The polo-like kinase Plx1 prevents premature inactivation of the APC(Fizzy)-dependent pathway in the early Xenopus cell cycle

Members of the polo-like family of protein kinases have been involved in th e control of APC (anaphase-promoting complex) during the cell cycle, Set ho w they activate APC is not understood in any detail. In Xenopus oocytes, Ca ti-dependent degradation of cyclin B associated with release from arrest at second meiotic metaphase was demonstrated to require the polo-like kinase Plx1. The aim of the present study mas to examine, beyond Ca2+-dependent re sumption of meiosis, the possible role of Plx1 in the control of cyclin deg radation during the early mitotic cell cycle, Plx1 was found to be dispensa ble for MPF to turn on the cyclin degradation machinery, However, it is req uired to prevent premature inactivation of the APC-dependent proteolytic pa thway. Microcystin suppresses the requirement for Plx1 in both Ca2+-depende nt exit from meiosis, associated with degradation of both cyclin B and A do wnstream of CaMK2 activation, and prevention of premature APC(Fizzy) inacti vation in the early mitotic cell cycle, These results are consistent with t he view that Plx1 antagonizes an unidentified microcystin-sensitive phospha tase that inactivates APC(Fizzy).