PURIFICATION AND CLONING OF A PROTEIN-KINASE THAT PHOSPHORYLATES AND ACTIVATES THE POLO-LIKE KINASE PLX1

The Xenopus polo-like kinase 1 (Plx1) is essential during mitosis for the activation of Cdc25C, for spindle assembly, and for cyclin B degra dation. Polo-Like kinases from various organisms are activated by phos phorylation by an unidentified protein kinase. A protein kinase, polo- like kinase kinase 1 or xPlkk1, that phosphorylates and activates Plx1 in vitro was purified to near homogeneity and cloned. Phosphopeptide mapping of Plx1 phosphorylated in vitro by recombinant xPlkk1 or in pr ogesterone-treated oocytes indicates that xPlkk1 may activate Plx1 in vivo. The xPlkk1 protein itself was also activated by phosphorylation on serine and threonine residues, and the kinetics of activation of xP lkk1 in vivo closely paralleled the activation of Plx1. Moreover, micr oinjection of xPlkk1 into Xenopus oocytes accelerated the timing of ac tivation of Plx1 and the transition from G(2) to M phase of the cell c ycle. These results define a protein kinase cascade that regulates sev eral events of mitosis.