Modulation in protein phosphorylation during G2 phase of the cell cycle in two-cell mouse embryos

The protein phosphorylation activities in extracts were assayed for 2-cell mouse embryos at three stages of the G2 phase of the cell cycle. The 2-cell embryos were unique in having a prolonged G2 phase and so easily staged at early G2 (EG2), middle G2 (MG2) and late G2 (LG2) by timing the embryo isolation from pregnant mice. The embryo extracts were used both as sources of protein kinases and their substrates. The phosphoproteins of the extracts were labelled with [gamma-32P]ATP and separated by electrophoresis on SDS-polyacrylamide gels. The present study revealed that protein phosphorylation increased 3-6-fold during the progression of 2-cell embryos from EG2 to LG2 and the level of protein phosphorylation at any stages was greatly decreased by the presence of cAMP. Thus, the protein phosphorylation system of 2-cell mouse embryos seems to differ from those reported systems in mammals in its negative dependence on cAMP.