Cancer Cell http://dx.doi.org/10.1016/j.ccell.2016.02.010 (2016)

Credit: ELSEVIER

The C helix of many protein kinases, including BRAF, has distinct conformational states that are linked to enzyme activity. Inactive kinases have an outward αC conformation; kinase activation shifts αC to an inward orientation, allowing a catalytic bridge to form with the β3 strand. This knowledge has informed the development of kinase inhibitors that specifically target the outward (vemurafenib) or the inward conformation (AZ-628) of the C helix. Recent large-scale sequencing of samples from patients with pancreatic tumors revealed a recurring five-amino-acid (NVTAP) deletion in the BRAF β3-αC loop, but the effects on kinase activity and inhibitor responsiveness were not known. Foster et al. found that cells expressing this shortened loop (BRAFΔNVTAP) exhibited constitutive signaling activity and were sensitive to AZ-628 but resistant to vemurafenib, suggesting that the deletion variant is in the inward conformation. This was verified by structural analysis of BRAFΔNVTAP complexed with AZ-628, which showed that the αC conformation was predominantly in the inward orientation and that steric restraints in the shortened loop prevented a shift to the outward conformation, explaining the inability of αC-out inhibitors to interact with the BRAFΔNVTAP. Interestingly, analogous β3-αC mutations in EGFR and HER2 were also resistant to C-helix-out inhibitors such as lapatinib. These results suggest that future inhibitor discovery should include careful consideration of mutational and conformational status.