The product of the mos proto-oncogene as a candidate "initiator" for oocyte maturation.

The endogenous c-mos product, pp39mos, is required for progesterone-induced meiotic maturation in Xenopus oocytes. Treatment of oocytes with progesterone induced a rapid increase in pp39mos that preceded both the activation of maturation promoting factor (MPF) and germinal vesicle breakdown (GVBD). Microinjection of synthetic mos RNA into oocytes activated MPF and induced GVBD in the absence of progesterone. Thus, the mos proto-oncogene product may qualify as a candidate "initiator" protein of MPF and is at least one of the "triggers" for G2 to M transition.