Structural and functional characterization of the bestrophin-2 anion channel.

Abstract

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 (Best1-4). Here we report cryo-EM structures of bovine Best2 (bBest2) bound and unbound by Ca2+ 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 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.