The amino acid sequence of the ram spermatidal protein 3--a transition protein TP3 or TP4?

As in other mammals, several nuclear basic proteins replace histones during the differentiation of germinal cells into spermatozoa in the ram. These proteins called transition proteins (TP) are later replaced by protamines. The amino acid sequence of the ram spermatidal protein 3 has been established by Edman degradation of the protein and of its fragments generated from digestion with endoproteinase Lys-C and pepsin and from the coding sequence of the gene and of the cDNA. The ram protein 3 is a basic protein of 109 residues (calculated Mr 13,200) with arginine and lysine residues uniformly distributed along the polypeptide chain. Of the 13 serine and threonine residues, 9 are located in structural motifs where they could be phosphorylated and, thus, modulate the binding of the protein to DNA. The tyrosine residues at position 33 and position 93, located in a basic environment, and the tryptophan residue at position 29 could be involved in the interactions of the protein with DNA through the stacking of their aromatic ring between the nucleotide bases. The ram protein 3 differs completely from the two well-defined transition protein families TP1 and TP2, which are also synthesised transiently during mammal spermiogenesis. In contrast with the rat TP3, ram protein 3 does not correspond to a precursor of a protamine. However, it shares structural similarities with both transition proteins TP3 and TP4 of the boar. The ram protein 3 and the boar transition proteins TP3 and TP4 probably belong to the same transition-protein group and would play similar functions in the chromatin remodelling during spermiogenesis. As protamine and transition-protein TP1 and TP2 genes from mammals, the coding sequence of the gene of ram protein 3 is interrupted by one intron but its organisation is different.