Identification of the modulatory Ca2+ binding sites of acid-sensing ion channel 1a
Acid-sensing ion channels (ASICs) are neuronal H+-gated, Na+-permeable channels involved in learning, fear sensing, pain sensation and neurodegeneration. An increase in the extracellular Ca2+ concentration shifts the pH dependence of ASIC1a to more acidic values. Here, we predicted candidate residues for Ca2+ binding on ASIC1a, based on available structural information and molecular dynamics simulations; the function of channels carrying mutations of these residues was then measured. We identify several residues in cavities previously associated with pH-dependent gating, whose mutation decreased the Ca2+-induced shift in ASIC1a pH dependence, likely due to a disruption of Ca2+ binding. We show also that Mg2+ shares some of the binding sites with Ca2+, and that some of the Ca2+ binding sites are functionally conserved in the splice variant ASIC1b. Our identification of divalent cation binding sites in ASIC1a shows how Ca2+ affects ASIC1a gating, elucidating a regulatory mechanism present in many ion channels.