RAF Binding Preferences Dictate RAS-Mediated Tumorigenesis

RAS family proteins, including HRAS, KRAS, and NRAS, are membrane-bound small GTPases that are activated upon receipt of external signals by exchanging GDP to GTP. Activated RAS proteins bind to downstream effectors and initiate cell signaling through these effector cascades, one of which is the ERK cascade comprised of the RAF, MEK and ERK protein kinases. Mutations in RAS genes resulting in a constitutively active RAS are known drivers of tumorigenesis. RAF family proteins, including ARAF, BRAF, and CRAF, contain a RAS-binding domain and instigate the phosphorylation cascade that results in ERK activation by binding to activated RAS. Terrell and colleagues utilized bioluminescence resonance energy transfer, co-immunoprecipitation, and live-cell imaging experiments to investigate the interactions between RAF kinases and RAS members in live cells. They found that CRAF binds to all RAS proteins with the highest affinity, followed by ARAF and BRAF, and that BRAF selectively binds KRAS. By generating different RAS and RAF chimeric proteins, they showed that the N-segment of BRAF, which carries an acidic charge and is larger than the N-segments of ARAF and CRAF by 100 to 150 amino acids, binds to polybasic residues (PBR) in the hypervariable region of KRAS. Following removal of its N-segment, BRAF bound to all RAS proteins with high affinity, suggesting an inhibitory role of this segment. In addition, CRAF was required for HRAS-driven transformation, and its dimerization with BRAF enhanced the interaction between BRAF and HRAS, a non–PBR-containing RAS protein. Collectively, these results demonstrate the potential implications of individual binding between RAS and RAF family members in cell growth and tumorigenesis and the need to design new strategies to target these specific interactions.

Terrell EM, Durrant DE, Ritt DA, Sealover NE, Sheffels E, Spencer-Smith R, et al. Distinct binding preferences between Ras and Raf family members and the impact on oncogenic Ras signaling. Mol Cell 2019 Oct 9 [Epub ahead of print].

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