MAP KINASE-ACTIVITY INCREASES DURING MITOSIS IN EARLY SEA-URCHIN EMBRYOS

A MBP kinase activity increases at mitosis during the first two embryo nic cell cycles of the sea urchin embryo. The activity profile of the MBP kinase is the same both in whole cell extracts and after immunopre cipitation with an anti-MAP kinase antibody (2199). An in-gel assay of MBP activity also shows the same activity profile. The activity is as sociated with the 44 kDa protein that cross-reacts with anti-MAP kinas e antibodies. The 44 kDa protein shows cross-reactivity to anti-phosph otyrosine and MAP kinase-directed anti-phosphotyrosine/phosphothreonin e antibodies at the times that MBP kinase activity is high. The 2199 a ntibody co-precipitates some histone H1 kinase activity, but the MBP k inase 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 s hows the protein to be homologous to MAP kinases from other species. T hese data demonstrate that MAP kinase activation during nuclear divisi on is not confined to meiosis, but also occurs during mitotic cell cyc les. MAP kinase activity in immunoprecipitates also increases immediat ely after fertilization, which in the sea urchin egg occurs at interph ase of the cell cycle. Treating unfertilized eggs with the calcium ion ophore A23187 stimulates the increase in MAP kinase activity, demonstr ating that a calcium signal can activate MAP kinase and suggesting tha t the activation of MAP kinase at fertilization is due to the fertiliz ation-induced increase in cytoplasmic free calcium concentration. This signalling pathway must differ from the pathway responsible for calci um-induced inactivation of MAP kinase activity that is found in eggs t hat are fertilized in meiotic metaphase.