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

Abstract

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 Vmax and 14 microM Km). 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 Km and a 5-fold lower Vmax as compared to the parent phosphopeptide. The relevance of the individual phosphoseryl residues has been assessed by comparing the ph..