FIGURE 5.

CK2 is required for phosphorylation of Ser-1067 and regulates Ser-1083 phosphorylation in response to IR and intra-S phase checkpoint. A, CK2 is knocked down by co-transfection of siRNA against both α and α′ subunits. B, quantitative mass spectrometric measurements of Ser-1067 phosphorylation. SMC3 was immunoprecipitated from siCK2 (both CK2α and CK2α′ was knocked down) and siVim (as a control)-transfected cells. The percentage of Ser-1067 phosphorylation was normalized to that in the siVim cell. C, quantitative mass spectrometric measurements of Ser-1083 phosphorylation. SMC3 was purified from siCK2 and siVim cells with or without IR treatment. Phosphorylation of Ser-1083 in indicated conditions was measured and normalized to the level in siVim-transfected cells without IR. D, RDS measurements in siCK2 cells and siVim cells. Sixty hours after siRNA transfection, HeLa cells were labeled with ¹⁴C for 36 h and then treated with 10 Gy IR. RDS was measured at the indicated times.