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Updated: Jul 16, 2026
Xenopus laevis as a Model to Identify Translation Impairment
Published on: September 27, 2015
B-Raf and C-Raf are required for Ras-stimulated p42 MAP kinase activation in Xenopus egg extracts
1Department of Molecular Pharmacology, Stanford University, CA 94305-5174, USA. jyue@stanford.edu
Abstract:
During mitosis, a select pool of MEK1 and p42/p44 MAPK becomes activated at the kinetochores and spindle poles, without substantial activation of the bulk of the cytoplasmic p42/p44 MAPK. Recently, we set out to identify the MAP kinase kinase kinase (MAPKKK) responsible for this mitotic activation, using cyclin-treated Xenopus egg extracts as a model system, and presented evidence that Mos was the relevant MAPKKK . However, a second MAPKKK distinct from Mos was readily detectable as well. Here, we partially purify this second MAPKKK and identify it as B-Raf. No changes in the activity of B-Raf were detectable during progesterone-induced oocyte maturation, after egg fertilization, or during the early embryonic cell cycle, arguing against a role for B-Raf in the mitotic activation of MEK1 and p42 MAPK. Ras proteins can bring about activation of MEK1 and p42 MAPK in extracts, and Ras may contribute to signaling from the classical progesterone receptor during oocyte maturation and from receptor tyrosine kinases during early embryogenesis. We found that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42 MAPK activation. These data indicate that two upstream stimuli, active Ras and active Cdc2, utilize different MAPKKKs to activate MEK1 and p42 MAPK.
Insights
Two distinct MAP kinase kinase kinases (MAPKKKs) activate MEK1 and p42/p44 MAPK during cell division. While Mos is involved, B-Raf and C-Raf are crucial for Ras-induced signaling, highlighting parallel activation pathways.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Mitosis
Background:
- Mitosis involves localized activation of MEK1 and p42/p44 MAPK at kinetochores and spindle poles.
- Previous studies identified Mos as a key MAP kinase kinase kinase (MAPKKK) in this process.
Purpose of the Study:
- To identify the MAPKKK responsible for mitotic MEK1 and p42/p44 MAPK activation.
- To investigate the role of B-Raf in mitotic signaling pathways.
Main Methods:
- Utilized cyclin-treated Xenopus egg extracts as a model system.
- Partially purified and identified a second MAPKKK as B-Raf.
- Assessed B-Raf activity during oocyte maturation, fertilization, and early embryogenesis.
Main Results:
- Identified B-Raf as a second MAPKKK, but its activity did not change during key cell cycle events.
- Demonstrated that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42/p44 MAPK activation.
- Showed that active Ras and active Cdc2 utilize distinct MAPKKKs for MEK1 and p42/p44 MAPK activation.
Conclusions:
- B-Raf does not appear to mediate the specific mitotic activation of MEK1 and p42/p44 MAPK.
- Ras signaling pathways rely on B-Raf and C-Raf for MEK1 and p42/p44 MAPK activation.
- Two parallel signaling pathways, one involving Cdc2 and another involving Ras, activate MEK1 and p42/p44 MAPK through different MAPKKKs.
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