Xenopus Polo-like kinase Plx1: a multifunctional mitotic kinase.

The Xenopus Polo-like kinase Plx1 plays multiple roles in mitosis. Accumulating evidence shows that Plx1 is the trigger kinase for the G2/M transition that phosphorylates and activates the phosphatase Cdc25C, which subsequently dephosphorylates Cdc2/cyclin B and initiates a positive feedback loop between Cdc25C and Cdc2/cyclin B. Recent findings indicate that Plx1 itself is also in a positive feedback loop. It phosphorylates and activates the protein kinase xPlkk1, which itself then phosphorylates and further activates Plx1. Plx1 functions on the centrosome to promote bipolar spindle formation. Plx1 associates with the anaphase-promoting complex/cyclosome (APC/C) and is required to activate the APC/C for degradation of mitotic regulators required for sister chromatid separation and exit from mitosis. Plx1 is also required for cytokinesis and is localized on the midbody of the contractile ring. All known functions of Plx1 require not only its kinase activity but also an intact polo box domain in the C-terminus.