Cellular distribution and molecular heterogeneity of MAC393 antigen (clusterin, beta-chain) on the surface membrane of bull spermatozoa.

The distribution and size of a surface membrane antigen identified by a monoclonal antibody (MAC9393) have been examined in testicular and epididymal bovine sperm preparations. Western blots indicated a substantial decrease in molecular mass of the antigen during epididymal maturation from approximately 87 kDa in the testis to approximately 35 kDa in the cauda epididymidis. This was accompanied by a change in its cellular localization from the neck and whole head to the acrosomal region. N-terminal microsequencing identified MAC393 antigen as the beta-chain of clusterin. A polyclonal antiserum to the alpha-chain of clusterin recognized both testicular and epididymal forms and revealed that the heterodimer was present on the sperm tail as well as the acrosome. These findings are explained by the co-existence of dimeric and monomeric pools of clusterin on spermatozoa. The polyclonal antiserum recognizes both testicular and epididymal forms of the heterodimer and although the monoclonal antibody binds to the testicular heterodimer, it only recognizes the beta-chain monomer of epididymal clusterin. These findings support previous observations made on human spermatozoa that two forms of clusterin, the beta-chain monomer and the heterodimer, are present on the surface membrane and in seminal plasma.