Inhibition of c-mos protein kinase blocks mouse zygotes at the pronuclei stage.

The c-mos gene product is required for activation of the maturation-promoting factor (MPF) during oocyte maturation. The c-mos protein also acts as a cytostatic factor which is responsible for meiotic metaphase arrest of vertebrate eggs via stabilization of MPF. Here we show that mouse zygotes contain the c-mos protein. Introduction of a kinase-inhibitory anti-mos antibody into mouse zygotes 12 h after fertilization prevented the first cleavage of zygotes at the pronuclei stage. A second anti-mos antibody, known to allow the mos kinase to function, did not interfere with the formation of two-cell embryos. In addition to its known role in MPF activation in oocyte maturation and meiotic metaphase arrest, our findings indicate that the c-mos protein kinase is also required for completion of pronuclei breakdown following fertilization of mouse eggs.