Related Experiment Videos
Activation of brain B-Raf protein kinase by Rap1B small GTP-binding protein
T Ohtsuka1, K Shimizu, B Yamamori
1Department of Molecular Biology and Biochemistry, Osaka University Medical School, Suita, Japan.
Abstract:
Rap1 small GTP-binding protein has the same amino acid sequence at its effector domain as that of Ras. Rap1 has been shown to antagonize the Ras functions, such as the Ras-induced transformation of NIH 3T3 cells and the Ras-induced activation of the c-Raf-1 protein kinase-dependent mitogen-activated protein (MAP) kinase cascade in Rat-1 cells, whereas we have shown that Rap1 as well as Ras stimulates DNA synthesis in Swiss 3T3 cells. We have established a cell-free assay system in which Ras activates bovine brain B-Raf protein kinase. Here we have used this assay system and examined the effect of Rap1 on the B-Raf activity to phosphorylate recombinant MAP kinase kinase (MEK). Recombinant Rap1B stimulated the activity of B-Raf, which was partially purified from bovine brain and immunoprecipitated by an anti-B-Raf antibody. The GTP-bound form was active, but the GDP-bound form was inactive. The fully post-translationally lipid-modified form was active, but the unmodified form was nearly inactive. The maximum B-Raf activity stimulated by Rap1B was nearly the same as that stimulated by Ki-Ras. Rap1B enhanced the Ki-Ras-stimulated B-Raf activity in an additive manner. These results indicate that not only Ras but also Rap1 is involved in the activation of the B-Raf-dependent MAP kinase cascade.
Insights
Rap1, similar to Ras, activates the B-Raf kinase and MAP kinase cascade. This suggests Rap1 also plays a key role in cell signaling pathways, impacting DNA synthesis.
Area of Science:
- Cellular signaling pathways
- Protein kinase activation
- Molecular biology
Background:
- Ras and Rap1 are small GTP-binding proteins with similar effector domains.
- Rap1 can antagonize Ras functions but also stimulates DNA synthesis.
- Ras is known to activate B-Raf protein kinase.
Purpose of the Study:
- To investigate the effect of Rap1 on B-Raf activity in a cell-free system.
- To determine if Rap1, like Ras, can activate the MAP kinase cascade.
- To elucidate the role of Rap1 in cellular signaling.
Main Methods:
- Established a cell-free assay system for Ras-mediated B-Raf activation.
- Examined the effect of recombinant Rap1B on B-Raf activity.
- Used partially purified and immunoprecipitated B-Raf from bovine brain.
- Assessed the phosphorylation of recombinant MAP kinase kinase (MEK) by B-Raf.
Main Results:
- Recombinant Rap1B stimulated B-Raf activity, leading to MEK phosphorylation.
- The GTP-bound and lipid-modified forms of Rap1B were active, while GDP-bound and unmodified forms were inactive.
- Rap1B-stimulated B-Raf activity was comparable to Ki-Ras.
- Rap1B showed additive effects with Ki-Ras on B-Raf activity.
Conclusions:
- Rap1, in addition to Ras, is involved in activating the B-Raf-dependent MAP kinase cascade.
- Rap1's activation of B-Raf suggests a broader role in cellular signaling than previously understood.
- These findings highlight the complex interplay between Ras and Rap1 in regulating downstream signaling pathways.