THE SIGNAL CASCADE FOR THE ACTIVATION OF PROTEIN-SYNTHESIS DURING THEMATURATION OF STARFISH OOCYTES - A ROLE FOR PROTEIN-KINASE-C AND HOMOLOGIES WITH MATURATION IN XENOPUS AND MAMMALIAN OOCYTES

The goal of a growing oocyte is to prepare the female gamete for the r apid cleavages that follow fertilization. The oocyte stores proteins a nd mRNAs required for meiosis and mitosis, since little or no transcri ption of new mRNAs occurs during the early cleavage stages of most ani mal embryos. Immature oocytes of mammals, amphibians, and starfish are arrested in the prophase of meiosis I. Meiotic maturation of these oo cytes is induced by a maturation hormone that reinitiates meiosis, com pletes the nuclear reduction of meiosis I and meiosis II, prepares the oocytes for activation by the male gametes, and activates stores of p roteins and mRNAs needed for the early cleavage stages. Maturation hor mones induce at least three distinct and parallel molecular pathways i n animal oocytes: the activation of the maturation promotion factor (M PF); the activation and polyadenylation of mRNAs stored during oogenes is; the activation of the protein synthesis machinery of the oocyte, w hich is required to translate the newly activated mRNAs. We review her e (1) biological changes that occur during oocyte maturation, (2) sign al pathways that stimulate and coordinate maturation in starfish, Xeno pus, and mammalian oocytes, (3) components of the protein synthesis ma chinery that are activated during oocyte maturation, including (4) the phosphorylation of mRNA cap binding protein, eukaryotic initiation fa ctor 4 (eIF-4E), and (5) a potential role for protein kinase C in phos phorylating eIF-4E and activating MPF in starfish oocytes. Finally, we propose that the modulation of a trimeric G-protein leads to the obse rved decrease in cyclic adenosine monophosphate (cAMP) levels and the subsequent activation of a pivotal serine/threonine kinase that activa tes MPF, stored mRNAs, and the protein synthesis machinery.