STRUCTURE-FUNCTION ANALYSIS OF THE PERIPLASMIC HISTIDINE-BINDING PROTEIN - MUTATIONS DECREASING LIGAND BINDING ALTER THE PROPERTIES OF THE CONFORMATIONAL CHANGE AND OF THE CLOSED FORM

The periplasmic histidine-binding protein, HisJ, is a receptor for the histidine permease of Salmonella typhimurium. Receptors of this type are composed of two lobes that are far apart in the unliganded structu re (open conformation) and drawn close together in the liganded struct ure (closed conformation), The binding of the ligand, in a cleft betwe en the lobes, stabilizes the closed conformation. Such receptors have several functions in transport: interaction with the membrane-bound co mplex, transmission of a transmembrane signal to hydrolyze ATP, and re ceiving a signal to open the lobes and release the ligand. In this stu dy the mechanism of action of HisJ was further investigated using muta nt proteins defective in ligand binding activity and closed form-speci fic monoclonal antibodies (Wolf, A., Shaw, E. W., Nikaido, K., and Ame s G. F.-L. (1994) J. Biol. Chem. 269, 23051-23058). Y14H is defective in stabilization of the closed form, does not assume the closed empty form, and assumes an altered closed liganded form. T121A and G119R are similar to Y14H, but assume a normal closed Liganded form. S72P binds the ligand to the open form, but does not assume a recognizable close d form. S92F is defective in the ability to undergo conformational cha nge and to stabilize the closed form. Ad other mutant proteins appear to fall within one of these four categories. The biochemical character ization of these mutant proteins agrees with the structural analysis o f the protein. We suggest that mutant proteins that do not assume the normal closed form, in addition to their defect in ligand binding, fai l to interact with the membrane-bound complex and/or to transmit trans membrane signals.