[Basic nuclear proteins of transition during spermiogenesis in Scylliorhinus caniculus]

During dog-fish spermiogenesis, 2 basic nuclear protein transitions occur: the first from histones to spermatid-specific proteins S1 and S2, the second leading to protamines. S1, the most abundant transition protein, is a polypeptide containing 87 residues (Mr = 11,179 Da) whereas S2, the minor transition protein, contains 80 residues (Mr = 9,726 Da). The 2 proteins are mainly characterized by an asymmetry of the molecule, a very high content of basic residues, a relatively high level of hydrophobic residues and a cluster of acidic residues in the carboxy-terminal quarter of the molecule. The 2 proteins are phosphorylated on serine residues and the degree of phosphorylation is relatively important in protein S1. The 2 transition proteins are structurally unrelated to testis histones or sperm protamines and cannot be considered either as their proteolytic degradation products or as their precursors.