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First published online 4 August 2009
doi: 10.1242/jcs.052977

Journal of Cell Science 122, 3153-3160 (2009)
Published by The Company of Biologists 2009
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Research Article

Functional consequences of cleavage, dissociation and exocytotic release of ZP3R, a C4BP-related protein, from the mouse sperm acrosomal matrix

Mariano G. Buffone1,*, Kye-Seong Kim2,*, Birgit J. Doak1, Esmeralda Rodriguez-Miranda3 and George L. Gerton1,{ddagger}

1 Center for Research on Reproduction and Women's Health, Department of Obstetrics and Gynecology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
2 Department of Anatomy and Cell Biology, Hanyang University College of Medicine, Seoul 135-081, Korea
3 Instituto Tecnológico Superior de Irapuato (ITESI), Colonia El Copal, Irapuato 36821, Guanajuato, México

{ddagger} Author for correspondence (gerton{at}mail.med.upenn.edu)

Accepted 16 June 2009

The acrosome is an exocytotic vesicle located on the apical tip of the sperm head. In addition to having different morphological regions, two biochemically distinct compartments can be defined within the acrosome: a particulate acrosomal matrix and a soluble partition. The domains within the acrosome participate in the release of acrosomal proteins from the sperm during exocytosis, depending on whether the proteins partition into either the soluble or matrix compartments of the acrosome. We have examined the mechanism of differential release by evaluating the solubilization of acrosomal matrix protein ZP3R (sp56) from mouse sperm during the course of spontaneous acrosomal exocytosis. Using indirect immunofluorescence and immunoblotting, we found that the ZP3R monomer is processed from 67,000 Mr to 43,000 Mr by proteases coincident with release from the acrosome. Sperm require a maturational step, termed capacitation, before they are competent for acrosomal exocytosis and the processing of ZP3R is dramatically reduced under non-capacitating conditions. The cleavage probably takes place in complement control protein domain (CCP) 6 or the bridge region between CCP6 and CCP7, which is not present in the guinea pig orthologue AM67. The cleaved form of ZP3R does not bind to unfertilized eggs. We have incorporated these structural considerations into a model to explain the functional consequences of acrosomal exocytosis on sperm-zona interactions.

Key words: ZP3R (sp56), Acrosomal matrix, Mouse fertilization, Sperm


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