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The nucleobase-cation-symport-1 (NCS1) transporters are essential components of salvage pathways for nucleobases and related metabolites. Here, we report the 2.85-angstrom resolution structure of the NCS1 benzyl-hydantoin transporter, Mhp1, from Microbacterium liquefaciens. Mhp1 contains 12 transmembrane helices, 10 of which are arranged in two inverted repeats of five helices. The structures of the outward-facing open and substrate-bound occluded conformations were solved, showing how the outward-facing cavity closes upon binding of substrate. Comparisons with the leucine transporter LeuT(Aa) and the galactose transporter vSGLT reveal that the outward- and inward-facing cavities are symmetrically arranged on opposite sides of the membrane. The reciprocal opening and closing of these cavities is synchronized by the inverted repeat helices 3 and 8, providing the structural basis of the alternating access model for membrane transport.

Original publication

DOI

10.1126/science.1164440

Type

Journal article

Journal

Science

Publication Date

31/10/2008

Volume

322

Pages

709 - 713

Keywords

Actinomycetales, Amino Acid Sequence, Bacterial Proteins, Binding Sites, Cations, Cell Membrane, Crystallography, X-Ray, Hydantoins, Ion Transport, Models, Molecular, Molecular Sequence Data, Nucleobase Transport Proteins, Protein Conformation, Protein Structure, Secondary, Sodium, Symporters