Alternating-access mechanism in conformationally asymmetric trimers of the betaine transporter BetP
Alternating-access mechanism in conformationally asymmetric trimers of the betaine transporter BetP
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DOI：
10.1038/nature11403
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发表时间：
2012-10-04
期刊：
影响因子：
64.8
通讯作者：
Ziegler, Christine
中科院分区：
文献类型：
--
作者：
Perez, Camilo;Koshy, Caroline;Ziegler, Christine
甜菜碱和 Na+ 的同向转运已在来自谷氨酸棒杆菌的渗透调节转运蛋白 BetP 中得到广泛研究，该转运蛋白是甜菜碱/胆碱/肉碱转运蛋白家族的成员，具有两个反向结构重复序列的保守 LeuT 样折叠 (1)。 BetP 通过感知细胞质 K+ 浓度来调整其转运活性，作为通过渗透传感羧基末端结构域测量高渗应激的指标 (2,3)。 BetP 需要处于三聚体状态，以便各个原聚体之间通过几个三聚体内相互作用位点进行通讯 (4)。最近，向内的 BetP 三聚体的晶体结构有助于我们理解分子水平上的活性调节(5,6)。在这里，我们报告了新的晶体结构，它揭示了两种构象不对称的 BetP 三聚体 (7)，捕获了它们之间的三种不同的传输态。我们同时观察到总共四种新构象：一个向外开放的apo和一个向外封闭的apo状态，以及两个封闭的过渡态——一种与甜菜碱复合，一种不含底物。在这些新结构的基础上，我们确定了 BetP 的局部和整体构象变化，这些变化是分子转运机制的基础，部分类似于其他钠偶联 LeuT 样折叠转运蛋白中观察到的结构变化，但显示出差异，我们将其归因于甜菜碱的渗透性质、独特的底物特异性和 BetP 的调节特性。
Betaine and Na+ symport has been extensively studied in the osmotically regulated transporter BetP from Corynebacterium glutamicum, a member of the betaine/choline/carnitine transporter family, which shares the conserved LeuT-like fold of two inverted structural repeats(1). BetP adjusts its transport activity by sensing the cytoplasmic K+ concentration as a measure for hyperosmotic stress via the osmosensing carboxy-terminal domain(2,3). BetP needs to be in a trimeric state for communication between individual protomers through several intratrimeric interaction sites(4). Recently, crystal structures of inward-facing BetP trimers have contributed to our understanding of activity regulation on a molecular level(5,6). Here we report new crystal structures, which reveal two conformationally asymmetric BetP trimers(7), capturing among them three distinct transport states. We observe a total of four new conformations at once: an outward-open apo and an outward-occluded apo state, and two closed transition states-one in complex with betaine and one substrate-free. On the basis of these new structures, we identified local and global conformational changes in BetP that underlie the molecular transport mechanism, which partially resemble structural changes observed in other sodium-coupled LeuT-like fold transporters, but show differences we attribute to the osmolytic nature of betaine, the exclusive substrate specificity and the regulatory properties of BetP.