Unphosphorylated Cdc2 associates with Chk1 kinase.

<p>(A) Cyclin B (Cdc13) was immunoprecipitated from untreated and CPT-treated (40μM CPT, 4h) <i>cdc13-HA</i> cells and the precipitated protein was detected with the anti-Cdc2 antibody. Wild type (<i>cdc13+</i>) cells were included as a negative control. (B) The DNA damage checkpoint kinase Chk1 was immunoprecipitated from untreated and CPT-treated (40μM CPT, 4h) <i>chk1-HA</i> cells and the precipitated protein was detected with the anti-Cdc2 antibody and an anti-HA antibody (Chk1). The arrow indicates the DNA damage-induced phosphorylation of Chk1. Cdc2-U indicates the unphosphorylated Cdc2 species Ua and Ub. (C) Model. Since all four forms (C-F) in the H range bind to cyclin B (Cdc13) they are likely to be T167 phosphorylated. Cdc2-Hf may carry phosphate residues at T14, Y15 and T157, whereas Cdc2-He may be only phosphorylated at Y15 and T167. Cdc2-He and Cdc2-Hf are both involved in cell cycle regulation. Cdc2-Hc and Cdc2-Hd associate with cyclin B and may therefore be T167 modified. The unphosphorylated forms Cdc2-Ua and Cdc2-Ub associate with Chk1 and Cyclin B and may act in the response to DNA damage. Cdc2-M may be serine phosphorylated.