We have shown previously that oncogenic Ras induces cell cycle arrest in activated Xenopus egg extracts [Pan, Chen and Lin (1994) J. Biol. Chem. 269, 5968-5975]. The cell cycle arrest correlates with the stimulation of a protein kinase activity that phosphorylates histone H2b in vitro (designated p96(h2bk)) [Chen and Pan (1994) J. Biol. Chem. 269, 28034-28043]. We report here that p96(h2bk) is likely to be p96(ram), a protein of approx. 96 kDa that immunoreacts with a monoclonal antibody (Mk-1) raised against a synthetic peptide derived from a sequence highly conserved in Erk1/Erk2 (where Erk is extracellular-signal-regulated kinase). This is supported by two lines of evidence. First, activation/inactivation of p96(h2bk) correlates with upward/downward bandshifts of p96(ram) in polyacrylamide gels. Secondly, both p96(h2bk) and p96(ram) can be immunoprecipitated by antibody Mk-1. We also studied the activity of p96(h2bk)/p96(ram) in Xenopus oocytes and eggs. p96(h2bk)/p96(ram) was inactive in stage 6 oocytes, was active in unfertilized eggs, and became inactive again in eggs after fertilization. Since stage 6 oocytes are at G2-phase of the cell cycle, unfertilized eggs arrest at M-phase and eggs exit M-phase arrest after fertilization, the results thus indicate that p96(h2bk)/p96(ram) activity is cell cycle dependent. Moreover, microinjection of oncogenic Ras into fertilized eggs at the one-cell stage arrests the embryos at the two-cell stage, and this induced arrest is correlated with an inappropriate activation of p96(h2bk)/p96(ram). The data are consistent with the concept that inappropriate activation of p96(h2bk)/p96(ram) plays a role in the cell cycle arrest induced by oncogenic Ras.
