SUBSTRATE-SPECIFICITY OF PROTEIN-KINASE CK2

Unlike most Ser/Thr protein kinases which recognize phosphoacceptor si tes specified by basic residues, protein kinase CK2 is extraordinarily acidophilic in nature. By combining the analysis of more than 100 CK2 natural phosphorylation sites with the kinetic behaviour of a large n umber of model peptide substrates, it can be concluded that although t he most crucial specificity determinant is an acidic residue (Glu, Asp , TyrP, or SerP) at position +3, additional acidic residues at positio ns spanning from -2 to +7 (and probably farther) also act as positive specificity determinants for CK2, whereas basic residues at these posi tions, prolyl residue at position +1, and a bulky hydrophobic doublet at position +1 and +2, are powerful negative determinants. It also app ears that the nature of the acidic determinants may variably influence their effect depending on the position occupied: Thus, multiple aspar tic acids are, in general, determinants as good as, or even better, th an an equivalent number of glutamic acids; an individual Asp at positi on +3 flanked by Glu residues is ineffective; and phosphorylated resid ues appear to be much more effective if adjacent to the target residue (positions -2 to +2). In some instances, the local determinants alone are insufficient to account for the phosphorylation efficiency of the substrate which is greatly improved by the overall protein conformati on, as illustrated by the examples of CK2 beta-subunit and protein p53 , the latter exhibiting no consensus sequence around its phosphorylati on site. In contrast, no phosphorylation was found, comparable to that of bona fide substrates of CK2, using c-jun and the DNA binding oligo peptides derived fragment EAESN, which have been described as substrat es for CK2 even though they lack the canonical requirements for CK2 me diated phosphorylation. The specificity of CK2 can be also altered by polycationic effecters, such as some basic polypeptides, and by the su bunit composition of CK2.