Two-state allosteric behavior in a single-domain signaling protein

Protein actions are usually discussed in terms of static structures, but fu nction requires motion. We find a strong correlation between phosphorylatio n-driven activation of the signaling protein NtrC and microsecond time-scal e backbone dynamics. Using nuclear magnetic resonance relaxation, we charac terized the motions of NtrC in three functional states: unphosphorylated (i nactive), phosphorylated (active), and a partially active mutant. These dyn amics are indicative of exchange between inactive and active conformations. Both states are populated in unphosphorylated NtrC, and phosphorylation sh ifts the equilibrium toward the active species. These results support a dyn amic population shift between two preexisting conformations as the underlyi ng mechanism of activation.