Acrosome biogenesis begins during meiosis: evidence from the synthesis and distribution of an acrosomal glycoprotein, acrogranin, during guinea pig spermatogenesis

Abstract

The biogenesis of the sperm-specific organelle, the acrosome, was investigated using an acrosomal glycoprotein as a marker of development. This component, which we have named acrogranin, was purified from an acid extract of guinea pig testes by standard chromatographic procedures. The molecular weight of reduced acrogranin was determined to be 67,000 by analytical sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunization of female rabbits with purified acrogranin produced an antiserum that recognized a single protein with Mr = 67,000 in an acid extract of guinea pig testes. By indirect immunofluorescence, acrogranin was found only in the acrosome of mature sperm. In haploid spermatids, acrogranin was localized in the developing acrosome and, weakly, in the cytoplasm. Acrogranin was also detected in the cytoplasm and juxtanuclear region in putative proacrosomal granules of meiotic cells (pachytene spermatocytes). Detergent extracts from different purified germ cell populations contained only the Mr = 67,000 form of acrogranin, but sperm extracts had four lower Mr immunoreactive forms not present in the testicular extracts. By two-dimensional gel electrophoresis, acrogranin was found to be an acidic glycoprotein. Analysis of glycosylated and trifluoromethanesulfonic acid-deglycosylated acrogranin indicated that the antibody recognized polypeptide determinants. After highly enriched germ cell populations were labeled overnight with [35S]methionine and extracted with detergent, anti-acrogranin immunoprecipitated a single protein of Mr = 67,000. The synthesis of acrogranin by pachytene spermatocytes and round spermatids was similar, but the synthesis of the glycoprotein by condensing spermatids was markedly reduced. These studies demonstrate that acrosome biogenesis, as determined by the synthesis of a specific acrosomal component, begins during meiosis and continues through the early stages of spermiogenesis.