title: Structural basis of the alternating-access mechanism in a bile acid transporter.
abstract: Bile acids are synthesized from cholesterol in hepatocytes and secreted through the biliary tract into the small intestine, where they aid in absorption of lipids and fat-soluble vitamins. Through a process known as enterohepatic recirculation, more than 90% of secreted bile acids are then retrieved from the intestine and returned to the liver for resecretion. In humans, there are two Na(+)-dependent bile acid transporters involved in enterohepatic recirculation, the Na(+)-taurocholate co-transporting polypeptide (NTCP; also known as SLC10A1) expressed in hepatocytes, and the apical sodium-dependent bile acid transporter (ASBT; also known as SLC10A2) expressed on enterocytes in the terminal ileum. In recent years, ASBT has attracted much interest as a potential drug target for treatment of hypercholesterolaemia, because inhibition of ASBT reduces reabsorption of bile acids, thus increasing bile acid synthesis and consequently cholesterol consumption. However, a lack of three-dimensional structures of bile acid transporters hampers our ability to understand the molecular mechanisms of substrate selectivity and transport, and to interpret the wealth of existing functional data. The crystal structure of an ASBT homologue from Neisseria meningitidis (ASBT(NM)) in detergent was reported recently, showing the protein in an inward-open conformation bound to two Na(+) and a taurocholic acid. However, the structural changes that bring bile acid and Na(+) across the membrane are difficult to infer from a single structure. To understand the structural changes associated with the coupled transport of Na(+) and bile acids, here we solved two structures of an ASBT homologue from Yersinia frederiksenii (ASBTYf) in a lipid environment, which reveal that a large rigid-body rotation of a substrate-binding domain gives the conserved 'crossover' region, where two discontinuous helices cross each other, alternating accessibility from either side of the cell membrane. This result has implications for the location and orientation of the bile acid during transport, as well as for the translocation pathway for Na(+).
authors: 1] Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA [2] Department of Physiology and Cellular Biophysics, Columbia University, New York, New York 10032, USA [3].
authors: Xiaoming
authors: X
authors: Zhou
authors: 1] Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA [2].
authors: Elena J
authors: EJ
authors: Levin
authors: Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
authors: Yaping
authors: Y
authors: Pan
authors: Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
authors: Jason G
authors: JG
authors: McCoy
authors: Department of Physiology and Cellular Biophysics, Columbia University, New York, New York 10032, USA.
authors: Ruchika
authors: R
authors: Sharma
authors: New York Consortium on Membrane Protein Structure, New York, New York 10027, USA.
authors: Brian
authors: B
authors: Kloss
authors: New York Consortium on Membrane Protein Structure, New York, New York 10027, USA.
authors: Renato
authors: R
authors: Bruni
authors: 1] Department of Psychiatry and Center for Molecular Recognition, Columbia University, New York, New York 10032, USA [2] New York State Psychiatric Institute, Division of Molecular Therapeutics, New York, New York 10032, USA.
authors: Matthias
authors: M
authors: Quick
authors: 1] Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA [2] Department of Physiology and Cellular Biophysics, Columbia University, New York, New York 10032, USA [3] Ion Channel Research and Drug Development Center, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming 650223, China.
authors: Ming
authors: M
authors: Zhou
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001647:Bile Acids and Salts / Q000378:metabolism; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D018360:Crystallography, X-Ray; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D011487:Protein Conformation; D015203:Reproducibility of Results; D012399:Rotation; D012964:Sodium / Q000378:metabolism; D013329:Structure-Activity Relationship; D015007:Yersinia / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; C037643:bile acid binding proteins; D012964:Sodium