DIFFERENTIAL PHOSPHORYLATION OF HUMAN THYMIDINE KINASE IN PROLIFERATING AND M-PHASE-ARRESTED HUMAN-CELLS

The expression of cytosolic human thymidine kinase (TK) occurs in a ce ll cycle-dependent manner. Here, we show that TK is hyperphosphorylate d during the M phase in several human cell lines. Our data from charac terizing TK phosphorylation in proliferating and M phase-arrested HeLa cells suggest that the polypeptide of TR is differentially phosphoryl ated during the progression of the cell cycle. TK in the M phase-arres ted HeLa cells was found to have a 10-fold lower affinity for its subs trate, thymidine, than in the proliferating cells. We propose that pho sphorylation of TK by the mitotic kinase(s) may provide an attenuating mechanism to prevent unnecessary synthesis of dTTP at the time of mit osis.