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

Abstract

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (α and/or α′) and two regulatory (β) subunits generally combined to form α₂β₂, αα′β₂, or α′₂β₂ heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits α, α′, or β was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2α catalytic subunit: CK2α-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (α, α′ or β) had no effect on cell proliferation. In contrast, overexpression of the CK2α kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2α-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive α subunit and as associated to the β subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.