Phosphorylation of human sperm protamines HP1 and HP2: identification of phosphorylation sites

Human sperm is characterized by a high heterogeneity of its basic nuclear protein complement of pro-protamines, protamines and histones. This heterogeneity is increased by the persistence of phosphorylated protamines in mature spermatozoa. Alkaline phosphatase treatment of whole protein indicated that protamines HP1 and HP2 were phosphorylated to various degrees. Presence of non-phosphorylated and phosphorylated protamines HP1 and HP2 was further demonstrated by electrospray mass spectrometry. Phosphorylation sites of mono- and di-phosphorylated protamine HP1 were identified by automatic Edman degradation of the protein after phosphoserine derivatization to S-ethylcysteine. In both phosphorylated forms, Ser-10 was found phosphorylated; in the di-phosphorylated form, Ser-8 was identified as the second site of phosphorylation. In protamine HP2, the unique site of phosphorylation (Ser-14) was located after limited acid hydrolysis of enzymic peptides and thin-layer electrophoresis.