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Mitogen-activated protein kinase (MAP kinase) activation in Xenopus oocytes: roles of MPF and protein synthesis
Abstract:
Mitogen-activated protein kinase (MAP kinase) is a serine/threonine kinase whose enzymatic activity is thought to play a crucial role in mitogenic signal transduction and also in the progesterone-induced meiotic maturation of Xenopus oocytes. We have purified MAP kinase from Xenopus oocytes and have shown that the protein is present in metaphase II oocytes under two different forms: an inactive 41-kD protein able to autoactivate and to autophosphorylate in vitro, and an active 42-kD kinase resolved into two tyrosine phosphorylated isoforms on 2D gels. During meiotic maturation, MAP kinase becomes tyrosine phosphorylated and activated following the activation of the M-phase promoting factor (MPF), a complex between the p34cdc2 kinase and cyclin B. In vivo, MAP kinase activity displays a different stability in metaphase I and in metaphase II: protein synthesis is required to maintain MAP kinase activity in metaphase I but not in metaphase II oocytes. Injection of either MPF or cyclin B into prophase oocytes promotes tyrosine phosphorylation of MAP kinase, indicating that its activation is a downstream event of MPF activation. In contrast, injection of okadaic acid, which induces in vivo MPF activation, promotes only a very weak tyrosine phosphorylation of MAP kinase, suggesting that effectors other than MPF are required for the MAP kinase activation. Moreover, in the absence of protein synthesis, cyclin B and MPF are unable to promote in vivo activation of MAP kinase, indicating that this activation requires the synthesis of new protein(s).
Insights
Mitogen-activated protein kinase (MAP kinase) is crucial for cell signaling and oocyte maturation. Its activation in Xenopus oocytes is a downstream event of M-phase promoting factor (MPF) and requires new protein synthesis.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- Mitogen-activated protein kinase (MAPK) is a key enzyme in cell signaling pathways.
- MAPK activity is essential for progesterone-induced meiotic maturation in Xenopus oocytes.
Purpose of the Study:
- To investigate the role and regulation of MAP kinase during meiotic maturation in Xenopus oocytes.
- To elucidate the relationship between MAP kinase activation and M-phase promoting factor (MPF).
Main Methods:
- Purification of MAP kinase from Xenopus oocytes.
- Analysis of MAP kinase forms and phosphorylation states using 2D gel electrophoresis.
- In vivo experiments involving injection of MPF, cyclin B, and okadaic acid into oocytes.
- Assessment of protein synthesis requirements for MAP kinase activation.
Main Results:
- MAP kinase exists in inactive (41-kD) and active (42-kD) forms in metaphase II oocytes.
- MAP kinase activation and tyrosine phosphorylation occur downstream of MPF activation during meiotic maturation.
- MAP kinase activity is stable in metaphase II but requires protein synthesis for maintenance in metaphase I.
- MPF and cyclin B injections promote MAP kinase tyrosine phosphorylation, but okadaic acid shows a weaker effect, suggesting other effectors are involved.
- Protein synthesis is necessary for MPF- and cyclin B-induced MAP kinase activation in vivo.
Conclusions:
- MAP kinase activation is a critical downstream event regulated by MPF during Xenopus oocyte meiosis.
- The activation and stability of MAP kinase are dependent on MPF and protein synthesis.
- Additional signaling components beyond MPF are likely involved in the full activation of MAP kinase.