The small multidrug resistance (SMR) family of membrane proteins is prominent because of its rare dual topology architecture, simplicity, and small size. Its best studied member, EmrE, is an important model system in several fields related to membrane protein biology, from evolution to mechanism. But despite decades of work on these multidrug transporters, the native function of the SMR family has remained a mystery, and many highly similar SMR homologs do not transport drugs at all. Here we establish that representative SMR proteins, selected from each of the major clades in the phylogeny, function as guanidinium ion exporters. Drug-exporting SMRs are all clustered in a single minority clade. Using membrane transport experiments, we show that these guanidinium exporters, which we term Gdx, are very selective for guanidinium and strictly and stoichiometrically couple its export with the import of two protons. These findings draw important mechanistic distinctions with the notably promiscuous and weakly coupled drug exporters like EmrE.

小的多药耐药 (SMR) 膜蛋白家族因其罕见的双拓扑结构、简单性和小尺寸而备受瞩目。其研究最好的成员 EmrE 是与膜蛋白生物学相关的多个领域的重要模型系统，从进化到机制。但尽管对这些多药转运蛋白进行了数十年的研究，SMR 家族的天然功能仍然是一个谜，许多高度相似的 SMR 同源物根本不转运药物。在这里，我们确定从系统发育中的每个主要进化枝中选择的代表性 SMR 蛋白作为胍离子输出物起作用。出口药物的 SMR 都聚集在一个少数进化枝中。使用膜转运实验，我们表明这些胍输出物，我们称之为 Gdx，对胍非常有选择性，并且严格和化学计量地将其输出与两个质子的输入耦合。这些发现与 EmrE 等显着混杂和弱耦合的药物出口商形成了重要的机制区别。