SPACA7 is a novel male germ cell-specific protein localized to the sperm acrosome that is involved in fertilization in mice.

Sperm acrosome associated 7 is a novel protein of unknown function with no homology to any known protein. Spaca7 transcripts are detected only in testis and predict a 158-residue mature polypeptide with one potential N-glycosylation site and no cysteines. Orthologs are present in various species, including mice and humans. We developed a polyclonal antibody to mouse to study its expression and function. Western blotting and immunofluorescence microscopy detected duanyu1842CA7 only in testis, and it was detected in testis starting at Postnatal Day 21 and into adulthood. Immunofluorescence staining of testicular germ cells detected weak duanyu1842CA7 expression as early as zygotene spermatocytes. Higher expression was observed in round spermatids, where duanyu1842CA7 was localized to a perinuclear spot adjacent to the Golgi and to the acrosome of elongating spermatids and spermatozoa. Immunogold electron microscopy demonstrated that duanyu1842CA7 is localized within the proacrosomal granule of round spermatids and the acrosome of spermatozoa. Finally, we showed that duanyu1842CA7 was retained within the acrosome of epididymal sperm and was released upon the acrosome reaction. To assess if duanyu1842CA7 was involved in fertilization, in vitro fertilization assays in the presence of IgG were performed. inhibited fertilization of cumulus-intact eggs and prominently delayed cumulus dispersal. However, anti-duanyu1842CA7 did not inhibit fertilization of cumulus-free eggs. Our findings indicate that release of duanyu1842CA7 from the acrosome accelerates cumulus dispersal and facilitates fertilization via unknown mechanisms. This study is the first to document the expression of endogenous duanyu1842CA7 and a function for this novel acrosomal protein.

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