MOS INDUCES THE IN-VITRO ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASES IN LYSATES OF FROG OOCYTES AND MAMMALIAN SOMATIC-CELLS

Mitogen-activated protein kinases (MAPKs) are rapidly and transiently activated when both quiescent Go-arrested cells and G2-arrested oocyte s are stimulated to reenter the cell cycle. We previously developed a cell-free system from lysates of quiescent Xenopus oocytes that respon ds to oncogenic H-ras protein by activating a MAPK, p42MAPK. Here, we show that the oncogenic protein kinase mos is also a potent activator of p42MAPK in these lysates. Mos also induces p42MAPK activation in ly sates of activated eggs taken at a time when neither mos nor p42MAPK i s normally active, showing that the mos-responsive MAPK activation pat hway persists beyond the stage where mos normally functions. Similarly , lysates of somatic cells (rabbit reticulocytes) also retain a mos-in ducible MAPK activation pathway. The mos-induced activation of MAPKs i n all three lysates leads to phosphorylation of the pp90rsk proteins, downstream targets of the MAPK signaling pathway in vivo. The in vitro activation of MAPKs by mos in cell-free systems derived from oocytes and somatic cells suggests that mos contributes to oncogenic transform ation by inappropriately inducing the activation of MAPKs.