title: Substrate specificity and ion coupling in the Na+/betaine symporter BetP.
abstract: BetP is an Na(+)-coupled betaine-specific transporter of the betaine-choline-carnitine (BCC) transporter family involved in the response to hyperosmotic stress. The crystal structure of BetP revealed an overall fold of two inverted structurally related repeats (LeuT-fold) that BetP shares with other sequence-unrelated Na(+)-coupled symporters. Numerous structures of LeuT-fold transporters in distinct conformational states have contributed substantially to our understanding of the alternating access mechanism of transport. Nevertheless, coupling of substrate and co-transported ion fluxes has not been structurally corroborated to the same extent. We converted BetP by a single-point mutation--glycine to aspartate--into an H(+)-coupled choline-specific transporter and solved the crystal structure of this mutant in complex with choline. The structure of BetP-G153D demonstrates a new inward-facing open conformation for BetP. Choline binding to a location close to the second, low-affinity sodium-binding site (Na2) of LeuT-fold transporters is facilitated by the introduced aspartate. Our data confirm the importance of a cation-binding site in BetP, playing a key role in a proposed molecular mechanism of Na(+) and H(+) coupling in BCC transporters.
authors.affiliation: Department of Structural Biology, Max-Planck Institute of Biophysics, Frankfurt am Main, Germany.
authors.forename: Camilo
authors.initials: C
authors.lastname: Perez
authors.forename: Caroline
authors.initials: C
authors.lastname: Koshy
authors.forename: Susanne
authors.initials: S
authors.lastname: Ressl
authors.forename: Sascha
authors.initials: S
authors.lastname: Nicklisch
authors.forename: Reinhard
authors.initials: R
authors.lastname: Krämer
authors.forename: Christine
authors.initials: C
authors.lastname: Ziegler
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001622:Betaine / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D018360:Crystallography, X-Ray; D007477:Ions / Q000378:metabolism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D017354:Point Mutation; D017434:Protein Structure, Tertiary; D012964:Sodium / Q000378:metabolism*; D013379:Substrate Specificity; D027981:Symporters / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; C101652:BetP protein, Corynebacterium glutamicum; D002352:Carrier Proteins; D007477:Ions; D050505:Mutant Proteins; D027981:Symporters; D001622:Betaine; D012964:Sodium