THE 96 KDA PROTEIN-KINASE ACTIVATED BY ONCOGENIC RAS IN XENOPUS EGG EXTRACTS IS ALSO ACTIVATED BY CONSTITUTIVELY ACTIVE MEK - ACTIVATION REQUIRES SERINE THREONINE PHOSPHORYLATION/

In the Xenopus egg and oocyte system, oncogenic Ras protein can induce cell cycle arrest. The effect of oncogenic Ras on the cell cycle seem s to be mediated by the Raf-Mek-Erk pathway of Ras signal transduction since constitutively active Raf, Mek, or Erk can mimic the effect of oncogenic Ras protein and since specific inhibition of these kinases c an block the effect of oncogenic Ras. Using activated Xenopus egg extr acts, we previously reported that the cell cycle arrest induced by onc ogenic Ras correlates with the activation of a 96 kDa protein that pho sphorylates histone h2b in vitro. This result raised the possibility t hat the 96 kDa kinase (designated as p96(h2bk)) is a potential target of the Raf-Mek-Erk pathway that links the pathway to the control of th e cell cycle. We report here that constitutively active Mek1 could act ivate p96(h2bk) in the absence of oncogenic Ras. Moreover, inhibition of endogenous Mek by a specific inhibitor, PD 098059, suppressed the a ctivation of p96(h2bk) by oncogenic Ras. These results are consistent with the concept that p96(h2bk) is a component or target of the Raf-Me k-Erk pathway. Furthermore, we have shown that activation of p96(h2bk) requires serine/threonine phosphorylation of p96(h2bk).