The extraordinary active site substrate specificity of pp60c-src. A multiple specificity protein kinase

We report the first active site substrate specificity analysis of a tyrosine-specific protein kinase, namely pp60c-src. Like the cAMP-dependent protein kinase and protein kinase C, pp60c-src will phosphorylate an assortment of achiral residues attached to active site-directed peptides. Furthermore, pp60c-src phosphorylates both aromatic and aliphatic alcohols. However, the substrate specificity of pp60c-src is much broader than that of the two previously examined serine/threonine-specific protein kinases. We have previously shown that both the cAMP-dependent protein kinase and protein kinase C will utilize a wide array of non-amino acid residues as substrates, as long as the distance between the hydroxyl moiety and the adjacent peptide backbone is comparable with that present in serine and threonine (Kwon, Y.-G., Mendelow, M., and Lawrence, D. S. (1994) J. Biol. Chem. 269, 4839-4844). In marked contrast, pp60c-src does not discriminate against substrates on the basis of chain length, catalyzing the phosphorylation of residues that contain anywhere from 2-12 carbons between the alcohol functional group and the adjacent peptide bond. In addition, pp60c-src phosphorylates L-serine in an active site-directed peptide. The possible structural basis for the multiple specificity of pp60c-src is discussed. Finally, the active site specificity of pp60c-src is not just limited to L-amino acid residues, but also extends into the realm of D-amino acids as well.