What does Mos do in oocytes and somatic cells?

Mos, a protein kinase, is specifically expressed and functions during meiotic maturation (or G2/M progression) of vertebrate oocytes. When expressed ectopically, however, it can also readily induce oncogenic transformation (or uncontrolled G1/S transitions) in somatic cells. In both of these cell types, Mos activates mitogen-activated protein kinase (MAPK), which seems largely to mediate its different functions in both oocyte maturation and cellular transformation. In oocyte maturation, the Mos-MAPK pathway probably serves to activate and stabilize M-phase promoting factor (MPF) (possibly by inhibiting some negative regulator(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 Mos in oocytes and somatic cells may arise chiefly from its different MAPK-mediated targets in the respective cell types. This review discusses the cellular basis that may enable Mos to act differently in oocytes and somatic cells.