Quantitative Membrane Binding Assays Reveal an Inhibitory Role for the BRAF-Specific Region in CRD and Lipid Interaction

BRAF is a serine/threonine kinase and a central effector of the mitogen-activated protein kinase (MAPK) signaling pathway, frequently mutated in cancer. Its activation is tightly controlled by autoinhibitory mechanisms that regulate membrane recruitment and dimerization. The BRAF-specific region (BSR), located at the N-terminus, is known to promote isoform-preferred RAS binding and facilitate dimerization with kinase suppressor of RAS (KSR), yet its role in regulating lipid interaction has remained unexplored. Here, we identify the BSR as a previously unrecognized inhibitory module that attenuates lipid binding by the cysteine-rich domain (CRD). Using quantitative in vitro reconstitution with supported lipid bilayers and fluorescence microscopy, we demonstrate that the BRAF CRD exhibits high intrinsic affinity for phosphatidylserine-rich membranes, but the inclusion of the BSR markedly reduces the membrane binding. We further demonstrate that the inhibitory function of the BSR correlates with its global electrostatic properties rather than a single defined sequence motif. This inhibitory effect of BSR was corroborated in live cells by quantifying plasma membrane localization of BRAF constructs, including the full-length protein. When canonical autoinhibition of CRD—mediated by sequestration within the 14-3-3 dimer—is disrupted by oncogenic mutation or RAF inhibitor treatment, the BSR assumes a compensatory role in repressing CRD–lipid interaction. This additional regulatory layer provided by the BSR prevents RAS-independent membrane recruitment under both physiological and pathological conditions.