Architecture of autoinhibited and active BRAF-MEK1-14-3-3 complexes


Eun Young Park, Shaun Rawson, Kunhua Li, Byeong-Won Kim, Scott B. Ficarro, Gonzalo Gonzalez-Del Pino, Humayun Sharif, Jarrod A. Marto, Hyesung Jeon, and Michael J. Eck. 2019. “Architecture of autoinhibited and active BRAF-MEK1-14-3-3 complexes.” Nature.


RAF family kinases are RAS-activated switches that initiate signaling through the MAP kinase cascade to control cellular proliferation, differentiation and survival1-3. RAF activity is tightly regulated, and inappropriate activation is a frequent cause of cancer4-6. At present, the structural basis of RAF regulation is poorly understood. Here we describe autoinhibited and active state structures of full-length BRAF in complexes with MEK1 and a 14-3-3 dimer, determined using cryo-electron microscopy (cryo-EM). A 4.1?\AA resolution cryo-EM reconstruction reveals an inactive BRAF-MEK1 complex restrained in a cradle formed by the 14-3-3 dimer, which binds the phosphorylated S365 and S729 sites that flank the BRAF kinase domain. The BRAF cysteine-rich domain (CRD) occupies a central position that stabilizes this assembly, but the adjacent RAS-binding domain (RBD) is poorly ordered and peripheral. The 14-3-3 cradle maintains autoinhibition by sequestering the membrane-binding CRD and blocking dimerization of the BRAF kinase domain. In the active state, these inhibitory interactions are released and a single 14-3-3 dimer rearranges to bridge the C-terminal pS729 binding sites of two BRAFs, driving formation of an active, back-to-back BRAF dimer. Our structural snapshots provide a foundation for understanding normal RAF regulation and its mutational disruption in cancer and developmental syndromes.