Phosphorylation of the N-Ethylmaleimide-sensitive Factor Is Associated with Depolarization-dependent Neurotransmitter Release from Synaptosomes

Critical to SNARE protein function in neurotransmission are the accessory proteins, soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP), and NSF, that play a role in activation of the SNAREs for membrane fusion. In this report, we demonstrate the depolarization-induced, calcium-dependent phosphorylation of NSF in rat synaptosomes. Phosphorylation of NSF is coincident with neurotransmitter release and requires an influx of external calcium. Phosphoamino acid analysis of the radiolabeled NSF indicates a role for a serine/threonine-specific kinase. Synaptosomal phosphorylation of NSF is stimulated by phorbol esters and is inhibited by staurosporine, chelerythrine, bisindolylmaleimide I, calphostin C, and Ro31-8220 but not the calmodulin kinase II inhibitor, Kn-93, suggesting a role for protein kinase C (PKC). Indeed, NSF is phosphorylated by PKC in vitro at Ser-237 of the catalytic D1 domain. Mutation of this residue to glutamic acid or to alanine eliminates in vitro phosphorylation. Molecular modeling studies suggest that Ser-237 is adjacent to an inter-subunit interface at a position where its phosphorylation could affect NSF activity. Consistently, mutation of Ser-237 to Glu, to mimic phosphorylation, results in a hexameric form of NSF that does not bind to SNAP-SNARE complexes, whereas the S237A mutant does form complex. These data suggest a negative regulatory role for PKC phosphorylation of NSF.