Fig. 3: The open monomer state is poised for activation.

The comparison of the autoinhibited (a), open monomer (b), and active dimer (c) states of the CRAF/MEK1/14-3-3 complex. Structures are shown in a surface representation and with the same orientation of the CRAF and MEK1 kinase domains. Residues of the CRAF kinase domain that form the dimer interface in active CRAF dimers is shaded violet in each structure. a In the autoinhibited state, the 14-3-3 blocks the CRAF dimer interface and also sequesters the CRD (purple). b In the open monomer state, the pSer259 segment and the CRD are released from the 14-3-3 and are therefore not visible in the cryo-EM maps. The pSer621 site remains engaged with the 14-3-3 dimer, which has reoriented to expose the CRAF dimer interface. c In the active, dimeric state, the 14-3-3 dimer engages the pSer621 sites of two CRAFs, stabilizing the activating back-to-back interaction of the CRAF kinase domains. The active dimer is drawn from a previously reported cryo-EM structure (PDB ID: 8CHF) that was obtained with a truncated CRAF construct lacking the RBD, CRD and pSer259 regions.