A Novel Site of Action for α-SNAP in the SNARE Conformational Cycle Controlling Membrane Fusion

Abstract

Regulated exocytosis in neurons and neuroendocrine cells requires the formation of a stable soluble N-ethylmaleimidesensitive factor attachment protein receptor (SNARE) complex consisting of synaptobrevin-2/vesicle-associated membrane protein 2, synaptosome-associated protein of 25 kDa (SNAP-25), and syntaxin 1. This complex is subsequently disassembled by the concerted action of ␣-SNAP and the ATPases associated with different cellular activities-ATPase N-ethylmaleimide-sensitive factor (NSF). We report that NSF inhibition causes accumulation of ␣-SNAP in clusters on plasma membranes. Clustering is mediated by the binding of ␣-SNAP to uncomplexed syntaxin, because cleavage of syntaxin with botulinum neurotoxin C1 or competition by using antibodies against syntaxin SNARE motif abolishes clustering. Binding of ␣-SNAP potently inhibits Ca 2؉ -dependent exocytosis of secretory granules and SNARE-mediated liposome fusion. Membrane clustering and inhibition of both exocytosis and liposome fusion are counteracted by NSF but not when an ␣-SNAP mutant defective in NSF activation is used. We conclude that ␣-SNAP inhibits exocytosis by binding to the syntaxin SNARE motif and in turn prevents SNARE assembly, revealing an unexpected site of action for ␣-SNAP in the SNARE cycle that drives exocytotic membrane fusion. INTRODUCTIONSoluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) comprise a superfamily of small, mostly membrane-anchored proteins that mediate membrane fusion in the secretory pathway of eukaryotic cells. They are characterized by the presence of SNARE motifs, homologous stretches of 60 -70 amino acids located next to the membrane anchor domains. Key to the understanding of SNARE function in membrane fusion was the discovery of an assembly-disassembly cycle that is associated with major conformational changes. SNARE motifs of appropriate sets of SNAREs are unstructured, but they spontaneously assemble into tight complexes of extraordinary stability, forming elongated coiled-coils. When residing in different membranes, SNARE assembly leads to the formation of metastable "trans"-complexes in which the N-terminal parts of the SNARE motifs are associated, whereas the C-terminal membrane anchors are still residing in separate membranes. Progression of assembly toward the C-terminal membrane anchors is thought to proceed down a steep energy gradient and force the membranes together, resulting in fusion, with the SNAREs being converted from trans to "cis" complexes (for reviews, see Söllner, 2004;Brunger, 2005;Hong, 2005;Jahn and Scheller, 2006).To be reused in another round of fusion, SNAREs need to be reactivated by disassembly of cis-complexes, which is mediated by the hexameric ATPase N-ethylmaleimide-sensitive factor (NSF), a member of the ATPases associated with different cellular activities protein superfamily. NSF operates on all SNARE complexes and prevents accumulation of "spent" cis-complexes, thus ensuring that sufficient concentrations of free SNAREs are available for the maint...

α-SNAP Interferes with the Zippering of the SNARE Protein Membrane Fusion Machinery

“…In large vesicles such as CGs (approximately 200 nm in diameter) this interference results in an arrest at a step prior to fusion during docking, a behavior that we could further demonstrate with SNARE-containing liposomes. Thus, our data provide a framework for rationalizing previous inhibitory effects of ␣-SNAP in the exocytosis of dense-core vesicles (100ϳ300 nm in diameter) in PC12 cells (21), in yeast vacuoles (Յ5 m in diameter) (19,20), and in acrosomal fusion (2ϳ5 m in diameter) (17,18) following capacitation of sperm. In contrast, no effect of ␣-SNAP was observed when SVs were used, an observation that agrees well with a previous in vivo study reporting no significant inhibitory effect on exocytosis of SVs (31).…”

“…Although the presumed role of ␣-SNAP is confined to disassembly and thus to re-generation of SNAREs, a few studies have reported that ␣-SNAP on its own can inhibit exocytosis of large vesicles in Drosophila (16), sperm acrosome fusion (17, l8), yeast vacuole fusion (19,20), and dense-core vesicle exocytosis in PC12 cells (21). The molecular mechanisms by which ␣-SNAP inhibits fusion of these vesicles are unclear, but it is known that this inhibition is prevented in the presence of NSF.…”

Evaluation of the Heterogeneous Reactivity of the Syntaxin Molecules on the Inner Leaflet of the Plasma Membrane

“…1). Based on the observations of Hanson et al (1995) and Barszczewski et al (2008), ␣SNAP interacts with syntaxin molecules, and this complex is disassembled by NSF. These two reactions were included in the disassembly dynamics model, as detailed in Figure 5.…”

“…The monomeric syntaxin molecules react with ␣SNAP, and the complex is disassembled by NSF. The rates of these reactions were treated as adjustable parameters (k 5 /k Ϫ5 and k 6 , respectively) (Barszczewski et al, 2008). Figure 4.…”

“…As shown by Hanson et al (1995) and Barszczewski et al (2008), ␣SNAP can react with individual molecules of syntaxin to form a stable complex that calls for catalytic dissociation by NSF. As shown in Figure 9, frames J and K, the rate constants of these reactions only partially converged within a range of approximately three orders of magnitude.…”