Regulation of p34cdc2 protein kinase activity by phosphorylation and cyclin binding.

Activation of the protein kinase p34cdc2 is required for entry into meiotic or mitotic M phase in all eukaryotic cells. One important mechanism regulating the activity of p34cdc2 during the cell cycle is based on phosphorylation/dephosphorylation. Avian p34cdc2 is phosphorylated on threonine 14 (Thr14), tyrosine 15 (Tyr15), threonine 161 (Thr161) and serine 277 (Ser277). Dephosphorylation of both Thr14 and Tyr15 is required for activation of p34cdc2 at the G2/M transition, indicating that phosphorylation of these residues negatively regulates p34cdc2 activity. Conversely, phosphorylation of Thr161 is required for kinase activity. Whether modification of this residue is due to intramolecular autophosphorylation or to the action of an as yet unidentified kinase remains unresolved. Likewise, the role of phosphorylation of p34cdc2 on Ser277 during G1 phase of the cell cycle remains to be determined. The function of p34cdc2 is regulated also by cell cycle-dependent complex formation with cyclin proteins. We found that chicken cyclin B2 undergoes a striking redistribution from the cytoplasm to the nucleus just prior to the onset of mitosis. Expression of a non-destructible cyclin B2 mutant causes HeLa cells to arrest in mitosis. Frequently, arrested cells displayed multiple mitotic spindles.