Identification of the modulatory Ca-binding sites of acid-sensing ion channel 1a.

Acid-sensing ion channels (ASICs) are neuronal Na-permeable ion channels activated by extracellular acidification. ASICs are involved in learning, fear sensing, pain sensation and neurodegeneration. Increasing the extracellular Ca concentration decreases the H sensitivity of ASIC1a, suggesting a competition for binding sites between H and Ca ions. Here, we predicted candidate residues for Ca binding on ASIC1a, based on available structural information and our molecular dynamics simulations. With functional measurements, we identified several residues in cavities previously associated with pH-dependent gating, whose mutation reduced the modulation by extracellular Ca of the ASIC1a pH dependence of activation and desensitization. This occurred likely owing to a disruption of Ca binding. Our results link one of the two predicted Ca-binding sites in each ASIC1a acidic pocket to the modulation of channel activation. Mg regulates ASICs in a similar way as does Ca. We show that Mg shares some of the binding sites with Ca. Finally, we provide evidence that some of the ASIC1a Ca-binding sites are functionally conserved in the splice variant ASIC1b. Our identification of divalent cation-binding sites in ASIC1a shows how Ca affects ASIC1a gating, elucidating a regulatory mechanism present in many ion channels.