Rat spermatid basic nuclear protein TP3 is the precursor of protamine 2

A second protamine, P2, synthesized as a precursor in late spermatids, is present in the sperm of some mammals such as the mouse. On the other hand, the sperm of other mammals such as the rat do not contain the mature P2, although they have the gene for the protein. Here we report that the P2 precursor is synthesized in late spermatids of the rat and is identical to the protein named TP3. TP3 and four other proteins present in the sonication-resistant spermatids of rat (steps 13-19) immunoreact with an antibody to human P2. By microsequencing the amino terminal ends of the immunoreacting bands, we show that TP3 is the primary translational product of the rat P2 gene and that at least two of the other bands are cleavage products of the precursor molecule. The cleavages occur between residues 10 and 11 and between 19 and 20, as is the case in the mouse. Basic nuclear protein extract of rat epididymal sperm also contains a protein that weakly immunoreacts with the antibody. This protein migrates along with the fastest immunoreacting band from late spermatids. By quantitating the amount of the precursor molecules, we conclude that appreciable levels of P2 precursors are present in rat late spermatids (about 9% of total protamines), but the processing appears to be impaired and most of the protein is lost from the chromatin by the spermatozoa stage.