Alternating-access mechanism in conformationally asymmetric trimers of the betaine transporter BetP

摘要

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. BetP adjusts its transport activity by sensing the cytoplasmic K~+ concentration as a measure for hyperosmotic stress via the osmosensing carboxy-terminal domain. BetP needs to be in a trimeric state for communication between individual protomers through several intratrimeric interaction sites. Recently, crystal structures of inward-facing BetP trimers have contributed to our understanding of activity regulation on a molecular level. Here we report new crystal structures, which reveal two conformationally asymmetric BetP trimers, 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.%渗透调控的共转运体BetP(甜菜碱／胆碱／肉毒碱转运体家族的一个成员)响应于细胞质中钾浓度的变化来共转运甜菜碱和钠。这篇论文报告了这种膜蛋白在四个新构形中的X-射线晶体结构：一个向外开放的apo构形、一个向外闭塞的apo构形和两个封闭状态-一个是与甜菜碱形成复合物的状态；另一个是没有任何基质的状态。根据这些新的结构，本文作者识别出了BetP在转运周期中发生的关键构形变化。