Mechanism of MEK1 phosphorylation by the N-terminal acidic motif-mediated asymmetric BRAF dimer.

The rapidly accelerated fibrosarcoma/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (RAF/MEK/ERK) signaling cascade regulates cell proliferation and differentiation and is frequently dysregulated in cancer. Approximately 90% of RAF-mutant cancers harbor mutations in B-type rapidly accelerated fibrosarcoma (BRAF). Its proto-oncogenicity is attributed to a four-residue N-terminal acidic (NtA) motif. Although a long-standing model proposes that the NtA promotes activating asymmetric RAF dimerization, the model lacks structural support. Here, we present structures of NtA-mediated asymmetric BRAF dimers bound to their substrate MEK1. Cellular and biochemical data show that the NtA is not required for KRAS-mediated BRAF recruitment to the plasma membrane but is required for the fully catalytically active state. The structure capturing BRAF in a post-catalytic state bound to Ser222-phosphorylated MEK1 further supports this model. The combination of structural and cellular data corroborates the model of NtA-driven asymmetry in BRAF activation and resolves a long-standing disconnect between RAF cancer genetics and structural biology.