Ca(2+)-activated Cl(−) Current from Human Bestrophin-4 in Excised Membrane Patches

Bestrophins are a newly discovered family of Cl(−) channels, some members of which are activated by intracellular Ca(2+). So far, all studies were carried out with whole-cell recordings from plasmid-transfected cultured cells, so it is unclear whether Ca(2+) activates bestrophin through a metabolic mechanism or in a more direct way. We report here experiments that addressed this question with excised, inside-out membrane patches. We chose human bestrophin-4 (hBest4) for heterologous expression because it gave particularly large Cl(−) currents when expressed, thus allowing detection even in excised membrane patches. hBest4 gave a negligible Cl(−) current in a Ca(2+)-free solution on the cytoplasmic (bath) side, but produced a Cl(−) current that was activated by Ca(2+) in a dose-dependent manner, with a K (1/2) of 230 nM. Thus, Ca(2+) appears to activate the bestrophin Cl(−) channel without going through a freely diffusible messenger or through protein phosphorylation. Because the activation and deactivation kinetics were very slow, however, we cannot exclude the involvement of a membrane-associated messenger.