INCREASED ASSOCIATION OF SYNAPTOSOME-ASSOCIATED PROTEIN OF 25 KDA WITH SYNTAXIN AND VESICLE-ASSOCIATED MEMBRANE-PROTEIN FOLLOWING ACROSOMALEXOCYTOSIS OF SEA-URCHIN SPERM

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a palmitoylated integral membrane protein expressed almost exclusively in neuronal an d neuroendocrine tissues. This protein forms a ternary complex with ve sicle-associated membrane protein (VAMP) and syntaxin, which is though t to regulate the fusion of plasma and vesicle membranes during exocyt osis, We report the identification of SNAP-25 expressed in sea urchin sperm. Sea urchin SNAP-25 shares greater identity with mammalian SNAP- 25 than with mammalian SNAP-23, a ubiquitously expressed homologue bel ieved to regulate membrane fusion in non-neuronal tissues. Sea urchin sperm contain a single exocytotic vesicle, the acrosomal vesicle, whos e contents are exposed during the acrosome reaction. Fusion of the pla sma membrane with the acrosomal vesicle membrane at multiple points (v esiculation) results in the release of SNAP-25 with the shed acrosome reaction vesicles. A complex containing SNAP-25, syntaxin, and VAMP is present in sperm, as detected by affinity chromatography and immunopr ecipitation. Although this complex is present prior to the acrosome re action, the amount of complex increases over 4-fold following acrosoma l exocytosis, These findings support the involvement of SNAP-25 in the invertebrate sperm acrosome reaction, possibly through increased asso ciation with VAMP and syntaxin driving the fusion of plasma and acroso mal membranes.