WHAT DOES MOS DO IN OOCYTES AND SOMATIC-CELLS

Mos, a protein kinase, is specifically expressed and functions during meiotic maturation (or G(2)/M progression) of vertebrate oocytes. When expressed ectopically, however, it can also readily induce oncogenic transformation (or uncontrolled G(1)/S transitions) in somatic cells. In both of these cell types, Mos activates mitogen-activated protein k inase (MAPK), which seems largely to mediate its different functions i n both oocyte maturation and cellular transformation, In oocyte matura tion, the Mos-MAPK pathway probably serves to activate and stabilize M -phase promoting factor (MPF) (possibly by inhibiting some negative re gulator(s) of this factor), while in cellular transformation, it seems to stabilize and activate the nuclear oncoprotein c-Fos as well as to induce transcription of its gene. Thus, the different functions of Mo s in oocytes and somatic cells may arise chiefly from its different MA PK-mediated targets in the respective cell types. This review discusse s the cellular basis that may enable Mos to act differently in oocytes and somatic cells.