RELEASE FROM THE METAPHASE-I BLOCK IN INVERTEBRATE OOCYTES - POSSIBLEINVOLVEMENT OF CA2+ CALMODULIN-DEPENDENT KINASE-III/

Full grown mature oocytes of the prosobranch gastropod mollusc Patella or the bivalves Mytilus or Ruditapes provide an excellent model for s tudying the mechanisms which trigger cyclin degradation and exit from the M phase. They are naturally arrested in metaphase of the first mat uration division and their fertilization or artificial activation rapi dly results in destruction of the cyclins and completion of meiosis. I n this paper, we establish the presence of Ca2+/calmodulin-dependent k inase III or eEF-2 kinase in these oocytes and describe how the protei n synthesis inhibitor emetine is able to release them from the metapha se block. Using the fluorescent Ca2+ indicator dye, fluo-3, we demonst rate moreover that both fertilization or KCl-dependent activation of R uditapes and Mytilus oocytes actually trigger a measurable transient i ncrease in cytosolic free Ca2+ concentration. We also show that the ac tivations triggered by these signals as well as by the ionophore A 231 87 can be reversibly blocked by the calmodulin antagonists TFP (30 muM ) and W7 (100 muM), while these drugs have no effect upon emetine-depe ndent activations. Finally, we report that the rate of protein synthes is, measured in pulse experiments, decreases at each meiotic and mitot ic cleavage following fertilization of metaphase I-arrested oocytes of Mytilus. On the basis of these experiments and as a working hypothesi s, we thus propose that the Ca2+ surge which activates the oocyte may inhibit protein synthesis by triggering a transient phosphorylation of eEF-2. This would result in disappearance of the putative short-lived proteins which protect cyclins from degradation during the metaphase block.