title: Crystal structures of the choline/acetylcholine substrate-binding protein ChoX from Sinorhizobium meliloti in the liganded and unliganded-closed states.
abstract: The ATP-binding cassette transporter ChoVWX is one of several choline import systems operating in Sinorhizobium meliloti. Here fluorescence-based ligand binding assays were used to quantitate substrate binding by the periplasmic ligand-binding protein ChoX. These data confirmed that ChoX recognizes choline and acetylcholine with high and medium affinity, respectively. We also report the crystal structures of ChoX in complex with either choline or acetylcholine. These structural investigations revealed an architecture of the ChoX binding pocket and mode of substrate binding similar to that reported previously for several compatible solute-binding proteins. Additionally the ChoX-acetylcholine complex permitted a detailed structural comparison with the carbamylcholine-binding site of the acetylcholine-binding protein from the mollusc Lymnaea stagnalis. In addition to the two liganded structures of ChoX, we were also able to solve the crystal structure of ChoX in a closed, substrate-free conformation that revealed an architecture of the ligand-binding site that is superimposable to the closed, ligand-bound form of ChoX. This structure is only the second of its kind and raises the important question of how ATP-binding cassette transporters are capable of distinguishing liganded and unliganded-closed states of the binding protein.
authors.affiliation: Institute of Biochemistry, Heinrich Heine University Duesseldorf, Universitaetsstrasse 1, 40225 Duesseldorf, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Oswald
authors.forename: Sander H J
authors.initials: SH
authors.lastname: Smits
authors.forename: Marina
authors.initials: M
authors.lastname: Höing
authors.forename: Linda
authors.initials: L
authors.lastname: Sohn-Bösser
authors.forename: Laurence
authors.initials: L
authors.lastname: Dupont
authors.forename: Daniel
authors.initials: D
authors.lastname: Le Rudulier
authors.forename: Lutz
authors.initials: L
authors.lastname: Schmitt
authors.forename: Erhard
authors.initials: E
authors.lastname: Bremer
mesh_terms: D000109:Acetylcholine / Q000737:chemistry*; D000255:Adenosine Triphosphate / Q000737:chemistry; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism; D002794:Choline / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008969:Molecular Sequence Data; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid; D016962:Sinorhizobium meliloti / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D008024:Ligands; D002244:Carbon; D000255:Adenosine Triphosphate; D002794:Choline; D000109:Acetylcholine