Crystal structure reveals the full Ras:Raf interface and advances mechanistic understanding of Raf activation

The interaction between Ras and Raf-kinase through the Ras-binding (RBD) and cysteine-rich domains (CRD) of Raf is essential for signaling through the mitogen-activated protein kinase (MAPK) pathway, yet the molecular mechanism leading to Raf activation has remained elusive. We present the 2.8 [A] crystal structure of the HRas/CRaf-RBD_CRD complex showing the Ras/Raf interface as a continuous surface on Ras. In the Ras dimer, with helices roughly perpendicular to the membrane, the CRD is located between the two Ras protomers and far from the membrane, where its dynamic nature in the Ras binding pocket is expected to accommodate BRaf and CRaf heterodimers. Our structure and its analysis by MD simulations, combined with work in the literature, result in a molecular model in which Ras binding is involved in the release of Raf autoinhibition while the Ras/Raf complex dimerizes to promote a platform for signal amplification, with Raf-CRD poised to have direct and allosteric effects on both the Ras active site and the dimerization interface.