Synthesis of sperm-specific basic nuclear proteins (SPs) in cultured spermatids from Xenopus laevis

The accumulation and synthesis of sperm-specific basic nuclear proteins (SPs) in Xenopus spermatids in vitro were studied by acid-urea-Triton polyacrylamide gel electrophoresis and fluorography. In synchronous cultures of round spermatids, the amount of SP2 and SP3-5 accumulated almost linearly with time, while that of SP1 remained almost constant. Fluorography showed that round spermatids incorporated [14C]arginine mostly into SP1 and SP3-5, very little into SP2, and none into histones. When [14C]arginine was incorporated into cells for 24 h on Days 0, 3, and 6, followed by immediate extraction of basic nuclear proteins, the SP1 band was detected faintly on Day 0 and the intensity increased to the maximum level by Day 3 and remained constant on Day 6; the SP3-5 bands were first detected on Day 3 and their intensity increased by Day 6. Thus, SP1 and SP3-5 were synthesized differentially during the culture period. When [14C]arginine or [14C]lysine was incorporated into round spermatids on Days 0, 3, and 6 for 15 h and chased for 3-12 days, the intensity of the SP2 band increased significantly, while the intensity of the SP1 band decreased concomitantly. This result indicates that SP2 was processed from a precursor protein which is probably SP1.