Primary substrate specificity determinants for the H4-specific protease-activated protein phosphotransferase.

The specificity of the histone-H4-specific, protease-activated protein kinase (H4-PK) was examined using two series of synthetic peptides corresponding to the phosphorylation sites in histone H4 and pyruvate kinase. Optimum kinetic constants for phosphorylation were observed using the peptide Val-Lys-Arg-Ile-Ser-Gly-Leu. Peptides in which the Lys was replaced by Arg or the Lys-Arg sequence was transposed were phosphorylated with less favorable kinetics. Peptides with either basic residue deleted did not serve as substrates. Only the H4 peptide, containing an Arg-Arg sequence, was phosphorylated by the cyclic-AMP-dependent protein kinase (CA-PK). Distinct specificity determinants for H4-PK and CA-PK were also observed using the pyruvate kinase peptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly). Collectively the data indicated that the primary substrate specificity determinants for H4-PK are Lys-Arg-Xaa-Ser whereas the CA-PK selectively phosphorylates the sequence Arg-Arg-Xaa-Ser.