Structural and Functional Characterization of the Bestrophin-2 Anion Channel

The bestrophin family of calcium (Ca(2+))-activated chloride (Cl(−)) channels, which mediate the influx and efflux of monovalent anions in response to the levels of intracellular Ca(2+), comprises four members in mammals (Best1-4). Here we report cryo-EM structures of bovine Best2 (bBest2) bound and unbound by Ca(2+) at 2.4 Å and 2.2 Å, 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 Ca(2+), it has substantial Ca(2+)-independent activity for Cl(−), reflecting the opening at the cytoplasmic restriction of the ion conducting pathway even when Ca(2+) is absent. Moreover, the ion selectivity of bBest2 is controlled by multiple residues including those involved in gating.