, we will review the current state of the COG interactome and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting, which plays a central role in maintaining glycosylation homeostasis in all eukaryotic cells.

Selective control of SNARE recycling by Golgi retention

Two distinct sets of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) catalyze membrane fusion in the cis-Golgi and trans-Golgi. The mechanism that controls Golgi localization of SNAREs remains largely unknown. Here we tested three potential mechanisms, including vesicle recycling between the Golgi and the endoplasmic reticulum, partitioning in Golgi lipid microdomains, and selective intra-Golgi retention. Recycling rates showed a linear relationship with intra-Golgi mobility of SNAREs. The cis-Golgi SNAREs had higher mobility than intra-Golgi SNAREs, whereas vesicle SNAREs had higher mobility than target membrane SNAREs. The differences in SNARE mobility were not due to preferential partitioning into detergent-resistant membrane microdomains. We propose that intra-Golgi retention precludes entropy-driven redistribution of SNAREs to the endoplasmic reticulum and endocytic compartments.

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.

COG complexes form spatial landmarks for distinct SNARE complexes

Vesicular tethers and SNAREs (soluble N-ethylmalemide-sensitive fusion attachment protein receptors) are two key protein components of the intracellular membrane-trafficking machinery. The conserved oligomeric Golgi (COG) complex has been implicated in the tethering of retrograde intra-Golgi vesicles. Here, using yeast two-hybrid and co-immunoprecipitation approaches, we show that three COG subunits, namely COG4, 6 and 8, are capable of interacting with defined Golgi SNAREs, namely STX5, STX6, STX16, GS27 and SNAP29. Comparative analysis of COG8-STX16 and COG4-STX5 interactions by a COG-based mitochondrial relocalization assay reveals that the COG8 and COG4 proteins initiate the formation of two different tethering platforms that can facilitate the redirection of two populations of Golgi transport intermediates to the mitochondrial vicinity. Our results uncover a role for COG sub-complexes in defining the specificity of vesicular sorting within the Golgi.

Presentation of phagocytosed antigens by MHC class I and II

Phagocytosis provides innate immune cells with a mechanism to take up and destroy pathogenic bacteria, apoptotic cells and other large particles. In some cases, however, peptide antigens from these particles are preserved for presentation in association with major histocompatibility complex (MHC) class I or class II molecules in order to stimulate antigen-specific T cells. Processing and presentation of antigens from phagosomes presents a number of distinct challenges relative to antigens internalized by other means; while bacterial antigens were among the first discovered to be presented to T cells, analyses of the cellular mechanisms by which peptides from phagocytosed antigens assemble with MHC molecules and by which these complexes are then expressed at the plasma membrane have lagged behind those of conventional model soluble antigens. In this review, we cover recent advances in our understanding of these processes, including the unique cross-presentation of phagocytosed antigens by MHC class I molecules, and in their control by signaling modalities in phagocytic cells.

New Ca(2+)-dependent regulators of autophagosome maturation

Autophagy is a membrane trafficking pathway responsible for the breakdown of unwanted intracellular materials and crucial for the cell healthiness and survival. In the autophagic flux, various dynamic membrane rearrangements occurs starting with the elongation of the phagophore and its closure to build an autophagosome and ending with its fusion with late endosomes and lysosomes to form an autolysosome. Although Ca²⁺ is a well established regulator of membrane fusion events, little is known about its role in these processes during autophagy. Recent studies, based on proteomic analyses of lysosomal membranes, have provided new insights into this field of study. Thus, the levels on lysosomal membranes of annexin A1, annexin A5 and copine 1, three proteins that bind to phospholipid membranes in a Ca²⁺-dependent manner, increased under nutrient deprivation, a condition that promotes autophagic degradation. In addition, two different studies showed that annexin A5 and annexin A1 are involved in autophagosome maturation. Here, we discuss the molecular mechanisms by which the fusion of autophagosomes with endosomes and lysosomes could be regulated by these three proteins and Ca²⁺.

Retrograde vesicle transport in the Golgi

The Golgi apparatus is the central sorting and biosynthesis hub of the secretory pathway, and uses vesicle transport for the recycling of its resident enzymes. This system must operate with high fidelity and efficiency for the correct modification of secretory glycoconjugates. In this review, we discuss recent advances on how coats, tethers, Rabs and SNAREs cooperate at the Golgi to achieve vesicle transport. We cover the well understood vesicle formation process orchestrated by the COPI coat, and the comprehensively documented fusion process governed by a set of Golgi localised SNAREs. Much less clear are the steps in-between formation and fusion of vesicles, and we therefore provide a much needed update of the latest findings about vesicle tethering. The interplay between Rab GTPases, golgin family coiled-coil tethers and the conserved oligomeric Golgi (COG) complex at the Golgi are thoroughly evaluated.

The SNARE protein SYP71 expressed in vascular tissues is involved in symbiotic nitrogen fixation in Lotus japonicus nodules

Soluble N-Ethylmaleimide Sensitive Factor Attachment Protein Receptor (SNARE) proteins are crucial for signal transduction and development in plants. Here, we investigate a Lotus japonicus symbiotic mutant defective in one of the SNARE proteins. When in symbiosis with rhizobia, the growth of the mutant was retarded compared with that of the wild-type plant. Although the mutant formed nodules, these exhibited lower nitrogen fixation activity than the wild type. The rhizobia were able to invade nodule cells, but enlarged symbiosomes were observed in the infected cells. The causal gene, designated LjSYP71 (for L. japonicus syntaxin of plants71), was identified by map-based cloning and shown to encode a Qc-SNARE protein homologous to Arabidopsis (Arabidopsis thaliana) SYP71. LjSYP71 was expressed ubiquitously in shoot, roots, and nodules, and transcripts were detected in the vascular tissues. In the mutant, no other visible defects in plant morphology were observed. Furthermore, in the presence of combined nitrogen, the mutant plant grew almost as well as the wild type. These results suggest that the vascular tissues expressing LjSYP71 play a pivotal role in symbiotic nitrogen fixation in L. japonicus nodules.

Secretory pathway-dependent localization of the Saccharomyces cerevisiae Rho GTPase-activating protein Rgd1p at growth sites

Establishment and maintenance of cell polarity in eukaryotes depends upon the regulation of Rho GTPases. In Saccharomyces cerevisiae, the Rho GTPase activating protein (RhoGAP) Rgd1p stimulates the GTPase activities of Rho3p and Rho4p, which are involved in bud growth and cytokinesis, respectively. Consistent with the distribution of Rho3p and Rho4p, Rgd1p is found mostly in areas of polarized growth during cell cycle progression. Rgd1p was mislocalized in mutants specifically altered for Golgi apparatus-based phosphatidylinositol 4-P [PtdIns(4)P] synthesis and for PtdIns(4,5)P(2) production at the plasma membrane. Analysis of Rgd1p distribution in different membrane-trafficking mutants suggested that Rgd1p was delivered to growth sites via the secretory pathway. Rgd1p may associate with post-Golgi vesicles by binding to PtdIns(4)P and then be transported by secretory vesicles to the plasma membrane. In agreement, we show that Rgd1p coimmunoprecipitated and localized with markers specific to secretory vesicles and cofractionated with a plasma membrane marker. Moreover, in vivo imaging revealed that Rgd1p was transported in an anterograde manner from the mother cell to the daughter cell in a vectoral manner. Our data indicate that secretory vesicles are involved in the delivery of RhoGAP Rgd1p to the bud tip and bud neck.

A tethering complex dimer catalyzes trans-SNARE complex formation in intracellular membrane fusion

SNARE complexes mediate membrane fusion in the endomembrane system. They consist of coiled-coil bundles of four helices designated as Qa, Qb, Qc and R. A critical intermediate in the fusion pathway is the trans-SNARE complex generated by the assembly of SNAREs residing in opposing membranes. Mechanistic details of trans-SNARE complex formation and topology in a physiological system remain largely unresolved. Our studies on native yeast vacuoles revealed that SNAREs alone are insufficient to form trans-SNARE complexes and that additional factors, potentially tethering complexes and Rab GTPases, are required for the process. Here we report a novel finding that a HOPS tethering complex dimer catalyzes Rab GTPase-dependent formation of a topologically preferred QbQcR-Qa trans-SNARE complex.

Sequential analysis of trans-SNARE formation in intracellular membrane fusion

SNARE complexes are required for membrane fusion in the endomembrane system. They contain coiled-coil bundles of four helices, three (Q(a), Q(b), and Q(c)) from target (t)-SNAREs and one (R) from the vesicular (v)-SNARE. NSF/Sec18 disrupts these cis-SNARE complexes, allowing reassembly of their subunits into trans-SNARE complexes and subsequent fusion. Studying these reactions in native yeast vacuoles, we found that NSF/Sec18 activates the vacuolar cis-SNARE complex by selectively displacing the vacuolar Q(a) SNARE, leaving behind a Q(bc)R subcomplex. This subcomplex serves as an acceptor for a Q(a) SNARE from the opposite membrane, leading to Q(a)-Q(bc)R trans-complexes. Activity tests of vacuoles with diagnostic distributions of inactivating mutations over the two fusion partners confirm that this distribution accounts for a major share of the fusion activity. The persistence of the Q(bc)R cis-complex and the formation of the Q(a)-Q(bc)R trans-complex are both sensitive to the Rab-GTPase inhibitor, GDI, and to mutations in the vacuolar tether complex, HOPS (HOmotypic fusion and vacuolar Protein Sorting complex). This suggests that the vacuolar Rab-GTPase, Ypt7, and HOPS restrict cis-SNARE disassembly and thereby bias trans-SNARE assembly into a preferred topology.

Sec24p and Sec16p cooperate to regulate the GTP cycle of the COPII coat

Vesicle budding from the endoplasmic reticulum (ER) employs a cycle of GTP binding and hydrolysis to regulate assembly of the COPII coat. We have identified a novel mutation (sec24-m11) in the cargo-binding subunit, Sec24p, that specifically impacts the GTP-dependent generation of vesicles in vitro. Using a high-throughput approach, we defined genetic interactions between sec24-m11 and a variety of trafficking components of the early secretory pathway, including the candidate COPII regulators, Sed4p and Sec16p. We defined a fragment of Sec16p that markedly inhibits the Sec23p- and Sec31p-stimulated GTPase activity of Sar1p, and demonstrated that the Sec24p-m11 mutation diminished this inhibitory activity, likely by perturbing the interaction of Sec24p with Sec16p. The consequence of the heightened GTPase activity when Sec24p-m11 is present is the generation of smaller vesicles, leading to accumulation of ER membranes and more stable ER exit sites. We propose that association of Sec24p with Sec16p creates a novel regulatory complex that retards the GTPase activity of the COPII coat to prevent premature vesicle scission, pointing to a fundamental role for GTP hydrolysis in vesicle release rather than in coat assembly/disassembly.

Organization of SNAREs within the Golgi stack

Antero- and retrograde cargo transport through the Golgi requires a series of membrane fusion events. Fusion occurs at the cis- and trans-side and along the rims of the Golgi stack. Four functional SNARE complexes have been identified mediating lipid bilayer merger in the Golgi. Their function is tightly controlled by a series of reactions involving vesicle tethering and SM proteins. This network of protein interactions spatially and temporally determines the specificity of transport vesicle targeting and fusion within the Golgi.

Schizosaccharomyces pombe Pep12p is required for vacuolar protein transport and vacuolar homotypic fusion

In eukaryotic cells, SNARE proteins are essential for intracellular vesicle trafficking. Several SNARE proteins are required for vacuolar protein transport and vacuolar biogenesis in Saccharomyces cerevisiae. Previously we demonstrated that one of the fission yeast SNARE proteins, Pep12p, is not required for vacuolar fusion process in Schizosaccharomyces pombe. We have re-examined the function of S. pombe Pep12p using the newly created pep12(+) deletion strain. Deletion of the fission yeast pep12(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y-and various ion- and drug-sensitivities. GFP-Pep12 fusion protein is mostly localized at the vacuolar membrane and the prevacuolar compartment. The S. pombe pep12Δ mutation phenocopies that of vps33Δ, suggesting that both Pep12p and Vps33p act at the same membrane fusion step in S. pombe, and both mutations cause vacuolar deficiency.

Entry and exit mechanisms at the cis-face of the Golgi complex

Vesicular transport of protein and lipid cargo from the endoplasmic reticulum (ER) to cis-Golgi compartments depends on coat protein complexes, Rab GTPases, tethering factors, and membrane fusion catalysts. ER-derived vesicles deliver cargo to an ER-Golgi intermediate compartment (ERGIC) that then fuses with and/or matures into cis-Golgi compartments. The forward transport pathway to cis-Golgi compartments is balanced by a retrograde directed pathway that recycles transport machinery back to the ER. How trafficking through the ERGIC and cis-Golgi is coordinated to maintain organelle structure and function is poorly understood and highlights central questions regarding trafficking routes and organization of the early secretory pathway.

The discovery of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex and the molecular regulation of synaptic vesicle transmitter release: the 2010 Kavli Prize in neuroscience

Brain function depends on a crucial feature: The ability of individual neurons to share packets of information, known as quantal transmission. Given the sheer number of tasks the brain has to deal with, this information sharing must be extremely rapid. Synapses are specialized points of contact between neurons, where fast transmission takes place. Though the basic elements and functions of the synapse had been established since the 1950s, the molecular basis for regulation of fast synaptic transmitter release was not known 20 years ago. However, around 1990, crucial discoveries were made by Richard Scheller, James Rothman, and Thomas Südhof, leading a few years later to the formulation of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) hypothesis and a new understanding of the molecular events controlling vesicular release of transmitter in synapses. The 2010 Kavli Prize in neuroscience was awarded to these three researchers, "for their work to reveal the precise molecular basis of the transfer of signals between nerve cells in the brain."

Application of immobilized thrombin for production of S-thanatin expressed in Escherichia coli

S-thanatin, a small antimicrobial peptide with 21 amino acid residues, was expressed as a fusion protein containing thrombin cleavage site in Escherichia coli BL21 (DE3). To reduce the production cost, immobilization of thrombin in polyacrylamide gel for cleavage was studied in this work. The immobilized thrombin exhibited excellent activity within wider ranges of pH value and temperature for reaction than free enzyme, and the residual activity could remain above 75% after ten times of usage. Tricine-SDS-PAGE result showed that the immobilized thrombin could cleave the S-thanatin fusion protein effectively. After cleavage, recombinant S-thanatin was purified by preparative reversed-phase high-performance liquid chromatography and mass spectrum showed that the molecular weight (2,448.86) was close to the theoretical value (2,448.98). After purification, about 7 mg of S-thanatin was obtained from 1 l of culture and the recombinant exhibited excellent bioactivity to E. coli ATCC 25922, with the minimum inhibitory concentration of 12 μg/ml. The purification method could be applied to prepare other peptides with similar properties at low cost.

Topological arrangement of the intracellular membrane fusion machinery

The topology of the SNARE complex is strictly restricted: of all the possible topological combinations, only one is fusogenic—the topology compatible with both the basal fusion and the SM activation. A fusogenic SNARE complex must contain a complete set of the QabcR SNARE helices.

Soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) form a four-helix coiled-coil bundle that juxtaposes two bilayers and drives a basal level of membrane fusion. The Sec1/Munc18 (SM) protein binds to its cognate SNARE bundle and accelerates the basal fusion reaction. The question of how the topological arrangement of the SNARE helices affects the reactivity of the fusion proteins remains unanswered. Here we address the problem for the first time in a reconstituted system containing both SNAREs and SM proteins. We find that to be fusogenic a SNARE topology must support both basal fusion and SM stimulation. Certain topological combinations of exocytic SNAREs result in basal fusion but cannot support SM stimulation, whereas other topologies support SM stimulation without inducing basal fusion. It is striking that of all the possible topological combinations of exocytic SNARE helices, only one induces efficient fusion. Our results suggest that the intracellular membrane fusion complex is designed to fuse bilayers according to one genetically programmed topology.

Fusing a lasting relationship between ER tubules

Atlastin is an integral membrane GTPase localized to the endoplasmic reticulum (ER). In vitro and in vivo analyses indicate that atlastin is a membrane fusogen capable of driving membrane fusion, suggesting a role in ER structure and maintenance. Interestingly, mutations in the human atlastin-1 gene, SPG3A, cause a form of autosomal dominant hereditary spastic paraplegia (HSP). The etiology of HSP is unclear, but two predominant forms of the disorder are caused by mutant proteins that affect ER structure, formation and maintenance in motor neurons. In this review, we describe the current knowledge about the molecular mechanism of atlastin function and its potential role in HSP. Greater understanding of the function of atlastin and associated proteins should provide important insight into normal ER biogenesis and maintenance, as well as the pathology of disease.

Inositol 1, 4, 5-trisphosphate receptor interacts with the SNARE domain of syntaxin 1B

Inositol 1, 4, 5-trisphosphate receptors (IP(3)Rs) are intracellular ligand-gated Ca(2+) channels that mediate Ca(2+) release from the endoplasmic reticulum (ER) into the cytosol and function in diverse cellular processes including fertilization, muscle contraction, apoptosis, secretion, and synaptic plasticity. The Ca(2+) release activity of IP(3)Rs is tightly regulated by many factors including IP(3)R-binding proteins. We show that IP(3)Rs interact with syntaxin 1 (Syx1), a membrane trafficking protein that regulates various plasma-membrane ion channels including N-, P/Q, and L-type voltage-gated Ca(2+) channels, voltage-gated potassium channels, and an epithelial sodium channel. We found that a SNARE-domain of Syx1B, one of the two Syx1 isoforms, directly interacted with the type1 IP(3)R (IP(3)R1) internal coupling domain, a known modulator for channel opening. These results indicate that Syx1B is an IP(3)R-interacting protein and that its interaction may play a crucial role in regulating the channel activity of IP(3)Rs in Syx1B-expressing cells.

The Dsl1 tethering complex actively participates in soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptor (SNARE) complex assembly at the endoplasmic reticulum in Saccharomyces cerevisiae

Intracellular transport is largely dependent on vesicles that bud off from one compartment and fuse with the target compartment. The first contact of an incoming vesicle with the target membrane is mediated by tethering factors. The tethering factor responsible for recruiting Golgi-derived vesicles to the ER is the Dsl1 tethering complex, which is comprised of the essential proteins Dsl1p, Dsl3p, and Tip20p. We investigated the role of the Tip20p subunit at the ER by analyzing two mutants, tip20-5 and tip20-8. Both mutants contained multiple mutations that were scattered throughout the TIP20 sequence. Individual mutations could not reproduce the temperature-sensitive phenotype of tip20-5 and tip20-8, indicating that the overall structure of Tip20p might be altered in the mutants. Using molecular dynamics simulations comparing Tip20p and Tip20-8p revealed that some regions, particularly the N-terminal domain and parts of the stalk region, were more flexible in the mutant protein, consistent with its increased susceptibility to proteolysis. Both Tip20-5p and Tip20-8p mutants prevented proper ER trans-SNARE complex assembly in vitro. Moreover, Tip20p mutant proteins disturbed the interaction between Dsl1p and the coatomer coat complex, indicating that the Dsl1p-coatomer interaction could be stabilized or regulated by Tip20p. We provide evidence for a direct role of the Dsl1 complex, in particular Tip20p, in the formation and stabilization of ER SNARE complexes.

Subcellular phosphoproteomics

Protein phosphorylation represents one of the most extensively studied post-translational modifications, primarily due to the emergence of sensitive methods enabling the detection of this modification both in vitro and in vivo. The availability of enrichment methods combined with sensitive mass spectrometry instrumentation has played a crucial role in uncovering the dynamic changes and the large expanding repertoire of this reversible modification. The structural changes imparted by the phosphorylation of specific residues afford exquisite mechanisms for the regulation of protein functions by modulating new binding sites on scaffold proteins or by abrogating protein-protein interactions. However, the dynamic interplay of protein phosphorylation is not occurring randomly within the cell but is rather finely orchestrated by specific kinases and phosphatases that are unevenly distributed across subcellular compartments. This spatial separation not only regulates protein phosphorylation but can also control the activity of other enzymes and the transfer of other post-translational modifications. While numerous large-scale phosphoproteomics studies highlighted the extent and diversity of phosphoproteins present in total cell lysates, the further understanding of their regulation and biological activities require a spatio-temporal resolution only achievable through subcellular fractionation. This review presents a first account of the emerging field of subcellular phosphoproteomics where cell fractionation approaches are combined with sensitive mass spectrometry methods to facilitate the identification of low abundance proteins and to unravel the intricate regulation of protein phosphorylation.

Protein determinants of SNARE-mediated lipid mixing

Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE)-mediated lipid mixing can be efficiently recapitulated in vitro by the incorporation of purified vesicle membrane (-v) SNARE and target membrane (t-) SNARE proteins into separate liposome populations. Despite the strong correlation between the observed activities in this system and the known SNARE physiology, some recent works have suggested that SNARE-mediated lipid mixing may be limited to circumstances where membrane defects arise from artifactual reconstitution conditions (such as nonphysiological high-protein concentrations or unrealistically small liposome populations). Here, we show that the previously published strategies used to reconstitute SNAREs into liposomes do not significantly affect either the physical parameters of the proteoliposomes or the ability of SNAREs to drive lipid mixing in vitro. The surface density of SNARE proteins turns out to be the most critical parameter, which controls both the rate and the extent of SNARE-mediated liposome fusion. In addition, the specific activity of the t-SNARE complex is significantly influenced by expression and reconstitution protocols, such that we only observe optimal lipid mixing when the t-SNARE proteins are coexpressed before purification.

The late stage of autophagy: cellular events and molecular regulation

Autophagy is an intracellular degradation system that delivers cytoplasmic contents to the lysosome for degradation. It is a "self-eating" process and plays a "house-cleaner" role in cells. The complex process consists of several sequential steps-induction, autophagosome formation, fusion of lysosome and autophagosome, degradation, efflux transportation of degradation products, and autophagic lysosome reformation. In this review, the cellular and molecular regulations of late stage of autophagy, including cellular events after fusion step, are summarized.

Manipulation of host membrane machinery by bacterial pathogens

Subversion of host membrane machinery is important for the uptake, survival, and replication of bacterial pathogens. Understanding how pathogens manipulate host membrane transport pathways provides mechanistic insight into how infection occurs and is also revealing new information on biochemical processes involved in the functioning of eukaryotic cells. In this review we discuss several of the canonical host pathways targeted by bacterial pathogens and emerging areas of investigation in this exciting field.

Minimal membrane docking requirements revealed by reconstitution of Rab GTPase-dependent membrane fusion from purified components

Rab GTPases and their effectors mediate docking, the initial contact of intracellular membranes preceding bilayer fusion. However, it has been unclear whether Rab proteins and effectors are sufficient for intermembrane interactions. We have recently reported reconstituted membrane fusion that requires yeast vacuolar SNAREs, lipids, and the homotypic fusion and vacuole protein sorting (HOPS)/class C Vps complex, an effector and guanine nucleotide exchange factor for the yeast vacuolar Rab GTPase Ypt7p. We now report reconstitution of lysis-free membrane fusion that requires purified GTP-bound Ypt7p, HOPS complex, vacuolar SNAREs, ATP hydrolysis, and the SNARE disassembly catalysts Sec17p and Sec18p. We use this reconstituted system to show that SNAREs and Sec17p/Sec18p, and Ypt7p and the HOPS complex, are required for stable intermembrane interactions and that the three vacuolar Q-SNAREs are sufficient for these interactions.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

Single-molecule studies of the neuronal SNARE fusion machinery

SNAREs are essential components of the machinery for Ca(2+)-triggered fusion of synaptic vesicles with the plasma membrane, resulting in neurotransmitter release into the synaptic cleft. Although much is known about their biophysical and structural properties and their interactions with accessory proteins such as the Ca(2+) sensor synaptotagmin, their precise role in membrane fusion remains an enigma. Ensemble studies of liposomes with reconstituted SNAREs have demonstrated that SNAREs and accessory proteins can trigger lipid mixing/fusion, but the inability to study individual fusion events has precluded molecular insights into the fusion process. Thus, this field is ripe for studies with single-molecule methodology. In this review, we discuss applications of single-molecule approaches to observe reconstituted SNAREs, their complexes, associated proteins, and their effect on biological membranes. Some of the findings are provocative, such as the possibility of parallel and antiparallel SNARE complexes or of vesicle docking with only syntaxin and synaptobrevin, but have been confirmed by other experiments.

A role of complexin-lipid interactions in membrane fusion

Complexins (Cpxs) and synaptotagmins regulate calcium-dependent exocytosis. A central helix in Cpx confers specific binding to the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) fusion machinery. An accessory helix in the amino-terminal region inhibits membrane fusion by blocking SNAREpin zippering. We now show that an amphipathic helix in the carboxy-terminal region of CpxI binds lipid bilayers and affects SNARE-mediated lipid mixing in a liposome fusion assay. The substitution of a hydrophobic amino acid within the helix by a charged residue abolishes the lipid interaction and the stimulatory effect of CpxI in liposome fusion. In contrast, the introduction of the bulky hydrophobic amino acid tryptophan stimulates lipid binding and liposome fusion. This data shows that local Cpx-lipid interactions can play a role in membrane fusion.

AtVPS45 is a positive regulator of the SYP41/SYP61/VTI12 SNARE complex involved in trafficking of vacuolar cargo

We report a functional characterization of AtVPS45 (for vacuolar protein sorting 45), a protein from the Sec1/Munc18 family in Arabidopsis (Arabidopsis thaliana) that interacts at the trans-Golgi network (TGN) with the SYP41/SYP61/VTI12 SNARE complex. A null allele of AtVPS45 was male gametophytic lethal, whereas stable RNA interference lines with reduced AtVPS45 protein levels had stunted growth but were viable and fertile. In the silenced lines, we observed defects in vacuole formation that correlated with a reduction in cell expansion and with autophagy-related defects in nutrient turnover. Moreover, transport of vacuolar cargo with carboxy-terminal vacuolar sorting determinants was blocked in the silenced lines, suggesting that AtVPS45 functions in vesicle trafficking to the vacuole. These trafficking defects are similar to those observed in vti12 mutants, supporting a functional relationship between AtVPS45 and VTI12. Consistent with this, we found a decrease in SYP41 protein levels coupled to the silencing of AtVPS45, pointing to instability and malfunction of the SYP41/SYP61/VTI12 SNARE complex in the absence of its cognate Sec1/Munc18 regulator. Based on its localization on the TGN, we hypothesized that AtVPS45 could be involved in membrane fusion of retrograde vesicles recycling vacuolar trafficking machinery. Indeed, in the AtVPS45-silenced plants, we found a striking alteration in the subcellular fractionation pattern of vacuolar sorting receptors, which are required for sorting of carboxy-terminal vacuolar sorting determinant-containing cargo. We propose that AtVPS45 is essential for recycling of the vacuolar sorting receptors back to the TGN and that blocking this step underlies the defects in vacuolar cargo trafficking observed in the silenced lines.

Membrane fusion: grappling with SNARE and SM proteins

The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.

Binding interactions control SNARE specificity in vivo

Saccharomyces cerevisiae contains two SNAP25 paralogues, Sec9 and Spo20, which mediate vesicle fusion at the plasma membrane and the prospore membrane, respectively. Fusion at the prospore membrane is sensitive to perturbation of the central ionic layer of the SNARE complex. Mutation of the central glutamine of the t-SNARE Sso1 impaired sporulation, but does not affect vegetative growth. Suppression of the sporulation defect of an sso1 mutant requires expression of a chimeric form of Spo20 carrying the SNARE helices of Sec9. Mutation of two residues in one SNARE domain of Spo20 to match those in Sec9 created a form of Spo20 that restores sporulation in the presence of the sso1 mutant and can replace SEC9 in vegetative cells. This mutant form of Spo20 displayed enhanced activity in in vitro fusion assays, as well as tighter binding to Sso1 and Snc2. These results demonstrate that differences within the SNARE helices can discriminate between closely related SNAREs for function in vivo.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

The syntaxins SYP31 and SYP81 control ER-Golgi trafficking in the plant secretory pathway

Overexpression of the Golgi and endoplasmic reticulum (ER) syntaxins SYP31 and SYP81 strongly inhibits constitutive secretion. By comparing the secreted reporter alpha-amylase with the ER-retained reporter alpha-amylase-HDEL, it was concluded that SYP81 overexpression inhibits both retrograde and anterograde transport, while SYP31 overexpression mainly affected anterograde transport. Of the other interacting SNAREs investigated, only the overexpression of MEMB11 led to an inhibition of protein secretion. Although the position of a fluorescent tag does not influence the correct localization of the fusion protein, only N-terminal-tagged SYP31 retained the ability of the untagged SNARE to inhibit transport. C-terminal-tagged SYP31 failed to exhibit this effect. Overexpression of both wild-type and N-terminal-tagged syntaxins caused standard Golgi marker proteins to redistribute into the ER. Nevertheless, green fluorescent protein (GFP)-SYP31 was still visible as fluorescent punctae, which, unlike SYP31-GFP, were resistant to brefeldin A treatment. Immunogold electron microscopy showed that endogenous SYP81 is not only present at the ER but also in the cis Golgi, indicating that this syntaxin cycles between these two organelles. However, when expressed at non-inhibitory levels, YFP-SYP81 was seen to locate principally to subdomains of the ER. These punctate structures were physically separated from the Golgi, suggesting that they might possibly reflect the position of ER import sites.

Reconstituted membrane fusion requires regulatory lipids, SNAREs and synergistic SNARE chaperones

The homotypic fusion of yeast vacuoles, each with 3Q- and 1R-SNARE, requires SNARE chaperones (Sec17p/Sec18p and HOPS) and regulatory lipids (sterol, diacylglycerol and phosphoinositides). Pairs of liposomes of phosphatidylcholine/phosphatidylserine, bearing three vacuolar Q-SNAREs on one and the R-SNARE on the other, undergo slow lipid mixing, but this is unaffected by HOPS and inhibited by Sec17p/Sec18p. To study these essential fusion components, we reconstituted proteoliposomes of a more physiological composition, bearing vacuolar lipids and all four vacuolar SNAREs. Their fusion requires Sec17p/Sec18p and HOPS, and each regulatory lipid is important for rapid fusion. Although SNAREs can cause both fusion and lysis, fusion of these proteoliposomes with Sec17p/Sec18p and HOPS is not accompanied by lysis. Sec17p/Sec18p, which disassemble SNARE complexes, and HOPS, which promotes and proofreads SNARE assembly, act synergistically to form fusion-competent SNARE complexes, and this synergy requires phosphoinositides. This is the first chemically defined model of the physiological interactions of these conserved fusion catalysts.

Munc18/SNARE proteins’ regulation of exocytosis in guinea pig duodenal Brunner’s gland acini

AIM: To examine the molecular mechanism of exocytosis in the Brunner’s gland acinar cell.

METHODS: We used a submucosal preparation of guinea pig duodenal Brunner’s gland acini to visualize the dilation of the ductal lumen in response to cholinergic stimulus. We correlated this to electron microscopy to determine the extent of exocytosis of the mucin-filled vesicles. We then examined the behavior of SNARE and interacting Munc18 proteins by confocal microscopy.

RESULTS: One and 6 &mgr;mol/L carbachol evoked a dose-dependent dilation of Brunner’s gland acini lumen, which correlated to the massive exocytosis of mucin. Munc18c and its cognate SNARE proteins Syntaxin-4 and SNAP-23 were localized to the apical plasma membrane, and upon cholinergic stimulation, Munc18c was displaced into the cytosol leaving Syntaxin-4 and SNAP-23 intact.

CONCLUSION: Physiologic cholinergic stimulation induces Munc18c displacement from the Brunner’s gland acinar apical plasma membrane, which enables apical membrane Syntaxin-4 and SNAP-23 to form a SNARE complex with mucin-filled vesicle SNARE proteins to affect exocytosis.

In vitro assays to measure SNARE-mediated vesicle fusion

Membrane fusion is fundamental for a broad variety of physiological processes, such as synaptic transmission, fertilization, and viral entry. Intracellular fusion along the secretory and endocytic pathway is mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. When recombinant v- and t-SNAREs are reconstituted into distinct liposome populations, membrane fusion can be monitored by either lipid or content mixing. The in vitro assays use fluorescence dequenching to measure vesicle fusion. The lipid-mixing assay is based on fluorescence resonance energy transfer between the fluorophores 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) and rhodamine, which are covalently coupled to lipids. Fusion of labeled v-SNARE liposomes with unlabeled t-SNARE liposomes increases the distance between NBD and rhodamine, increasing the NBD fluorescence. In the content-mixing assay, the water-soluble fluorophore 8-Hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) (pyranine) and its quencher p-Xylene-bis-pyridinium bromide (DPX) are incorporated into v-SNARE vesicles. The fusion of labeled v-SNARE vesicles with unlabeled t-SNARE vesicles dilutes the quencher and thus increases HPTS fluorescence. By controlling the lipid and protein composition, these assays provide important tools to detect fusion intermediates (e.g., hemifusion), and to elucidate the molecular mechanisms that regulate membrane fusion.

Sec18p and Vam7p remodel trans-SNARE complexes to permit a lipid-anchored R-SNARE to support yeast vacuole fusion

Intracellular membrane fusion requires SNARE proteins in a trans-complex, anchored to apposed membranes. Proteoliposome studies have suggested that SNAREs drive fusion by stressing the lipid bilayer via their transmembrane domains (TMDs), and that SNARE complexes require a TMD in each docked membrane to promote fusion. Yeast vacuole fusion is believed to require three Q-SNAREs from one vacuole and the R-SNARE Nyv1p from its fusion partner. In accord with this model, we find that fusion is abolished when the TMD of Nyv1p is replaced by lipid anchors, even though lipid-anchored Nyv1p assembles into trans-SNARE complexes. However, normal fusion is restored by the addition of both Sec18p and the soluble SNARE Vam7p. In restoring fusion, Sec18p promotes the disassembly of trans-SNARE complexes, and Vam7p enhances their assembly. Thus, either the TMD of this R-SNARE is not essential for fusion, and TMD-mediated membrane stress is not the only mode of trans-SNARE complex action, or these SNAREs have more flexibility than heretofore appreciated to form alternate functional complexes that violate the 3Q:1R rule.

Syntaxin 8 has two functionally distinct di-leucine-based motifs

Syntaxin 8 has been shown to form the SNARE complex with syntaxin 7, vti1b and endobrevin. These have been shown to function as the machinery for the homotypic fusion of late endosomes. Recently, we showed that syntaxins 7 and 8 cycle through the plasma membrane, and that the di-leucine-based motifs in the cytoplasmic domain of syntaxins 7 and 8 respectively function in their endocytic and exocytic processes. However, we could not elucidate the mechanism by which syntaxin 8 cycles through the plasma membrane. In this study, we constructed several different syntaxin 8 molecules by mutating putative di-leucine-based motifs, and analyzed their intracellular localization and trafficking. We found a di-leucine-based motif in the cytoplasmic domain of syntaxin 8. It is similar to that of syntaxin 7, and functions in its endocytosis. These results suggest that in the cytoplasmic domain, syntaxin 8 has two functionally distinct di-leucine-based motifs that act independently in its endocytic and exocytic processes. This is the first report on two di-leucine-based motifs in the same molecule acting independently in distinct transport pathways.

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.