Cdc2, an evolutionarily conserved serine/threonine-specific protein kinase, is essential in the cell cycle transition from G2 to M phase. Cdc2 is regulated by association with B-type cyclins and by reversible phosophorylation. Cyclin B binding facilitates the phosphorylation of Cdc2 p34 on three regulatory sites: threonine 14, tyrosine 15, and threonine 161. In higher eukaryotes, Cdc2 is negatively regulated by phosphorylation of two residues located in the ATP-binding site, Thr 14 and Tyr 15. Cdc2 is positively regulated by the cyclin-dependent phosphorylation of Thr 161. Both phosphorylation and de- phosphorylation at Thr 161 are required for progression through the cell cycle.
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