Nuclear Basic Proteins of Xenopus laevis Sperm: Their Characterization and Synthesis during Spermatogenesis

Nuclear basic proteins from morphologically and functionally mature sperm of Xenopus laevis were analyzed by acid/urea/Triton X-100 polyacrylamide gel electrophoresis (AUT-PAGE). Six sperm-specific proteins (SP1-6) were identified in addition to somatic histones H3, H4 and smaller amount of H2A and H2B, but not H1. Of these, SP3–6 were unique in containing 33–41% arginine and having very low lysine/arginine ratios, while SP2 was more similar to H3 and H4 in having a lower arginine and higher lysine content. Fractionations of testicular cells at different spermatogenic stages by unit gravity sedimentation showed that primary spermatocytes and acrosomal vesicle spermatids possess typical somatic type histones but no SPs. Injection of ¹⁴C]-arginine into the testis and its tracing by fluorography on AUT-PAGE gels indicated that all somatic histones are synthesized during the stages between spermatogonia and primary spermatocytes, whereas SPs are synthesized at differentially regulated rates during the stages after acrosomal vesicle formation. In indirect immunofluorescence studies with anti-SP3-5 rabbit antiserum, a positive reaction was observed in the last step of spermiogenesis after the commencement of nuclear coiling.