Structural basis of the alternating-access mechanism in a bile acid transporter

Inhibitors of the bile acid transporter ASBT may be useful therapeutics for treating hypercholesterolaemia and type 2 diabetes; here, two X-ray crystal structures of an ASBT homologue from Yersinia frederiksenii are solved. This paper reports two X-ray crystal structures of a bacterial homologue of the human apical sodium-dependent bile salt transporter (ASBT, also known as SLC10A2), one of two transporters involved in retrieving secreted bile acids from the intestine. The homologue (termed ASBTYf), from Yersinia frederiksenii, was crystallized in a lipid environment. The structures reveal that a large rigid-body rotation of a substrate-binding domain gives alternate accessibility to the highly conserved 'crossover' region, where two discontinuous transmembrane helices cross each other. This result has implications for the location and orientation of the bile acid during transport, as well as for the translocation pathway for sodium ions. The authors cite evidence that implies that overall fold and transport mechanism are similar between ASBT and ASBTYf and they suggest that ASBTYf may serve as a useful model system for understanding mechanisms of transport and inhibition in the mammalian ASBT homologues. ASBT inhibitors are being studied as potential therapeutics for the treatment of hypercholesterolaemia and type II diabetes. 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 resecretion1. 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 ileum2. 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 consumption3,4. 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 data2,5,6,7,8. The crystal structure of an ASBT homologue from Neisseria meningitidis (ASBTNM) in detergent was reported recently9, 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+.