MAP kinase activity increases during mitosis in early sea urchin embryos.

A MBP kinase activity increases at mitosis during the first two embryonic cell cycles of the sea urchin embryo. The activity profile of the MBP kinase is the same both in whole cell extracts and after immunoprecipitation with an anti-MAP kinase antibody (2199). An in-gel assay of MBP activity also shows the same activity profile. The activity is associated with the 44 kDa protein that cross-reacts with anti-MAP kinase antibodies. The 44 kDa protein shows cross-reactivity to anti-phosphotyrosine and MAP kinase-directed anti-phosphotyrosine/phosphothreonine antibodies at the times that MBP kinase activity is high. The 2199 antibody co-precipitates some histone H1 kinase activity, but the MBP kinase activity cannot be accounted for by histone H1 kinase-dependent phosphorylation of MBP. The MAP kinase 2199 antibody was used to purify the MBP kinase activity. Peptide sequencing after partial digestion shows the protein to be homologous to MAP kinases from other species. These data demonstrate that MAP kinase activation during nuclear division is not confined to meiosis, but also occurs during mitotic cell cycles. MAP kinase activity in immunoprecipitates also increases immediately after fertilization, which in the sea urchin egg occurs at interphase of the cell cycle. Treating unfertilized eggs with the calcium ionophore A23187 stimulates the increase in MAP kinase activity, demonstrating that a calcium signal can activate MAP kinase and suggesting that the activation of MAP kinase at fertilization is due to the fertilization-induced increase in cytoplasmic free calcium concentration. This signalling pathway must differ from the pathway responsible for calcium-induced inactivation of MAP kinase activity that is found in eggs that are fertilized in meiotic metaphase.