THE EXOCYTOSIS REGULATORY PROTEINS SYNTAXIN AND VAMP ARE SHED FROM SEA-URCHIN SPERM DURING THE ACROSOME REACTION

Syntaxin is a cytoplasmically oriented plasma membrane protein and VAM P (vesicle-associated membrane protein; synaptobrevin) is a protein as sociated with the secretory vesicle membrane. These two proteins form part of a complex which is thought to mediate the fusion of plasma and vesicle membranes during exocytosis. This paper reports the identific ation of syntaxin and VAMP homologues in sea urchin sperm. During fert ilization, sea urchin sperm release the contents of a single vesicle, the acrosomal vesicle, exposing the membrane destined to fuse with the egg. During acrosomal exocytosis, the plasma membrane over the acroso mal vesicle fuses at multiple points with the acrosomal membrane (vesi culation) and syntaxin and VAMP are shed with the resulting membrane v esicles. Sea urchin sperm syntaxin and VAMP are associated in a comple x as detected by immunoprecipitation. Following acrosomal exocytosis, syntaxin and VAMP cosediment to denser fractions on sucrose gradients showing that they have undergone associative changes during or after t he acrosome reaction. Syntaxin and VAMP localization and loss during a crosomal exocytosis support a role for these proteins in regulating th e acrosome reaction. (C) 1997 Academic Press.