The ability of neurotrophins to modulate the survival and differentiation of neuronal populations involves the Trk/MAP (mitogen-activated protein kinase) kinase signaling pathway. More

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Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/ threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44 pk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44 Pk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation ofXenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of cultured mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERKI) and peptide sequences from sea star p44"!Ik indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the GI stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.Progesterone induces Xenopus laevis oocytes to maturethat is, to leave first meiotic prophase and progress part way through meiosis and arrest as eggs, in second meiotic metaphase. A burst in the phosphorylation of numerous proteins is triggered near the time of nuclear envelope (germinal vesicle) breakdown (GVBD) (46). Similarly, with sea star oocytes, 1-methyladenine induces both maturation and protein phosphorylation (19). Only a few of the phosphoproteins in maturing oocytes have been identified, such as the 40S ribosomal protein S6 (33,38,39,51), nucleoplasmin (17), nuclear lamins (48), and possibly histone Hi. Using these and other proteins as substrates, several maturation-activated protein-serine/threonine kinases have been characterized from sea star oocytes (6, 42, 52, 55) and Xenopus oocytes (12, 44, 47; reviewed in reference 53).The major histone Hi kinases that are activated during Xenopus (20, 28) and sea star (6, 42) oocyte maturation are the amphibian and echinoderm homologs of the 34-kDa protein kinase encoded by the Schizosaccharomyces pombe mitotic control gene cdc2. Microinjection of active preparations of histone Hi kinase into immature oocytes is sufficient to induce GVBD (44). Two ribosomal S6 kinases (S6K-I and S6K-II) are also activated at GVBD in progesterone-treated Xenopus oocytes (12,44,47) or following microinjection of the frog homolog of p34cdc2 (68). Both S6 kinases appear to be activated as a consequ...