THE KINASE EG2 IS A COMPONENT OF THE XENOPUS OOCYTE PROGESTERONE-ACTIVATED SIGNALING PATHWAY

Quiescent Xenopus oocytes are activated by progesterone, which binds t o an unidentified surface-associated receptor. Progesterone activates a poorly understood signaling pathway that results in the translationa l activation of mRNA encoding Mos, a MAP kinase kinase kinase necessar y for the activation of MAP kinase and MPF; the resumption of meiosis, and maturation of the oocyte into the sperm-responsive egg. We have d esigned a screen to identify early signaling proteins based on the pre mise that some of these proteins would be phosphorylated or otherwise modified within minutes of progesterone addition. This screen has reve aled Eg2, a Ser/Thr kinase, We find that Eg2 is phosphorylated soon af ter progesterone stimulation and provide evidence that it functions in the signaling pathway. Overexpression of Eg2 via mRNA microinjection shortens the time between progesterone stimulation and the appearance of new Mos protein, accelerates activation of MAP kinase and advances entry into the meiotic cell cycle. Finally, overexpression of Eg2 dram atically reduces the concentration of progesterone needed do trigger o ocyte activation. These results argue that the kinase Eg2 is a compone nt of the progesterone-activated signaling pathway that releases frog oocytes from cell cycle arrest.