title: Flexibility in the ABC transporter MsbA: Alternating access with a twist.
abstract: ATP-binding cassette (ABC) transporters are integral membrane proteins that translocate a wide variety of substrates across cellular membranes and are conserved from bacteria to humans. Here we compare four x-ray structures of the bacterial ABC lipid flippase, MsbA, trapped in different conformations, two nucleotide-bound structures and two in the absence of nucleotide. Comparison of the nucleotide-free conformations of MsbA reveals a flexible hinge formed by extracellular loops 2 and 3. This hinge allows the nucleotide-binding domains to disassociate while the ATP-binding half sites remain facing each other. The binding of the nucleotide causes a packing rearrangement of the transmembrane helices and changes the accessibility of the transporter from cytoplasmic (inward) facing to extracellular (outward) facing. The inward and outward openings are mediated by two different sets of transmembrane helix interactions. Altogether, the conformational changes between these structures suggest that large ranges of motion may be required for substrate transport.
authors.affiliation: Departments of Cell Biology and Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, CB-105, La Jolla, CA 92037, USA.
authors.forename: Andrew
authors.initials: A
authors.lastname: Ward
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Reyes
authors.forename: Jodie
authors.initials: J
authors.lastname: Yu
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Roth
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Chang
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000266:Adenylyl Imidodiphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D001693:Biological Transport, Active; D018360:Crystallography, X-Ray; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009038:Motion; D011485:Protein Binding; D011487:Protein Conformation; D012486:Salmonella typhimurium / Q000737:chemistry; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D014734:Vibrio cholerae / Q000737:chemistry
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C079726:MsbA protein, Bacteria; D000266:Adenylyl Imidodiphosphate; D000255:Adenosine Triphosphate