MSK1 activation in WT ES cells. WT ES cells were serum-starved for 8 h and incubated with 25 μM PD98059 during the last hour of starvation where indicated. Cells were stimulated with 15% FCS or 20 μM LPA for 10 min and MSK1 activity was determined by immunoprecipitation kinase assays. Error bars represent S.E.M. of four points.