Structural insights into the BRAF monomer-to-dimer transition mediated by RAS binding

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• RAF kinases are crucial downstream effectors of RAS signaling pathways.
• The precise mechanism by which RAS binding triggers RAF activation from an autoinhibited state has been elusive.
• Understanding RAF kinase activation is vital for comprehending cellular signaling and developing targeted therapies.

Purpose of the Study:

• To elucidate the structural basis of RAF kinase activation by RAS.
• To investigate the role of the RAS binding domain (RBD) in RAF monomer inhibition and dimer formation.
• To characterize the conformational changes involved in RAS-RAF complex assembly.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine high-resolution structures of BRAF complexes.
• Mammalian cell-derived complexes included autoinhibited monomeric BRAF:14-3-3:MEK and BRAF:14-3-3, and an inhibitor-bound dimeric BRAF2:14-3-3.
• Structure-guided mutational analyses were performed to validate functional roles of identified interfaces.

Main Results:

• Cryo-EM structures revealed the autoinhibited monomeric BRAF complex, with the RBD forming a significant interface with 14-3-3 proteins.
• Exposed key basic residues within the RBD suggest readiness for RAS interaction in the monomeric state.
• The study identified a dual function for RBD residues at the RBD:14-3-3 interface, participating in both autoinhibition and RAS binding.

Conclusions:

• RAS binding to RAF is a dynamic process involving conformational rearrangements.
• The RBD is central to both the maintenance of RAF autoinhibition and the subsequent activation by RAS.
• These findings provide critical structural insights into the regulation of RAF kinases and their activation by RAS GTPases.