Biochemical characterization and epididymal localization of the maturation-dependent ram sperm surface antigen ESA152.

We examine here the biochemical properties and epididymal localization of a maturation dependent ram sperm surface antigen. A monoclonal antibody, ESA152, identifies an antigen that is present on the surface of ejaculated sperm, but is absent from testicular sperm. Crosslinking of the ESA152 antigen with bivalent antibodies induces the acrosome reaction, redistributing the antigen into the anterior region of the sperm head where it associates with the fusion product of the plasma membrane and the outer acrosomal membrane. The ESA152 antigen appears as a polypeptide of 18 kDa on immunoblots of SDS-polyacrylamide gels. The ESA152 epitope includes the sialic acid termini of N-linked oligosaccharides, as shown by its sensitivity to neuraminidase and endoglycosidase F. The ESA152 antigen is a highly hydrophobic integral membrane protein that resists aqueous extraction, partitions into the detergent phase of Triton-X-114, and solubilizes in chloroform-methanol mixtures. The anchoring of ESA152 is unaffected by phosphtidylinositol specific phospholipase C. The antigen is absent from extracts of caput and corpus epididymidis but appears abruptly in the first segment of the cauda. Immunofluorescence reveals that the ESA152 epitope first appears in clusters of cells in the luminal epithelium of the proximal cauda, prior to or concurrent with its appearance on sperm.