Phosphotyrosine as a specificity determinant for casein kinase‐2, a growth related Ser/Thr‐specific protein kinase

Abstract

The motif Ser‐Ser‐Ser‐Glu‐Glu is readily phosphorylated by casein kinase‐2 (CK‐2), a growth‐related protein kinase whose consensus sequence is Ser(Thr)‐Xaa‐Xaa‐Glu(Asp) [(1990) Biochim. Biophys. Acta. 1054, 267–283]. Here we show that phosphotyrosine can replace carboxylic acids as specificity determinant for CK‐2 phosphorylation, the phosphotyrosyl peptide Ser‐Ser‐Ser‐TyrP‐TyrP actually being a substrate more efficient than Ser‐Ser‐Ser‐Glu‐Glu itself both in terms of K _m (0.69 vs 2.43 mM) and V???. Prior dephosphorylation of phosphotyrosine entirely prevents the subsequent phosphorylation of serine by CK‐2. While Ser‐Ser‐Ser‐TyrP‐TyrP is better than Ser‐Ser‐Ser‐SerP‐SerP, which in turn is better than Ser‐Ser‐Ser‐Glu‐Glu, Ser‐Ser‐Ser‐ThrP‐ThrP is a less efficient substrate than Ser‐Ser‐Ser‐Glu‐Glu. Thus the order of efficiency of phosphoamino acids as specificity determinants for CK‐2 appears to be TyrP>SerP>ThrP.