Abstract
Reversible protein phosphorylation directs essential cellular processes including cell-division, cell-growth, cell-death, inflammation, and differentiation. Because of this, small-molecule-competitive kinase inhibitors have achieved remarkable clinical success, often achieving target specificity by binding to inactive kinase conformations (type II inhibitors). These inactive conformations often have displaced conformation of the kinase activation-loop, which is itself a hub for signaling control, dictating kinase activity towards target proteins. We hypothesized that the protein phosphatases that are the cells natural kinase inhibitors may similarly recognize distinct activation-loop conformations. Using a set of type II inhibitors targeting p38α MAP kinase, we discovered inhibitors that trap activation-loop conformations that modulate phosphatase recognition in biochemically reconstituted reactions and in human cells. Our X-ray crystal structures of inhibitor bound p38α reveal a flipped conformation of the activation loop that presents the activation loop phosphothreonine for dephosphorylation that explains increased dephosphorylation rate. These findings reveal an unexpected dual-action mechanism for kinase inhibitors, suggesting an approach to achieve greater potency and specificity.