Atomic level characterization of the nonproton ligand-sensing domain of ASIC3 channels

Acid-sensing ion channels (ASICs) are known to be primarily activated by extracellular protons. Recently, we characterized a novel nonproton ligand (2-guanidine-4-methylquinazoline, GMQ), which activates the ASIC3 channel subtype at neutral pH. Using an interactive computational-experimental approach, here we extend our investigation to delineate the architecture of the GMQ-sensing domain in the ASIC3 channels. We first established a GMQ binding mode and revealed that residues Glu-423, Glu-79, Leu-77, Arg-376, Gln-271, and Gln-269 play key roles in forming the GMQ-sensing domain. We then verified the GMQ binding mode using ab initio calculation and mutagenesis and demonstrated the critical role of the above GMQ-binding residues in the interplay among GMQ, proton, and Ca(2+) in regulating the function of ASIC3. Additionally, we showed that the same residues involved in coordinating GMQ responses are also critical for activation of the ASIC3(E79C) mutant by thiol-reactive compound DTNB. Thus, a range of complementary techniques provide independent evidence for the structural details of the GMQ-sensing domain at atomic level, laying the foundation for further investigations of endogenous nonproton ligands and gating mechanisms of the ASIC3 channels.