title: myo-inositol and D-ribose ligand discrimination in an ABC periplasmic binding protein.
abstract: The periplasmic binding protein (PBP) IbpA mediates the uptake of myo-inositol by the IatP-IatA ATP-binding cassette transmembrane transporter. We report a crystal structure of Caulobacter crescentus IbpA bound to myo-inositol at 1.45 Å resolution. This constitutes the first structure of a PBP bound to inositol. IbpA adopts a type I PBP fold consisting of two α-β lobes that surround a central hinge. A pocket positioned between the lobes contains the myo-inositol ligand, which binds with submicromolar affinity (0.76 ± 0.08 μM). IbpA is homologous to ribose-binding proteins and binds D-ribose with low affinity (50.8 ± 3.4 μM). On the basis of IbpA and ribose-binding protein structures, we have designed variants of IbpA with inverted binding specificity for myo-inositol and D-ribose. Five mutations in the ligand-binding pocket are sufficient to increase the affinity of IbpA for D-ribose by 10-fold while completely abolishing binding to myo-inositol. Replacement of ibpA with these mutant alleles unable to bind myo-inositol abolishes C. crescentus growth in medium containing myo-inositol as the sole carbon source. Neither deletion of ibpA nor replacement of ibpA with the high-affinity ribose binding allele affected C. crescentus growth on D-ribose as a carbon source, providing evidence that the IatP-IatA transporter is specific for myo-inositol. This study outlines the evolutionary relationship between ribose- and inositol-binding proteins and provides insight into the molecular basis upon which these two related, but functionally distinct, classes of periplasmic proteins specifically bind carbohydrate ligands.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois, USA.
authors.forename: Julien
authors.initials: J
authors.lastname: Herrou
authors.forename: Sean
authors.initials: S
authors.lastname: Crosson
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D016935:Caulobacter crescentus / Q000235:genetics / Q000378:metabolism*; D007294:Inositol / Q000378:metabolism*; D008954:Models, Biological; D033902:Periplasmic Binding Proteins / Q000235:genetics / Q000378:metabolism*; D012266:Ribose / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D033902:Periplasmic Binding Proteins; D007294:Inositol; D012266:Ribose