Abstract

Bestrophin (BEST1-4 in humans) channels are ligand gated chloride (Cl-) channels that are activated by calcium (Ca2+). Mutations in BEST1 cause retinal degenerative diseases. Partly because these channels have no sequence or structural similarity to other ion channels, the molecular mechanisms underlying gating are unknown. Here, we present a series of cryo-electron microscopy (cryo-EM) structures of chicken BEST1, determined at 3.1 [A] resolution or better, that represent the principal gating states of the channel. Unlike other channels, opening of the pore is due to the repositioning of tethered pore-lining helices within a surrounding protein shell that dramatically widens a \"neck\" of the pore through a concertina of amino acid rearrangements within the protein core. The neck serves as both the activation and the inactivation gate. The binding of Ca2+ to a cytosolic domain instigates pore opening and the structures reveal that, unlike voltage-gated Na+ and K+ channels, similar molecular rearrangements are responsible for inactivation and deactivation. A single aperture within the 95 [A]-long opened pore separates the cytosol from the extracellular milieu and controls anion permeability. The studies define the basis for Ca2+-activated Cl- channel function and reveal a new molecular paradigm for gating in ligand-gated ion channels.