Liganded and Unliganded Receptors Interact with Equal Affinity with the Membrane Complex of Periplasmic Permeases, a Subfamily of Traffic ATPases

• Published in: The Journal of biological chemistry 1996-06, Vol.271 (24), p.14264-14270
• Main Authors: Ames, G F, Liu, C E, Joshi, A K, Nikaido, K
• Format: Article
• Language: English
• Subjects: Adenosine Triphosphatases - metabolism; Amino Acid Transport Systems, Basic; ATP-Binding Cassette Transporters; Bacterial Proteins; Biological Transport, Active; Carrier Proteins - isolation & purification; Carrier Proteins - metabolism; Cell Membrane - metabolism; Cross-Linking Reagents - pharmacology; Formaldehyde - pharmacology; Histidine - metabolism; Kinetics; Ligands; Membrane Proteins - metabolism; Membrane Transport Proteins - isolation & purification; Membrane Transport Proteins - metabolism; Models, Structural; Periplasmic Binding Proteins; Salmonella typhimurium; Salmonella typhimurium - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.271.24.14264

The histidine-binding protein, HisJ, is the soluble receptor for the periplasmic histidine permease of Salmonella typhimurium. The receptor binds the substrate in the periplasm, interacts with the membrane-bound complex, transmits a transmembrane signal to hydrolyze ATP, and releases the ligand for translocation. HisJ, like other periplasmic receptors, has two lobes that are apart in the unliganded structure (open conformation) and drawn close together in the liganded structure (closed conformation), burying deeply the ligand. Such receptors are postulated to interact with the membrane-bound complex with high affinity in their liganded conformation, and, upon substrate translocation, to undergo a reduction in affinity and therefore be released. Here we show that in contrast to the current postulate, liganded and unliganded receptors have equal affinity for the membrane-bound complex. The affinity is measured both by chemical cross-linking and co-sedimentation procedures. An ATPase activity assay is also used to demonstrate the interaction of unliganded receptor with the membrane-bound complex. These findings support a new model for the transport mechanism, in which the soluble receptor functions independently of the commonly accepted high-low affinity switch.