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 VAMP (vesicle-associated membrane protein; synaptobrevin) is a protein associated 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 identification of syntaxin and VAMP homologues in sea urchin sperm. During fertilization, 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 acrosomal vesicle fuses at multiple points with the acrosomal membrane (vesiculation) and syntaxin and VAMP are shed with the resulting membrane vesicles. Sea urchin sperm syntaxin and VAMP are associated in a complex 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 the acrosome reaction. Syntaxin and VAMP localization and loss during acrosomal exocytosis support a role for these proteins in regulating the acrosome reaction.