Casein kinase II phosphorylates p34 super(cdc2) kinase in G1 phase of the HeLa cell division cycle

The activity of p34 super(cdc2) kinase is regulated in the phases of vertebrate cell cycle by mechanisms of phosphorylation and dephosphorylation. We demonstrate that casein kinase II (CKII) phosphorylates p34 super(cdc2) in vivo and in vitro at Ser super(39) during the G1 phase of HeLa cell division cycle. Human p34 super(cdc2) shows a typical phosphorylation sequence motif site for CKII at Ser super(39) (ES super(39)EEE). In our experiments, either p34 super(cdc2) expressed and purified from bacteria or p34 super(cdc2) immunoprecipitated from HeLa cells enriched in G1 by elutriation were substrates for in vitro phosphorylation by CKII. Phosphoamino acid analysis, N-chlorosuccinimide mapping, and two-dimensional tryptic mapping of p34 super(cdc2) phosphorylated in vitro were performed to determine the phosphorylation site. A synthetic peptide spanning residues 33-50 of human p34 super(cdc2), including the CKII site, was used to map the site.