Protein-protein interaction in transport

The transport of histidine in the gram negative bacterium S. typhimurium has been studied over a number of years and found to occur through five transport systems (Ames, 1972). Of these, the one with the highest affinity has been studied in detail from the genetic, physiological and biochemical point of view. This system, known as the high-affinity histidine permease, is composed of two subsystems, the J-P and K-P systems, which have a component in common, the P protein, presumed to be membrane-bound. The J-P system, moreover, is known to require the presence of a periplasmic histidine-binding protein, the J protein. The J protein is coded for by the hisJ gene and the P protein is coded for by the hisP gene. Both of these genes have been mapped at 75 min on the Salmonella chromosomal map. Adjacent to them is a regulatory gene, the dhuA gene. The periplasmic histidine-binding protein J has been shown to interact directly with the second component of transport, the P protein (Ames and Spudich, 1976). In accordance with this, histidine-binding protein J has been shown to contain, besides the histidine-binding site, a second site, essential for function, the interaction site (Kustu and Ames, 1974). We have recently shown that a mutant J protein with a defective interaction site but an intact histidine-binding site cannot function in histidine transport, unless an appropriate compensating mutation is introduced in the P protein. The interaction between the J and P proteins is an obligatory step in transport. The mutation in the interaction site of the J protein has been shown to map in the hisJ gene, and the compensating supressor mutation in the P protein has been shown to map in the hisP gene. Our contention that the J and P proteins engage in a functional interaction assumes further strength from other studies on protein-protein interaction in bacteriophage development and in ribosomal structure. Among the possible functions of the J-P interaction in histidine transport, a likely one is the transmission of information to the P protein, concerning whether or not the histidine-binding site on the J protein is occupied. Appropriate conformational changes then can occur in either the J or the P protein, or both, such that the histidine is released in the correct location and direction on the inside of the cell. This could occur either by a pore-formation mechanism or by binding-site translocation. Another alternative is that the P protein is part of an energy transducing mechanism in which energy is transmitted to the J protein, through the interaction site, as a prerequisite for the J protein participation in translocation. Among the interesting findings coming out of this work, is also the fact that the P protein performs a central function in transport being involved in the permeation of other substrates besides histidine. It is likely that other binding proteins besides the J protein require the P protein. Thus an interesting question which we are trying to answer at present is whether the P protein has separate interaction sites for each of these other binding proteins requiring its function, or whether they all interact at one common site.