A role for protein kinase CK2 in cell proliferation: evidence using a kinase-inactive mutant of CK2 catalytic subunit alpha

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (B) subu nits generally combined to form alpha (2)beta (2), alpha alpha'beta (2), or alpha'(2)beta (2) heterotetramers, To gain more insight into the role of C K2 in the control of proliferation in mammalian cells, overexpression of is olated CK2 subunits alpha, alpha', or beta was carried out in two fibroblas t cell lines: NIH3T3 and CCL39, To interfere with CK2 cellular functions, c ells were also transfected with a kinase-inactive mutant of CK2 alpha catal ytic subunit: CK2 alpha -K68A. In NIH3T3 cells, overexpression of either wi ld-type subunit (alpha, alpha' or beta) had no effect on cell proliferation , In contrast, overexpression of the CK2 alpha kinase-deficient mutant indu ced a marked inhibition of cell proliferation, This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments i n both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2 alpha -K68A mutant by growth cur ve analysis. We demonstrated that the kinase-negative mutant has the capaci ty to integrate the endogenous CK2 subunit pool both as an isolated kinase- inactive alpha subunit and as associated to the beta subunit in a kinase-in active tetramer. Finally we showed that expression of the kinase-inactive m utant interferes with phosphorylation of an endogenous CK2 substrate; we sp eculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.