mos proto-oncogene function.

Maturation promoting factor (MPF) is a cytoplasmic activity that causes oocytes arrested in prophase to resume meiosis. An inactive form of MPF termed pre-MPF exists in fully grown oocytes. In Xenopus oocytes, progesterone induces maturation and pre-MPF activation. These early maturation events require protein synthesis. We have shown that p39mos synthesis is rapidly induced in progesterone-treated Xenopus oocytes during the protein synthesis sensitive period and prior to activation of pre-MPF or germinal vesicle breakdown (GVBD). p39mos may qualify, therefore, as an 'initiator' of maturation. Mouse oocytes undergoing meiotic maturation also express p39mos. Microinjection of antisense mos oligodeoxynucleotides into fully grown mouse and Xenopus oocytes results in the block of meiotic maturation. In Xenopus, antisense-injected oocytes not only lack p39mos, but also lack MPF and fail to undergo GVBD. In the mouse, the microinjected oocytes progress through GVBD, but fail to produce the first polar body; cytogenetic analysis shows they are arrested at the bivalent chromosome stage of metaphase I. This and additional studies with Xenopus oocytes indicate that p39mos is also required throughout maturation. We have shown that p39mos is indistinguishable from the protein product constitutively expressed in NIH/3T3 cells transformed with activated c-mos. It is likely that its activity as a transforming gene may be due to activation of pre-MPF activities in interphase in the somatic cell cycle.