Increased Association of Synaptosome-associated Protein of 25 kDa with Syntaxin and Vesicle-associated Membrane Protein following Acrosomal Exocytosis of Sea Urchin Sperm

Joseph R Schulz,Jun D Sasaki,Victor D Vacquier

doi:10.1074/jbc.273.38.24355

Joseph R Schulz, Jun D Sasaki + Show 1 more

Open Access

https://doi.org/10.1074/jbc.273.38.24355

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Abstract

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a palmitoylated integral membrane protein expressed almost exclusively in neuronal and neuroendocrine tissues. This protein forms a ternary complex with vesicle-associated membrane protein (VAMP) and syntaxin, which is thought to regulate the fusion of plasma and vesicle membranes during exocytosis. 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 believed to regulate membrane fusion in non-neuronal tissues. Sea urchin sperm contain a single exocytotic vesicle, the acrosomal vesicle, whose contents are exposed during the acrosome reaction. Fusion of the plasma membrane with the acrosomal vesicle membrane at multiple points (vesiculation) 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 immunoprecipitation. Although this complex is present prior to the acrosome reaction, the amount of complex increases over 4-fold following acrosomal exocytosis. These findings support the involvement of SNAP-25 in the invertebrate sperm acrosome reaction, possibly through increased association with VAMP and syntaxin driving the fusion of plasma and acrosomal membranes.

Highlights

During fertilization in many animals, including sea urchins and mammals, outer investments of the egg trigger the exocytosis of a single exocytotic vesicle in sperm, the acrosomal vesicle [1,2,3]
Exocytosis of the acrosomal vesicle during the acrosome reaction is unique in that the plasma and acrosomal membranes fuse at multiple points resulting in formation of acrosome reaction vesicles (ARVs),1 which are subsequently shed from sperm (4 – 6)
An intracellular protein integral to the plasma membrane [12, 13], and vesicleassociated membrane protein (VAMP), which is integral to the vesicle membrane, are expressed in sea urchin sperm and are shed with the ARVs during the acrosome reaction

Summary

Introduction

During fertilization in many animals, including sea urchins and mammals, outer investments of the egg trigger the exocytosis of a single exocytotic vesicle in sperm, the acrosomal vesicle [1,2,3]. 14 and 15), which is integral to the vesicle membrane, are expressed in sea urchin sperm and are shed with the ARVs during the acrosome reaction. Sperm SNAP-25 is found in a complex with syntaxin and VAMP and is shed with the ARVs. The amount of complex of these three proteins increases following the acrosome reaction.

Results

Conclusion