Casein kinase 2 (CK2)-mediated reduction of the activities of Src family tyrosine kinases in vitro.

The physiological correlation between casein kinase 2 (CK2) and two Src family tyrosine kinases (Src-TKs, Fyn and Src) was mainly investigated in vitro. It was found that (i) Thr-residues of these two Src-TKs were preferentially phosphorylated by CK2 using [gamma-32P]GTP as a phosphate donor; (ii) this phosphorylation was highly stimulated in the presence of poly-Arg; (iii) full phosphorylation of two Src-TKs by CK2 resulted in significant reduction of their TK activities; and (iv) quercetin (a CK2 inhibitor) inhibited the CK2-mediated reduction of their Src-TK activities in vitro. Under the same experimental conditions, similar results were obtained with Yes. These results suggest that CK2 may be a protein kinase responsible for the suppression of at least three Src-TKs (Fyn, Src and Yes) through the specific phosphorylation of their Thr-residues at the cellular level.