Structural insight into regulation of myosin phosphatase by CPI-17.

Cellular activity of type-1 Ser/Thr phosphatase (PP1) is controlled by endogenous phospho-inhibitor proteins, which transduce kinase signals into PP1. Inhibition of myosin phosphatase, a PP1 holoenzyme, by a specific inhibitor protein CPI-17 is required for a robust smooth muscle contraction, long-term synaptic depression, and cell transformation. In response to G-protein-mediated signaling, phosphorylation of CPI-17 at Thr38 triggers a > 1000-fold increase of inhibitory potency. However, mechanisms for phospho-inhibitor-mediated inhibition of PP1 have yet to be elucidated. We solved the NMR structure of phospho-CPI-17 (P-CPI-17), which reveals how phosphorylation triggers a conformational change to convert the protein into a potent inhibitor for myosin phosphatase. Upon phosphorylation, P-Thr38 is moved 8.1 A to solvent, and two pairs of helices undergo re-alignment into parallel. This helix re-alignment results in the formation of a hydrophobic core of intercalated side-chains that is needed for the pot...