A synthetic β-casein phosphopeptide and analogues as model substrates for casein kinase-1, a ubiquitous, phosphate directed protein kinase

Abstract

The phosphopeptide Ser (P)-Ser(P)-Ser-(P)-Glu-Glu-Ser11-Ilc-Thr, reproducing the 17-24 segment of β-casein Λ1 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 β-casein (similar Vmax and 14 μM vs 50 μM 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 pho..