Dynamic allostery in the ring protein TRAP

We have discovered distinct, characteristic differences in the thermodynamic signatures of tryptophan binding by trp RNA-binding attenuation protein (TRAP) from two different bacterial species. The TRAP 11mer ring binds 11 molecules of tryptophan at symmetry-related sites. Tryptophan binding to Bacillus stearothermophilus TRAP is not cooperative, but isothermal titration calorimetry shows that filling the first tryptophan binding sites of Bacillus subtilis TRAP has a marked effect on the thermodynamics of subsequent ligand binding. We have identified a single, conservative amino acid replacement (Ile to Leu) in B. subtilis TRAP that abolishes this effect, and suggest the initial ligand binding causes a change throughout the wild-type protein ring.