GUINEA-PIG PROACROSIN IS SYNTHESIZED PRINCIPALLY BY ROUND SPERMATIDS AND CONTAINS O-LINKED AS WELL AS N-LINKED OLIGOSACCHARIDE SIDE-CHAINS

Proacrosin is the zymogen precursor of acrosin, a sperm protease believed to play an essential role in fertilization. In this study, we used primary cultures of guinea pig spermatogenic cells to examine the temporal appearance and mechanisms of synthesis and processing of proacrosin during acrosome development. Following [S-35]methionine incorporation and immunoprecipitation, cultured spermatogenic cells were found to synthesize two forms of proacrosin (M(r) 54,000 and 57,000). Proacrosin was synthesized mainly by round spermatids. By immunoblotting, proacrosin became very prominent in round spermatids and persisted throughout spermiogenesis. Pulse-chase experiments demonstrated that the M(r) 54,000 form of proacrosin was converted to the M(r) 57,000 form, presumably reflecting posttranslational processing of carbohydrate side chains. When spermatogenic cells were cultured in the presence of tunicamycin, the synthesized proacrosin had an M(r) of 54,000. However, in vitro translation of mRNA extracted from guinea pig testis followed by immunoprecipitation indicated that the core polypeptide of proacrosin has an M(r) of 44,000. Guinea pig spermatogenic cells incorporated glucosamine and fucose into the oligosaccharides of proacrosin. Treatment of guinea pig testis proacrosin with N-glycosidase or O-glycosidase reduced the M(r) by 3-7%. These results indicate that proacrosin is synthesized by postmeiotic cells and the enzyme contains N- and O-linked oligosaccharides.