Identification of phosphoseryl residues in protamines from mature mammalian spermatozoa.

Protamines isolated from ejaculated human, stallion, bull, boar, and ram spermatozoa were subjected to phosphoserine conversion reaction and protein sequencing. Phosphoserines were detected as S-ethylcysteines. Endogenously phosphorylated protamines have previously been found only in ejaculated human sperm. In this study, we demonstrate that ejaculated sperm from other species also contain protamines phosphorylated at serine residues. In P1-protamines, the endogenously phosphorylated serines were located at the N-terminal region in all species studied, whereas in major forms of human and stallion P2-protamines, the serine residues located in the middle region of the molecule were predominantly phosphorylated. These results support the current DNA binding model in the case of the P1-protamines. The internal location of the phosphorylated serines in the P2-protamines indicates, however, that the binding of these proteins to DNA or their interaction with other protamine molecules may differ from that of P1-protamines. This also suggests that, during sperm maturation, P2-protamines may have a role different from that of P1-protamines.