LIGAND-DEPENDENT CONFORMATIONAL PLASTICITY OF THE PERIPLASMIC HISTIDINE-BINDING PROTEIN HISJ - INVOLVEMENT IN TRANSPORT SPECIFICITY

The periplasmic histidine permease of Salmonella typhimurium is compos ed of a membrane-bound complex and a soluble histidine-binding protein (the periplasmic receptor), HisJ. Liganded receptor interacts with th e membrane-bound complex, inducing ATP hydrolysis and substrate transl ocation. Preliminary evidence had shown a lack of direct correlation b etween the affinity of HisJ for a ligand and translocation efficiency, suggesting that the precise form of the receptor is important in dete rmining its interaction with the membrane-bound complex. We have inves tigated the nature of the conformations assumed by HisJ upon binding a variety of ligands by tryptophan fluorescence enhancement, reaction w ith a closed form-specific monoclonal antibody, and changes in UV abso rption spectra. It is demonstrated that although HisJ binds all the li gands and undergoes a conformational change, it assumes measurably dif ferent conformations. We also show that the interaction between HisJ a nd the membrane-bound complex depends on the nature of the ligand. Tra nsport specificity appears to be defined, at least in part, by the con formation of the bound receptor, manifested either by the effect of a given ligand on the closed structure per se, or by the effect of ligan d association on the equilibrium constant relating the open and the cl osed liganded forms.