The comparative efficiencies of the Ser(P)-, Thr(P)- and Tyr(P)-residues as specificity determinants for casein kinase-1.

The beta-casein derived phosphopeptide, Glu-Glu-Ser(P)-Glu-Glu-Ser-Ile-Thr-NHMe and two derivatives in which the Ser(P)-residue is replaced by the Thr(P)- and Tyr(P)-residue have been compared for their susceptibility to phosphorylation by casein kinase-1. While both the Ser(P)- and Thr(P)-peptides are good substrates with similar kinetic constants, the Tyr(P)-peptide is a substrate as poor as the unphosphorylated derivative EEEEESIT, exhibiting a 21-fold higher Km and 6-fold lower Vmax values. While prior dephosphorylation of the Ser(P)-peptide caused a marked loss in its phosphoacceptor capacity, prior dephosphorylation of the Tyr(P)-peptide caused no significant change in its poor phosphoacceptor capacity. Thus the order of efficiency of phosphoaminoacids as specificity determinants for casein kinase-1 was found to be Ser(P)=Thr(P) much greater than Tyr(P) and this order is markedly different from Tyr(P) greater than Ser(P) much greater than Thr(P) which was previously established for casein kinase-2 [Meggio et al. (1991) FEBS Lett. 279, 307-309].