Phospholipid (PL) scramblases disrupt the lipid asymmetry of the plasma membrane, externalizing phosphatidylserine to trigger blood coagulation and mark apoptotic cells. Recently, members of the TMEM16 family of Ca²⁺-gated channels have been shown to be involved in Ca²⁺-dependent scrambling. It is however controversial whether they are scramblases or channels regulating scrambling. Here we show that purified afTMEM16, from Aspergillus fumigatus, is a dual-function protein: it is a Ca²⁺-gated channel, with characteristics of other TMEM16 homologues, and a Ca²⁺-dependent scramblase, with the expected properties of mammalian PL scramblases. Remarkably, we find that a single Ca²⁺ site regulates separate transmembrane pathways for ions and lipids. Two other purified TMEM16-channel homologues do not mediate scrambling, suggesting that the family diverged into channels and channel/scramblases. We propose that the spatial separation of the ion and lipid pathways underlies the evolutionary divergence of the TMEM16 family, and that other homologues, such as TMEM16F, might also be dual-function channel/scramblases.

Significance: Calcium-activated chloride channels play key roles in physiology, from mediating sensory transduction and nociception to regulating mucine secretion in airway epithelia and controlling excitability of smooth muscle fibers. Recently, TMEM16A was identified as the pore-forming subunit of these channels. It remains unclear, however, whether this protein is sufficient to form calcium-activated chloride channels, or whether association with other subunits is required for function. Recently, association with calmodulin has been proposed to be required for the calcium-dependent activation and ion selectivity of these channels. Here we show that purified and reconstituted TMEM16A is necessary and sufficient to recapitulate the properties of native and heterologously expressed calcium-activated chloride currents. Thus, association of TMEM16A with other proteins is not required for function.

The crucial ion-binding events that drive H+/Cl− exchange in the bacterial transporter CLC-ec1 are now probed by isothermal calorimetry and detergent-solubilized proteins. The results indicate that transport via CLC channels have an inherent directionality, rather than being driven by an electrochemical gradient. Ion binding to secondary active transporters triggers a cascade of conformational rearrangements resulting in substrate translocation across cellular membranes. Despite the fundamental role of this step, direct measurements of binding to transporters are rare. We investigated ion binding and selectivity in CLC-ec1, a H+-Cl− exchanger of the CLC family of channels and transporters. Cl− affinity depends on the conformation of the protein: it is highest with the extracellular gate removed and weakens as the transporter adopts the occluded configuration and with the intracellular gate removed. The central ion-binding site determines selectivity in CLC transporters and channels. A serine-to-proline substitution at this site confers NO3− selectivity upon the Cl−-specific CLC-ec1 transporter and CLC-0 channel. We propose that CLC-ec1 operates through an affinity-switch mechanism and that the bases of substrate specificity are conserved in the CLC channels and transporters.