Candidate amino acids involved in H⁺ gating of acid-sensing ion channel 1a

Acid-sensing ion channels are ligand-gated cation channels, gated by extracellular H⁺. H⁺ is the simplest ligand possible, and whereas for larger ligands that gate ion channels complex binding sites in the three-dimensional structure of the proteins have to be assumed, H⁺ could in principle gate a channel by titration of a single amino acid. Experimental evidence suggests a more complex situation, however. For example, it has been shown that extracellular Ca²⁺ ions compete with H ⁺; probably Ca²⁺ ions bound to the extracellular loop of ASICs stabilize the closed state of the channel and have to be displaced before the channel can open. In such a scheme, amino acids contributing to Ca ²⁺ binding would also be candidates contributing to H⁺ gating. In this study we systematically screened more than 40 conserved, charged amino acids in the extracellular region of ASIC1a for a possible contribution to H⁺ gating. We identified four amino acids where substitution strongly affects H⁺ gating: Glu⁶³, His⁷²/ His⁷³, and Asp⁷⁸. These amino acids are highly conserved among H⁺-sensitive ASICs and are candidates for the "H ⁺ sensor" of ASICs.