Voltage-Switchable HCl Transporters: The Effect of Lipid Headgroup Binding

Synthetic anion transporters that facilitate transmembrane H<sup>+</sup>/Cl<sup>-</sup>symport (cotransport)have anti-cancer potential due to their ability to neutralize pH gradients and inhibit autophagy in cells. However, compared to the natural product prodigiosin, synthetic anion transporters have low-to-modest H<sup>+</sup>/Cl<sup>-</sup>symportactivity and their mechanism of action remains less well understood. We here report a chloride-selective tetraurea macrocycle that has a record-high H<sup>+</sup>/Cl<sup>-</sup>symportactivity similar to prodigiosin and most importantly demonstrates unprecedented voltage-switchable transport properties that is linked to the lack of uniport activity. By studying anion binding affinity and transport mechanisms of four other anion transporters, we show that the lack of uniport and the voltage-dependent H<sup>+</sup>/Cl<sup>-</sup>symport originate from strong binding to lipid phosphate headgroup that hampers the diffusion of the free transporters through the membranes, leading to an unusual H<sup>+</sup>/Cl<sup>-</sup>symport mechanism that involves only charged species. Our work provides important mechanistic insights into different classes of anion transporters and a new approach to achieve voltage-switchability in artificial membrane transport systems.