Synthetic anion transporters that facilitate transmembrane H+/Cl-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+/Cl-symportactivity and their mechanism of action remains less well understood. We here report a chloride-selective tetraurea macrocycle that has a record-high H+/Cl-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+/Cl-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+/Cl-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.

Ein Tetraharnstoff-Makrocyclus und andere synthetische Anionentransporter erleichtern den H+/Cl−-Symport und zeigen abgeschwächte Transportraten in Gegenwart eines Membranpotentials, wie P. A Gale und Mitarbeiter in ihrem Forschungsartikel auf S. 15286 beschreiben. Diese spannungsabhängige Eigenschaft, die an spannungsgesteuerte Ionenkanäle erinnert, steht im Zusammenhang mit der Bindung von Anionentransportern an Lipidphosphat-Kopfgruppen, die die Transmembrandiffusion von Anionentransportern hemmen.

A tetraurea macrocycle and other synthetic anion transporters facilitate transmembrane H+/Cl− symport and show attenuated transport rates in the presence of a membrane potential, as described by P. A. Gale et al. in their Research Article on page 15142 ff. This voltage-dependent property, reminiscent of voltage-gated ion channels, is related to the binding of anion transporters to lipid phosphate headgroups that inhibits the transmembrane diffusion of anion transporters.

Synthetic anion transporters that facilitate transmembrane H+/Cl- 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+/Cl- symport activity and their mechanism of action remains less well understood. We report a chloride-selective tetraurea macrocycle that has a record-high H+/Cl- symport activity similar to that of prodigiosin and most importantly demonstrates unprecedented voltage-switchable transport properties that are linked to the lack of uniport activity. By studying the anion binding affinity and transport mechanisms of four other anion transporters, we show that the lack of uniport and voltage-dependent H+/Cl- symport originate from strong binding to phospholipid headgroups that hampers the diffusion of the free transporters through the membrane, leading to an unusual H+/Cl- 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.

Synthetic transmembrane anion transporters (anionophores) have potential as tools for biomedical research and as therapeutic agents for diseases associated with anion-channel dysfunction. However, the possibility of H+ or OH- transport by anionophores has received little attention, and an anionophore selective for Cl- over H+/OH- is currently unavailable. Here, we show that depending on anionophore acidity, many anionophores facilitate electrogenic H+ or OH- transport, potentially leading to toxicity. Nevertheless, using several liposome-membrane-based assays, we identified two newly developed small molecules that promote electrogenic Cl- transport without effectively dissipating the transmembrane pH gradient, essentially mimicking the electrogenic cationophore valinomycin. The Cl- > H+/OH- selectivity of anionophores showed a consistent positive correlation with the degree of Cl- encapsulation and a negative correlation with the acidity of hydrogen-bond donors. Our study demonstrates that a valinomycin equivalent for Cl--selective transport is achievable.