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Regular Article |
a Departments of Cellular and Molecular Medicine and
b Obstetrics and Gynecology (Division of Reproductive Medicine), Ottawa Health Research Institute, University of Ottawa, Ottawa, Ontario, Canada K1Y 4E9
c Department of Biology, University of California, Riverside, California 92521
The acrosome is a large secretory granule that undergoes exocytosis when receptors on the sperm surface bind ligands in the egg extracellular matrix. Acrosomal exocytosis resembles stimulated secretion in neurons in that it is triggered by a rise in intracellular Ca2+. Synaptotagmins (Syt) comprise proteins thought to transduce this Ca2+ signal to the fusion machinery. In this study, we showed that Syt VIII is present in spermatogenic cDNA libraries. Antiserum raised against a Syt VIII-specific peptide, which recognizes Syt VIII but does not cross-react with other Syt isoforms, labeled a single prominent band on Western immunoblots of mouse sperm homogenate. Syt VIII was restricted to the sperm membrane fraction enriched in markers associated with the mouse sperm head. Fluorescent immunocytochemistry on intact mouse sperm showed that Syt VIII is localized to the acrosomal crescent and is lost upon acrosome reaction. Moreover, the amount of Syt VIII remaining with the sperm decreased proportionately with the extent of acrosome-reacted sperm. Thus, Syt VIII is a candidate for the Ca2+ sensor that regulates acrosomal exocytosis in mammalian sperm.
1 This work was supported by a grant from the Natural Sciences and Engineering Council of Canada (NSERC). D.M.H. is a recipient of a NSERC postgraduate scholarship.
2 Correspondence: Johnny K. Ngsee, Department of Cellular and Molecular Medicine, Ottawa Health Research Institute, University of Ottawa, 725 Parkdale Ave., Ottawa, ON, Canada K1Y 4E9. FAX: 613 761 5365; jngsee{at}ohri.ca
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