Structural basis of the alternating-access mechanism in a bile acid transporter.
Structural basis of the alternating-access mechanism in a bile acid transporter.
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胆汁酸是由肝细胞中的胆固醇合成的，通过胆道分泌到小肠，在那里它们有助于脂质和脂溶性维生素的吸收。通过一个被称为肠肝循环的过程，90%以上的分泌胆汁酸从肠道中回收并返回肝脏进行再分泌。在人类中，有两种Na+依赖性胆汁酸转运体参与肠肝再循环，在肝细胞中表达的Na+-牛磺酸胆酸共转运多肽（NTCP或SLC10A1）和在回肠末端肠细胞中表达的根尖钠依赖性胆汁酸转运体（ASBT或SLC10A2）。近年来，ASBT作为治疗高胆固醇血症的潜在药物靶点引起了人们的极大兴趣，因为抑制ASBT可以减少胆汁酸的重吸收，从而增加胆汁酸的合成，从而增加胆固醇的消耗。然而，缺乏胆汁酸转运体的三维结构阻碍了我们理解底物选择性和转运的分子机制，并解释丰富的现有功能数据。最近报道了一种来自脑膜炎奈瑟菌（ASBTNM）洗涤剂中的ASBT同源物的晶体结构，显示该蛋白与两个Na+和一个牛磺酸结合为内向开放的构象。然而，胆汁酸和Na+穿过细胞膜的结构变化很难从单一结构中推断出来。为了更好地理解与Na+和胆汁酸耦合运输相关的结构变化，我们在脂质环境中结晶并解决了来自frederiksenii耶尔森菌ASBT同源物（ASBTYf）的两个结构，这表明底物结合域的大刚体旋转提供了高度保守的“交叉”区域的替代可达性，其中两个不连续的跨膜螺旋相互交叉。这一结果对胆汁酸在运输过程中的位置和方向以及Na+的转运途径都有影响。
Bile acids are synthesized from cholesterol in hepatocytes and secreted via 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 re-secretion. In humans, there are two Na+-dependent bile acid transporters involved in enterohepatic recirculation, the Na+-taurocholate co-transporting polypeptide (NTCP or SLC10A1) expressed in hepatocytes, and the apical sodium-dependent bile acid transporter (ASBT or SLC10A2) expressed on enterocytes in the terminal ileum. In recent years, ASBT has attracted much interest as a potential drug target for treatment of hypercholesterolemia, because inhibition of ASBT reduces reabsorption of bile acids, thus increasing bile acid synthesis and consequently cholesterol consumption. However, a lack of 3-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 homolog from Neisseria meningitidis (ASBTNM) 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 better the structural changes associated with the coupled transport of Na+ and bile acids, we crystallized and solved two structures of a ASBT homolog from Yersinia frederiksenii (ASBTYf) in a lipid environment, which 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 Na+.