PROTHYMOSIN-ALPHA IS PHOSPHORYLATED BY CASEIN KINASE-2

Prothymosin alpha (ProTalpha) is a 12.5 kDa acidic polypeptide that is considered to have a nuclear function related to cell proliferation. Inspection of its amino acid sequence revealed the presence of sequences that may serve as targets for phosphorylation by casein kinase-2 (CK-2). ProTalpha isolated from calf thymocytes was phosphorylated in vitro by CK-2. The phosphorylation sites are Ser and Thr residues located among the first 14 amino acid residues in the ProTalpha sequence. Another site that is theoretically suitable for phosphorylation by CK-2, at the C-terminus of the polypeptide, is not, in fact, phosphorylated. Thymosin alpha1 (Talpha1), a peptide whose sequence corresponds to the first 28 amino acids of ProTalpha, is also phosphorylated by CK-2 at the same phosphorylation sites as ProTalpha. In cultured splenic lymphocytes ProTalpha was phosphorylated at Thr residues located at positions 7, 12 and/or 13. Based on these observations we conclude that CK-2, or another cellular kinase with similar sequence specifity, is responsible for phosphorylation of ProTalpha in vivo.