Abstract
The Ras-Raf-MEK signaling cascade is critical for normal development and is activated in many forms of cancer. We have recently shown that B-Raf kinase interacts with and is inhibited by Rheb, the target of the GTPase-activating domain of the tuberous sclerosis complex 2 gene product tuberin. Here, we demonstrate for the first time that activation of Rheb is associated with decreased B-Raf and C-Raf phosphorylation at residues Ser-446 and Ser-338, respectively, concomitant with a decrease in the activities of both kinases and decreased heterodimerization of B-Raf and C-Raf. Importantly, the impact of Rheb on B-Raf/C-Raf heterodimerization and kinase activity are rapamycin-insensitive, indicating that they are independent of Rheb activation of the mammalian target of rapamycin-Raptor complex. In addition, we found that Rheb inhibits the association of B-Raf with H-Ras. Taken together, these results support a central role of Rheb in the regulation of the Ras/B-Raf/C-Raf/MEK signaling network.
Highlights
We have recently shown that B-Raf kinase interacts with and is inhibited by Rheb, the target of the GTPase-activating domain of the tuberous sclerosis complex 2 gene product tuberin
B-Raf, A-Raf, and C-Raf are members of the Raf kinase family, which regulate the activation of the MEK2 (MAPK/ERK-activating kinase)/MAPK signaling cascade and share Ras as a common activator
Activation of Rheb results in activation of the mammalian target of rapamycin (14 –19, 20), resulting in translational initiation and cell growth
Summary
B-Raf, A-Raf, and C-Raf ( called Raf-1) are members of the Raf kinase family, which regulate the activation of the MEK2 (MAPK/ERK-activating kinase)/MAPK signaling cascade and share Ras as a common activator. Rapamycin treatment did not impact the magnitude of Rheb inhibition of C-Raf Ser-338 phosphorylation (Fig. 1A), indicating that the inhibition is mTOR-Raptor- independent.
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