Interspecies immunologic cross-reactivity of mammalian sperm basic proteins

The species uniqueness of the sperm basic nuclear proteins (SBNPs) of mammals is in sharp contrast to the highly conserved function of those proteins, to mediate extreme condensation of the sperm chromatin. Despite their molecular uniqueness, however, the SBNPs of all eutherian mammals share certain characteristics: low molecular weight, high percentage of arginine and cysteine residues and, in the final step of chromatin condensation, extensive SS cross-linking. By radioimmunoassay, we have demonstrated that rabbit antiserum to mouse SBNP is reactive with purified mouse SBNP and mouse sperm and is cross-reactive with purified bull SBNP and sperm of bull, rat, rabbit, dog and man. The immunologically cross-reactive sites, therefore, may represent the homologous regions of the various species of SBNPs that serve the function of chromatin condensation. Immunologic reactivity was displayed only by sperm made to swell by treatment with 2-mercaptoethanol. Unswollen sperm were not reactive, confirming previous observations by immunofluorescence [1] indicating that reduction of SS bonds is necessary to render the antigenic sites of SBNP available for immunologic recognition, and suggesting that formation of SS bonds within that protein may be a molecular mechanism of antigen sequestration in vivo.