Volume 271, Number 35, Issue of August 30, 1996 pp. 21243-21250
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Ligand-dependent Conformational Plasticity of the Periplasmic Histidine-binding Protein HisJ
INVOLVEMENT IN TRANSPORT SPECIFICITY

(Received for publication, April 3, 1996, and in revised form, May 29, 1996)

From the Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California at Berkeley, Berkeley, California 94720

The periplasmic histidine permease of Salmonella typhimurium is composed of a membrane-bound complex and a soluble histidine-binding protein (the periplasmic receptor), HisJ. Liganded receptor interacts with the membrane-bound complex, inducing ATP hydrolysis and substrate translocation. Preliminary evidence had shown a lack of direct correlation between the affinity of HisJ for a ligand and translocation efficiency, suggesting that the precise form of the receptor is important in determining its interaction with the membrane-bound complex. We have investigated the nature of the conformations assumed by HisJ upon binding a variety of ligands by tryptophan fluorescence enhancement, reaction with a closed form-specific monoclonal antibody, and changes in UV absorption spectra. It is demonstrated that although HisJ binds all the ligands and undergoes a conformational change, it assumes measurably different conformations. We also show that the interaction between HisJ and the membrane-bound complex depends on the nature of the ligand. Transport specificity appears to be defined, at least in part, by the conformation of the bound receptor, manifested either by the effect of a given ligand on the closed structure per se, or by the effect of ligand association on the equilibrium constant relating the open and the closed liganded forms.

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