Cellular functions of NSF: not just SNAPs and SNAREs

N-ethylmaleimide sensitive factor (NSF) is an ATPases associated with various cellular activities protein (AAA), broadly required for intracellular membrane fusion. NSF functions as a SNAP receptor (SNARE) chaperone which binds, through soluble NSF attachment proteins (SNAPs), to SNARE complexes and utilizes the energy of ATP hydrolysis to disassemble them thus facilitating SNARE recycling. While this is a major function of NSF, it does seem to interact with other proteins, such as the AMPA receptor subunit, GluR2, and beta2-AR and is thought to affect their trafficking patterns. New data suggest that NSF may be regulated by transient post-translational modifications such as phosphorylation and nitrosylation. These new aspects of NSF function as well as its role in SNARE complex dynamics will be discussed.     

Endobrevin/VAMP8 mediates exocytotic release of hexosaminidase from rat basophilic leukaemia cells

Mast cells are important players in innate immunity and mediate allergic responses. Upon stimulation, they release biologically active mediators including histamine, cytokines and lysosomal hydrolases. We used permeabilized rat basophilic leukaemia cells as model to identify R-SNAREs (soluble NSF (N-ethylmaleimide-sensitive fusion protein)) mediating exocytosis of hexosaminidase from mast cells. Of a complete set of recombinant mammalian R-SNAREs, only vesicle associated membrane protein (VAMP8)/endobrevin consistently blocked hexosaminidase release, which was also insensitive to treatment with clostridial neurotoxins. Thus, VAMP8, which also mediates fusion of late endosomes and lysosomes, plays a major role in hexosaminidase release, strengthening the view that mast cell granules share properties of both secretory granules and lysosomes.     

Down-regulated expression of exocytotic proteins in pancreatic islets of diabetic GK rats

Exocytosis is regulated by exocytotic proteins, which are present in insulin-secreting beta-cells and play regulatory roles in insulin secretion. Non-insulin dependent diabetes mellitus (type 2 diabetes) is a disease characterized by impaired insulin secretion and insulin resistance. Exocytotic protein immunoreactivities were studied in pancreatic islets of type 2 diabetic Goto-Kakizaki (GK) rats using immunofluorescence histochemistry. The immunoreactivities for vesicle-associated membrane protein-2 (VAMP-2), synaptotagmin III, cysteine string protein (CSP), mammalian homologue of the unc-18 gene (Munc-18), alpha-soluble N-ethylmaleimide-sensitive attachment protein (alpha-SNAP), N-ethylmaleimide-sensitive factor (NSF) and synaptosomal-associated protein of 25 kDa (SNAP-25) exhibited weaker immunofluorescence intensity in islets of GK rats as compared to control Wistar rats. Insulin immunoreactivity was also decreased in GK rat beta-cells, whereas no detectable alterations in the expression of actin immunoreactivity could be detected. The data suggest that reduced expression of exocytotic proteins and decreased insulin content may contribute to the diabetic syndrome in the GK rat.     

Systematic analysis of SNARE localization in the filamentous fungus Aspergillus oryzae

In spite of their great importance for both applied and basic biology, studies on vesicular trafficking in filamentous fungi have been so far very limited. Here, we identified 21 genes, which might be a total set, encoding putative SNARE proteins that are key factors for vesicular trafficking, taking advantage of available whole genome sequence in the filamentous fungus Aspergillus oryzae. The subsequent systematic analysis to determine the localization of putative SNAREs using EGFP-fused chimeras revealed that most putative SNAREs show similar subcellular distribution to their counterparts in the budding yeast. However, there existed some characteristic features of SNAREs in A. oryzae, such as SNARE localization at/near the septum and the presence of apparently non-redundant plasma membrane Qa-SNAREs. Overall, this analysis allowed us to provide an overview of vesicular trafficking and organelle distribution in A. oryzae.     

The Habc Domain of the SNARE Vam3 Interacts with the HOPS Tethering Complex to Facilitate Vacuole Fusion

Membrane fusion at vacuoles requires a consecutive action of the HOPS tethering complex, which is recruited by the Rab GTPase Ypt7, and vacuolar SNAREs to drive membrane fusion. It is assumed that the Sec1/Munc18-like Vps33 within the HOPS complex is largely responsible for SNARE chaperoning. Here, we present direct evidence for HOPS binding to SNAREs and the H_abc domain of the Vam3 SNARE protein, which may explain its function during fusion. We show that HOPS interacts strongly with the Vam3 H_abc domain, assembled Q-SNAREs, and the R-SNARE Ykt6, but not the Q-SNARE Vti1 or the Vam3 SNARE domain. Electron microscopy combined with Nanogold labeling reveals that the binding sites for vacuolar SNAREs and the H_abc domain are located in the large head of the HOPS complex, where Vps16 and Vps33 have been identified before. Competition experiments suggest that HOPS bound to the H_abc domain can still interact with assembled Q-SNAREs, whereas Q-SNARE binding prevents recognition of the H_abc domain. In agreement, membranes carrying Vam3ΔH_abc fuse poorly unless an excess of HOPS is provided. These data suggest that the H_abc domain of Vam3 facilitates the assembly of the HOPS/SNARE machinery at fusion sites and thus supports efficient membrane fusion.     

Inhibition of SNARE-mediated membrane traffic impairs cell migration

Cell migration occurs as a highly-regulated cycle of cell polarization, membrane extension at the leading edge, adhesion, contraction of the cell body, and release from the extracellular matrix at the trailing edge. In this study, we investigated the involvement of SNARE-mediated membrane trafficking in cell migration. Using a dominant-negative form of the enzyme N-ethylmaleimide-sensitive factor as a general inhibitor of SNARE-mediated membrane traffic and tetanus toxin as a specific inhibitor of VAMP3/cellubrevin, we conducted transwell migration assays and determined that serum-induced migration of CHO-K1 cells is dependant upon SNARE function. Both VAMP3-mediated and VAMP3-independent traffic were involved in regulating this cell migration. Inhibition of SNARE-mediated membrane traffic led to a decrease in the protrusion of lamellipodia at the leading edge of migrating cells. Additionally, the reduction in cell migration resulting from the inhibition of SNARE function was accompanied by perturbation of a Rab11-containing alpha(5)beta(1) integrin compartment and a decrease in cell surface alpha(5)beta(1) without alteration to total cellular integrin levels. Together, these observations suggest that inhibition of SNARE-mediated traffic interferes with the intracellular distribution of integrins and with the membrane remodeling that contributes to lamellipodial extension during cell migration.     

SNARE-mediated membrane traffic modulates RhoA-regulated focal adhesion formation

In the present study, we examined the role of soluble NSF attachment protein receptor (SNARE)-mediated membrane traffic in the formation of focal adhesions during cell spreading. CHO-K1 cells expressing a dominant-negative form of N-ethylmaleimide-sensitive factor (E329Q-NSF) were unable to spread as well as control cells and they formed focal adhesions (FAs) that were larger than those in control cells. FA formation was impaired in cells transfected with a dominant-negative form of RhoA, but, significantly, not in cells simultaneously expressing dominant-negative NSF. Treatment of E329Q-NSF-expressing cells with the ROCK inhibitor Y-27632 did inhibit FA formation. The results are consistent with a model of cell adhesion in which SNARE-mediated membrane traffic is required for both the elaboration of lamellipodia and the modulation of biochemical signals that control RhoA-mediated FA assembly.     

NSF independent fusion of Salmonella-containing late phagosomes with early endosomes

Initial characterizations of live-Salmonella-containing early (LSEP) and late phagosomes (LSLP) in macrophages show that both phagosomes retain Rab5 and EEA1. In addition, LSEP specifically contain transferrin receptor whereas LSLP possess relatively more rabaptin-5. In contrast to LSLP, late-Salmonella-containing vacuoles in epithelial cells show significantly reduced levels of Rab5 and EEA1. Subsequent results demonstrate that both phagosomes efficiently fuse with early endosomes (EE). In contrast to LSEP, fusion between LSLP and EE is insensitive to ATPγS treatment. Furthermore, LSLP fuses with EE in absence of NEM-sensitive fusion factor (NSF) as well as in the presence of NSF:D1EQ mutant demonstrating that LSLP fusion with EE is NSF independent.     

DBC2 is essential for transporting vesicular stomatitis virus glycoprotein

DBC2 is a tumor suppressor gene linked to breast and lung cancers. Although DBC2 belongs to the RHO GTPase family, it has a unique structure that contains a Broad-Complex/Tramtrack/Bric a Brac (BTB) domain at the C terminus instead of a typical CAAX motif. A limited number of functional studies on DBC2 have indicated its participation in diverse cellular activities, such as ubiquitination, cell-cycle control, cytoskeleton organization and protein transport. In this study, the role of DBC2 in protein transport was analyzed using vesicular stomatitis virus glycoprotein (VSVG) fused with green fluorescent protein. We discovered that DBC2 knockdown hinders the VSVG transport system in 293 cells. Previous studies have demonstrated that VSVG is transported via the microtubule motor complex. We demonstrate that DBC2 mobility depends also on an intact microtubule network. We conclude that DBC2 plays an essential role in microtubule-mediated VSVG transport from the endoplasmic reticulum to the Golgi apparatus.     

Mechanism of neurotransmitter release coming into focus

Research for three decades and major recent advances have provided crucial insights into how neurotransmitters are released by Ca²⁺‐triggered synaptic vesicle exocytosis, leading to reconstitution of basic steps that underlie Ca²⁺‐dependent membrane fusion and yielding a model that assigns defined functions for central components of the release machinery. The soluble N‐ethyl maleimide sensitive factor attachment protein receptors (SNAREs) syntaxin‐1, SNAP‐25, and synaptobrevin‐2 form a tight SNARE complex that brings the vesicle and plasma membranes together and is key for membrane fusion. N‐ethyl maleimide sensitive factor (NSF) and soluble NSF attachment proteins (SNAPs) disassemble the SNARE complex to recycle the SNAREs for another round of fusion. Munc18‐1 and Munc13‐1 orchestrate SNARE complex formation in an NSF‐SNAP‐resistant manner by a mechanism whereby Munc18‐1 binds to synaptobrevin and to a self‐inhibited “closed” conformation of syntaxin‐1, thus forming a template to assemble the SNARE complex, and Munc13‐1 facilitates assembly by bridging the vesicle and plasma membranes and catalyzing opening of syntaxin‐1. Synaptotagmin‐1 functions as the major Ca²⁺ sensor that triggers release by binding to membrane phospholipids and to the SNAREs, in a tight interplay with complexins that accelerates membrane fusion. Many of these proteins act as both inhibitors and activators of exocytosis, which is critical for the exquisite regulation of neurotransmitter release. It is still unclear how the actions of these various proteins and multiple other components that control release are integrated and, in particular, how they induce membrane fusion, but it can be expected that these fundamental questions can be answered in the near future, building on the extensive knowledge already available.