A DOMAIN-SPECIFIC 50-KILODALTON STRUCTURAL PROTEIN OF THE ACROSOMAL MATRIX IS PROCESSED AND RELEASED DURING THE ACROSOME REACTION IN THE GUINEA-PIG

The sperm acrosome contains a variety of hydrolytic enzymes that exhib it differential release during the acrosome reaction. The interaction between hydrolases and the acrosomal matrix, and their compartmentaliz ation into chemically unique matrix domains may represent mechanisms b y which this ordered release is achieved. This study characterizes an acrosomal matrix protein that is restricted to the ventral-most region of the apical segment of guinea pig cauda epididymal spermatozoa. Pol yclonal antiserum, prepared against an SDS-PAGE-purified apical segmen t protein, recognized a 50-kDa band, termed AM50, on Western blots. Na tive AM50 is resistant to solubilization. However, during the acrosome reaction, AM50 is converted into a 42-43-kDa doublet protein (AM50(AR )) and released into the incubation medium. AM50 and AM50(AR) are not recognized by antiserum to guinea pig proacrosin and do not exhibit pr otease activity in a gelatinolytic SDS-PAGE assay. However, AM50(AR) d oes bind to p-aminobenzamidine affinity columns, suggesting that it ma y remain associated with proteases after the acrosome reaction. Light and electron microscopic immunocytochemistry established that AM50 was exclusively localized to the ventral aspect of the apical segment of the acrosome.