Comparative analysis of the nuclear basic proteins in rat, human, guinea pig, mouse and rabbit spermatozoa.

Cysteine-rich protamines (Arg = 47-61%, Cys = 8-16%) were isolated from the sperm of an individual guinea pig, human and rabbit and from pooled samples of mouse and rat sperm. Appreciable concentrations of histones were not found in the sperm nuclei of these species. In addition to the protamines, a substance of relatively low molecular weight, which reacted with the Lowry reagent, appeared in crude acid-soluble extracts of sperm nucleoprotein. This unidentified contaminent was resolved from the protamines by chromatography on Bio-Rex 70. Heterogeneity of human and mouse protamines was revealed by electrophoresis at pH 2.7, in the presence of 2.5 M urea, and confirmed by amino acid analysis, which also suggested the presence of 2 or more species of protamine in the rabbit. By contrast, the guinea pig and rat preparations displayed nearly stoichiometric ratios of amino acid residues, approaching homogeneity by this criterion. The functional consequences of crosslinks between cysteine residues of these proteins and the possible species-specific significance of their differing percentages of histidine are discussed. Potentially analogous functions are suggested for phosphorylated serine and threonine, and for ionized cysteine and tyrosine, within the protamines of developing spermatids. Their amino acid compositions indicate that the protamines of eutherian mammals are coded by a C.G-rich genome which has been unusually susceptible to genetic drift. An especially high rate of G leads to A transitions seems to have occurred in the human protamine genes.