Distinct regions that control ion selectivity and calcium-dependent activation in the bestrophin ion channel

Proc Natl Acad Sci U S A. 2016 Nov 22;113(47):E7399-E7408. doi: 10.1073/pnas.1614688113. Epub 2016 Nov 7.


Cytoplasmic calcium (Ca2+) activates the bestrophin anion channel, allowing chloride ions to flow down their electrochemical gradient. Mutations in bestrophin 1 (BEST1) cause macular degenerative disorders. Previously, we determined an X-ray structure of chicken BEST1 that revealed the architecture of the channel. Here, we present electrophysiological studies of purified wild-type and mutant BEST1 channels and an X-ray structure of a Ca2+-independent mutant. From these experiments, we identify regions of BEST1 responsible for Ca2+ activation and ion selectivity. A "Ca2+ clasp" within the channel's intracellular region acts as a sensor of cytoplasmic Ca2+. Alanine substitutions within a hydrophobic "neck" of the pore, which widen it, cause the channel to be constitutively active, irrespective of Ca2+. We conclude that the primary function of the neck is as a "gate" that controls chloride permeation in a Ca2+-dependent manner. In contrast to what others have proposed, we find that the neck is not a major contributor to the channel's ion selectivity. We find that mutation of a cytosolic "aperture" of the pore does not perturb the Ca2+ dependence of the channel or its preference for anions over cations, but its mutation dramatically alters relative permeabilities among anions. The data suggest that the aperture functions as a size-selective filter that permits the passage of small entities such as partially dehydrated chloride ions while excluding larger molecules such as amino acids. Thus, unlike ion channels that have a single "selectivity filter," in bestrophin, distinct regions of the pore govern anion-vs.-cation selectivity and the relative permeabilities among anions.

Keywords: CaCC; calcium-activated chloride channel; channel gating; ion channel biophysics; ion selectivity.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alanine / genetics
  • Amino Acid Substitution
  • Animals
  • Bestrophins / chemistry*
  • Bestrophins / genetics
  • Bestrophins / metabolism*
  • Calcium / metabolism*
  • Chickens / metabolism*
  • Chloride Channels / metabolism
  • Crystallography, X-Ray
  • Cytoplasm / metabolism
  • Models, Molecular
  • Protein Conformation


  • Bestrophins
  • Chloride Channels
  • Alanine
  • Calcium

Associated data

  • PDB/5T5N