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 amonoclonal antibody (MAC9393) have been examined in testicular andepididymal bovine sperm preparations. Western blots indicated a substantialdecrease in molecular mass of the antigen during epididymal maturation fromapproximately 87 kDa in the testis to approximately 35 kDa in the caudaepididymidis. This was accompanied by a change in its cellular localizationfrom the neck and whole head to the acrosomal region. N-terminalmicrosequencing identified MAC393 antigen as the beta-chain of clusterin. Apolyclonal antiserum to the alpha-chain of clusterin recognized bothtesticular and epididymal forms and revealed that the heterodimer waspresent on the sperm tail as well as the acrosome. These findings areexplained by the co-existence of dimeric and monomeric pools of clusterinon spermatozoa. The polyclonal antiserum recognizes both testicular andepididymal forms of the heterodimer and although the monoclonal antibodybinds to the testicular heterodimer, it only recognizes the beta-chainmonomer of epididymal clusterin. These findings support previousobservations made on human spermatozoa that two forms of clusterin, thebeta-chain monomer and the heterodimer, are present on the surface membraneand in seminal plasma.