Anoctamin-4 is a bona fide Ca(2+)-dependent non-selective cation channel

Changes in cell function occur by specific patterns of intracellular Ca(2+), activating Ca(2+)-sensitive proteins. The anoctamin (TMEM16) protein family has Ca(2+)-dependent ion channel activity, which provides transmembrane ion transport, and/or Ca(2+)-dependent phosphatidyl-scramblase activity. Using amino acid sequence analysis combined with measurements of ion channel function, we clarified the so far unknown Ano4 function as Ca(2+)-dependent, non-selective monovalent cation channel; heterologous Ano4 expression in HEK293 cells elicits Ca(2+) activated conductance with weak selectivity of K(+) > Na(+) > Li(+). Endogenously expressed Ca(2+)-dependent cation channels in the retinal pigment epithelium were identified as Ano4 by KO mouse-derived primary RPE cells and siRNA against Ano4. Exchanging a negatively charged amino acid in the putative pore region (AA702–855) into a positive one (E775K) turns Ano4-elicited currents into Cl(−) currents evidencing its importance for ion selectivity. The molecular identification of Ano4 as a Ca(2+)-activated cation channel advances the understanding of its role in Ca(2+) signaling.