Structural and functional characterization of the bestrophin-2 anion channel

The bestrophin family of calcium (Ca²⁺)-activated chloride (Cl^-) channels, which mediate the influx and efflux of monovalent anions in response to the levels of intracellular Ca²⁺, comprises four members in mammals (bestrophin 1-4). Here we report cryo-EM structures of bovine bestrophin-2 (bBest2) bound and unbound by Ca²⁺ at 2.4- and 2.2-Å resolution, respectively. The bBest2 structure highlights four previously underappreciated pore-lining residues specifically conserved in Best2 but not in Best1, illustrating the differences between these paralogs. Structure-inspired electrophysiological analysis reveals that, although the channel is sensitive to Ca²⁺, it has substantial Ca²⁺-independent activity for Cl^-, reflecting the opening at the cytoplasmic restriction of the ion conducting pathway even when Ca²⁺ is absent. Moreover, the ion selectivity of bBest2 is controlled by multiple residues, including those involved in gating.