A SYNTHETIC BETA-CASEIN PHOSPHOPEPTIDE AND ANALOGS AS MODEL SUBSTRATES FOR CASEIN KINASE-1, A UBIQUITOUS, PHOSPHATE DIRECTED PROTEIN-KINASE

The phosphopeptide Ser(P)-Ser(P)-Ser-(P)-Glu-Glu-Ser22-Ile-Thr, reproducing the 17-24 segment of beta-casein A2 including the seryl residue (Ser-22) which is targeted by casein kinase-1 was synthesized and used as model substrate for this enzyme. Its phosphorylation efficiency is actually higher than that of intact beta-casein (similar V(max) and 14-mu-M vs 50-mu-M K(m)). Conversely the fully dephosphorylated peptide SSSEESIT is not affected by CK-1 to any detectable extent and its glutamyl derivative EEEEESIT displays a more than 50-fold higher K(m) and a 5-fold lower V(max) as compared to the parent phosphopeptide. The relevance of the individual phosphoseryl residues has been assessed by comparing the phosphorylation efficiencies of the phosphopeptides EESpEESIT, ESpEEESIT and SpEEEESIT: while the first is a substrate almost as good as the tri Ser (P)-peptide (K(m) = 62-mu-M), and the third one is almost as poor as EEEEESIT (K(m) = 1.55 mM), ESpEEESIT displays a intermediate efficiency (K(m) = 277-mu-M). These data in conjunction with the finding that the phosphopentapeptide Ser(P)-Ser(P)-Ser-(P)-Ser-Ser(P), but neither Ser(P)-Ser(P)-Ser-Ser(P) nor Ser-Ser(P)-Ser(P)-Glu-Glu and Ser-Ala-Ala-Ser(P)-Ser(P), is readily phosphorylated by CK-1, support the concept that CK-1 is a phosphate directed protein kinase recognizing the Ser(P)-X-X-Ser-X and, less efficiently, the Ser(P)-X-X-X-Ser-X motifs.