title: Liganded and unliganded receptors interact with equal affinity with the membrane complex of periplasmic permeases, a subfamily of traffic ATPases.
abstract: 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.
authors.affiliation: Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California, Berkeley, California 94720-3202, USA.
authors.forename: G F
authors.initials: GF
authors.lastname: Ames
authors.forename: C E
authors.initials: CE
authors.lastname: Liu
authors.forename: A K
authors.initials: AK
authors.lastname: Joshi
authors.forename: K
authors.initials: K
authors.lastname: Nikaido
mesh_terms: D018528:ATP-Binding Cassette Transporters; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D026923:Amino Acid Transport Systems, Basic; D001426:Bacterial Proteins; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000302:isolation & purification / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000494:pharmacology; D005557:Formaldehyde / Q000494:pharmacology; D006639:Histidine / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000302:isolation & purification / Q000378:metabolism*; D008961:Models, Structural; D033902:Periplasmic Binding Proteins; D012486:Salmonella typhimurium / Q000378:metabolism*
chemical_list: D018528:ATP-Binding Cassette Transporters; D026923:Amino Acid Transport Systems, Basic; D001426:Bacterial Proteins; D002352:Carrier Proteins; D003432:Cross-Linking Reagents; D008024:Ligands; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033902:Periplasmic Binding Proteins; C018963:histidine-binding protein; D005557:Formaldehyde; D006639:Histidine; C043828:histidine permease, Bacteria; D000251:Adenosine Triphosphatases