MOS
MOS is a germ cell-restricted serine/threonine kinase and a MAP kinase kinase. In oocytes it is synthesized in response to progesterone, activates MEK1 and hence ERK, and forms part of cytostatic factor together with Emi2, maintaining metaphase II arrest until fertilization. Mouse oocytes lacking MOS fail to establish this arrest and show spindle defects, while MOS in testis has been linked to spermatogenesis and to meiotic progression. Historically MOS was one of the first retroviral oncogenes, and its transforming potential has been described in thyroid cells. Xenopus oocyte maturation assays remain the classical functional readout for recombinant MOS protein.
