The mechanism of Raf activation through dimerization

Raf, a threonine/serine kinase in the Raf/MEK/ERK pathway, regulates cell proliferation. Raf's full activation requires dimerization. Aberrant activation through dimerization is an important therapeutic target. Despite its clinical importance, fundamental questions, such as how the side-to-side dimerization promotes the OFF-to-ON transition of Raf's kinase domain and how the fully activated ON-state kinase domain is stabilized in the dimer for Raf signaling, remain unanswered. Herein, we decipher an atomic-level mechanism of Raf activation through dimerization, clarifying this enigma. The mechanism reveals that the replacement of intramolecular pi-pi stacking by intermolecular pi-pi stacking at the dimer interface releases the structural constraint of the alpha C-helix, promoting the OFF-to-ON transition. During the transition, the inhibitory hydrophobic interactions were disrupted, making the phosphorylation sites in A-loop approach the HRD motif for cis-autophosphorylation. Once fully activated, the ON-state kinase domain can be stabilized by a newly identified functional N-terminal basic (NtB) motif in the dimer for Raf signaling. This work provides atomic level insight into critical steps in Raf activation and outlines a new venue for drug discovery against Raf dimerization.

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