Regulation of Bestrophins by Ca(2+): A Theoretical and Experimental Study

Bestrophins are a recently discovered family of Cl(−) channels, for which no structural information is available. Some family members are activated by increased intracellular Ca(2+) concentration. Bestrophins feature a well conserved Asp-rich tract in their COOH terminus (Asp-rich domain), which is homologous to Ca(2+)-binding motifs in human thrombospondins and in human big-conductance Ca(2+)- and voltage-gated K(+) channels (BK(Ca)). Consequently, the Asp-rich domain is also a candidate for Ca(2+) binding in bestrophins. Based on these considerations, we constructed homology models of human bestrophin-1 (Best1) Asp-rich domain using human thrombospondin-1 X-ray structure as a template. Molecular dynamics simulations were used to identify Asp and Glu residues binding Ca(2+) and to predict the effects of their mutations to alanine. We then proceeded to test selected mutations in the Asp-rich domain of the highly homologous mouse bestrophin-2. The mutants expressed in HEK-293 cells were investigated by electrophysiological experiments using the whole-cell voltage-clamp technique. Based on our molecular modeling results, we predicted that Asp-rich domain has two defined binding sites and that D301A and D304A mutations may impact the binding of the metal ions. The experiments confirmed that these mutations do actually affect the function of the protein causing a large decrease in the Ca(2+)-activated Cl(−) current, fully consistent with our predictions. In addition, other studied mutations (E306A, D312A) did not decrease Ca(2+)-activated Cl(−) current in agreement with modeling results.