Discrete cytosolic macromolecular BRAF complexes exhibit distinct activities and composition

Area of Science:

• Molecular Biology
• Cell Signaling
• Cancer Research

Background:

• The serine/threonine kinase BRAF is crucial in the RAS/ERK pathway, vital for cellular functions.
• BRAF mutations are frequent in cancers, making it a key therapeutic target.
• The precise mechanisms of BRAF activation and inhibitor action remain incompletely understood.

Purpose of the Study:

• To investigate the protein-protein interactions and complex formation of wild-type BRAF (BRAF^WT) and the oncogenic BRAF^V600E mutant.
• To elucidate how cellular context, such as oncogenic Ras expression, and pharmacological interventions influence BRAF complex quaternary structure and activity.

Main Methods:

• Utilized Size Exclusion Chromatography coupled with Protein Complex Profiling and Stable Isotope Labeling by Amino acid in Cell culture (SEC-PCP-SILAC).
• Analyzed protein-protein interactions and molecular composition of BRAF complexes in different cellular conditions.

Main Results:

• Identified two distinct macromolecular cytosolic BRAF complexes with varying molecular composition and phosphorylation status.
• Found that hyperactive BRAF^V600E forms larger, more active complexes than BRAF^WT.
• Demonstrated that oncogenic K-Ras^G12V expression or RAF dimer-promoting inhibitors promote BRAF^WT incorporation into large, active complexes, while BRAF^V600E inhibitors reduce complex size.

Conclusions:

• The quaternary structure of BRAF complexes is dynamically regulated by its activation state (mutant vs. wild-type) and kinase domain conformation.
• Cellular factors like oncogenic Ras and therapeutic agents significantly influence BRAF complex assembly and activity.
• Understanding BRAF complex dynamics is essential for deciphering RAF inhibitor mechanisms and developing more effective cancer therapies.