N-ethylmaleimide-sensitive factor-dependent α-SNAP release, an early event in the docking/fusion process, is not regulated by Rab GTPases

The N-ethylmaleimide-sensitive factor (NSF) is required for multiple intracellular vesicle transport events. In vitro biochemical studies have demonstrated that NSF, soluble NSF attachment proteins (SNAPs), and SNAP receptors form a 20 S particle. This complex is disassembled by the ATPase activity of NSF. We have studied particle disassembly in a membrane environment by examining the binding of recombinant SNAPs and NSF to endosomal membranes. We present evidence that α-SNAP is released from the membranes in a temperature- and time-dependent manner and that this release is mediated by the ATPase activity of NSF. Our results indicate that NSF mutants in the first ATP binding domain completely abrogate α-SNAP release, whereas no inhibitory effect is observed with a mutant in the second ATP binding domain. Interestingly, neither β-SNAP nor γ-SNAP are released by the ATPase activity of NSF, indicating that these proteins are retained on the membranes by interactions that differ from those that retain α-SNAP. Although the small Rab GTPases are known to play a role in SNARE complex assembly, our results indicate that these GTPases do not regulate the NSF- dependent release of α-SNAP.