Phosphorylation of SNAP-23 by the novel kinase SNAK regulates t-SNARE complex assembly

The docking and fusion of cargo-containing vesicles with target membranes o f eukaryotic cells is mediated by the interaction of SNARE proteins present on both vesicle;and target membranes. In many cases, the target membrane S NARE, or t-SNARE,exists as a complex of syntaxin with a member of the SNAP- 25 family of palmitoylated proteins. We have identified a novel human kinas e SNAK (SNARE kinase) that specifically phosphorylates the nonneuronal t-SN ARE SNAP-23 in vivo. Interestingly, only SNAP-23 that:is not assembled into t-SNARE complexes is phosphorylated by SNAK, and phosphorylated SNAP-23 re sides exclusively in the cytosol. Coexpression with SNAK significantly enha nces-the stability of unassembled SNAP-23, and as a consequence, the assemb ly of newly synthesized SNAP-23 with syntaxin is augmented. These;data demo nstrate that phosphorylation of SNAP-23 by,SNAK enhances the kinetics of t- SNARE assembly in vivo.