PURIFICATION OF A SERINE KINASE THAT ASSOCIATES WITH AND PHOSPHORYLATES HUMAN CDC25C ON SERINE-216

Human Cdc25C is a protein phosphatase that dephosphorylates and activa tes Cdc2-cyclin B to trigger entry into mitosis. Cdc25C is itself regu lated by phosphorylation. In asynchronously growing HeLa cells, we hav e determined that serine 216 is the major site of Cdc25C phosphorylati on. We have isolated a protein kinase that binds to Cdc25C and phospho rylates serine 216. The kinase binds within amino acids 200-256 of Cdc 25C. This region is conserved in some Cdc25 homologues and contains a putative bipartite nuclear localization signal just downstream from se rine 216. Finally, the Cdc25C-associating kinase was purified over 800 0-fold from rat liver as a 36-38-kDa doublet of proteins.