The Escherichia coli uracil:proton symporter UraA is a prototypical member of the nucleobase/ascorbate transporter (NAT) or nucleobase/cation symporter 2 (NCS2) family, which corresponds to the human solute carrier family SLC23. UraA consists of 14 transmembrane segments (TMs) that are organized into two distinct domains, the core domain and the gate domain, a structural fold that is also shared by the SLC4 and SLC26 transporters. Here we present the crystal structure of UraA bound to uracil in an occluded state at 2.5 Å resolution. Structural comparison with the previously reported inward-open UraA reveals pronounced relative motions between the core domain and the gate domain as well as intra-domain rearrangement of the gate domain. The occluded UraA forms a dimer in the structure wherein the gate domains are sandwiched by two core domains. In vitro and in vivo biochemical characterizations show that UraA is at equilibrium between dimer and monomer in all tested detergent micelles, while dimer formation is necessary for the transport activity. Structural comparison between the dimeric UraA and the recently reported inward-facing dimeric UapA provides important insight into the transport mechanism of SLC23 transporters.

中文翻译：

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尿嘧啶：质子转运蛋白UraA的二聚体结构提供了对SLC4 / 23/26转运蛋白的机械学见解。

大肠杆菌尿嘧啶：质子转运蛋白UraA是核碱基/抗坏血酸转运蛋白（NAT）或核碱基/阳离子转运蛋白2（NCS2）家族的原型成员，其对应于人类溶质载体家族SLC23。UraA由14个跨膜片段（TM）组成，这些跨膜片段被组织为两个不同的域，即核心域和门域，这也是SLC4和SLC26转运蛋白所共有的结构折叠。在这里，我们介绍了在2.5Å分辨率下，UraA的晶体结构与尿嘧啶结合时处于闭塞状态。与先前报道的向内开放的UraA的结构比较揭示了核心域和门域之间的明显相对运动以及门域的域内重排。被封闭的UraA在结构中形成二聚体，其中栅极结构域被两个核心结构域夹在中间。体外和体内生化特征表明，在所有测试的洗涤剂胶束中，UraA在二聚体和单体之间处于平衡状态，而二聚体的形成对于转运活性是必需的。二聚体UraA与最近报道的向内的二聚体UapA之间的结构比较为SLC23转运蛋白的转运机制提供了重要的见识。