Thermodynamics and dynamics of histidine‐binding protein, the water‐soluble receptor of histidine permease: Implications for the transport of high and low affinity ligands

The bacterial histidine permease is a model system for ABC transporters (traffic ATPases). The water‐soluble receptor of this permease, HisJ, binds l‐histidine and l‐arginine (tightly) and l‐lysine and l‐ornithine (less tightly) in the periplasm, interacts with the membrane‐bound complex (HisQMP2) and induces its ATPase activity, which results in ligand translocation. HisJ is a two‐domain protein; in the absence of ligand, the cleft between two domains is open and binding of substrate stabilizes the closed conformation. Surprisingly, various liganded HisJ forms display substantial differences in their physicochemical characteristics and capacity to induce the ATPase. This is due to either different effects of the individual ligands on the respective closed structures, or to different equilibria being reached for each ligand between the open liganded form and the closed liganded form [Wolf, A., Lee, K.C., Kirsch, J.F. & Ames, G.F.‐L. (1996) J. Biol. Chem.271, 21243–21250]. In this work, time‐resolved measurements of the decay of intrinsic HisJ fluorescence and of the decay of the anisotropy of the fluorescence, as well as the analysis of the steady‐state near UV CD and fluorescence spectra, rule out the model in which the differences between liganded complexes reflect different equilibria. The decay of the anisotropy of the fluorescence shows that liganded complexes differ dramatically in their large‐scale conformational dynamics. Differential scanning calorimetry (DSC) curves for the HisJ thermal unfolding are well described by a scheme of equilibrium two‐state unfolding of two independent domains, which can be ascribed to the two‐domain structure of HisJ. This is true both for apo‐HisJ at various pH values, and for HisJ in the presence of its ligands at varying concentrations, at pH 8.3. The DSC and structural data suggest that all ligands interact more extensively with the larger domain. A qualitative model for the HisJ conformational dynamics employing the idea of a twisting movement of the domains is proposed, which explains the difference in the efficacy of the ATPase induction by the various liganded HisJ forms.