Cyclin G-associated kinase regulates protein phosphatase 2A by phosphorylation of its B'γ subunit

Protein phosphatase 2A (PP 2A) bearing the B'γ (= B'α/B56γ1/PR61γ) subunit is recruited to dephosphorylation targets by cyclin G. We demonstrate here that cyclin G-associated kinase (GAK), a component of the GAK/B'γ/cyclin G complex, directly phosphorylates the B'γ-Thr104 residue and regulates PP 2A activity. Indeed, an anti-B'γ-pT104 antibody detected immunofluorescence signals at the chromosome and centrosome during mitosis; these signals were reduced by siRNAmediated GAK knockdown. After DNA damage by γ-irradiation, the chromosome signals formed foci that colocalized with a DNA double-strand break (DSB) marker H2AX-pS139 (γH2AX) and CHK2-pT68. Moreover, B'γ-pT104 enhanced PP 2A holoenzyme assembly and PP 2A activity, as shown by the results of an in vitro phosphatase assay. These results suggest a novel role for GAK as a regulator of dephosphorylation events under the control of the PP2A B'γ subunit.