Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, United States.
Program in Chemical Biology, University of Michigan, Ann Arbor, United States.
Elife. 2022 Mar 7;11:e76766. doi: 10.7554/eLife.76766.
Proteins from the bacterial small multidrug resistance (SMR) family are proton-coupled exporters of diverse antiseptics and antimicrobials, including polyaromatic cations and quaternary ammonium compounds. The transport mechanism of the transporter, EmrE, has been studied extensively, but a lack of high-resolution structural information has impeded a structural description of its molecular mechanism. Here, we apply a novel approach, multipurpose crystallization chaperones, to solve several structures of EmrE, including a 2.9 Å structure at low pH without substrate. We report five additional structures in complex with structurally diverse transported substrates, including quaternary phosphonium, quaternary ammonium, and planar polyaromatic compounds. These structures show that binding site tryptophan and glutamate residues adopt different rotamers to conform to disparate structures without requiring major rearrangements of the backbone structure. Structural and functional comparison to Gdx-Clo, an SMR protein that transports a much narrower spectrum of substrates, suggests that in EmrE, a relatively sparse hydrogen bond network among binding site residues permits increased sidechain flexibility.
细菌小分子多药耐药（SMR）家族的蛋白质是多种防腐剂和抗菌药物的质子偶联外排泵，包括多环阳离子和季铵化合物。转运蛋白 EmrE 的转运机制已被广泛研究，但缺乏高分辨率的结构信息阻碍了对其分子机制的结构描述。在这里，我们应用一种新的方法，多功能结晶伴侣，来解决 EmrE 的几个结构，包括在低 pH 值下没有底物的 2.9 Å 结构。我们报告了另外五个与结构不同的转运底物结合的结构，包括季鏻、季铵和平面多环化合物。这些结构表明，结合部位的色氨酸和谷氨酸残基采用不同的构象，以适应不同的结构，而不需要骨架结构的主要重排。与 Gdx-Clo 的结构和功能比较，Gdx-Clo 是一种运输范围较窄的 SMR 蛋白，表明在 EmrE 中，结合部位残基之间相对稀疏的氢键网络允许侧链更大的灵活性。
