Epithelial Na(+) channel (ENaC)/degenerin family members are involved in mechanosensation, blood pressure control, pain sensation, and the expression of fear. Several of these channel types display a form of desensitization that allows the channel to limit Na(+) influx during prolonged stimulation. We used site-directed mutagenesis and chemical modification, functional analysis, and molecular dynamics simulations to investigate the role of the lower palm domain of the acid-sensing ion channel 1, a member of the ENaC/degenerin family. The lower palm domains of this trimeric channel are arranged around a central vestibule, at ∼20 Å above the plasma membrane and are covalently linked to the transmembrane channel parts. We show that the lower palm domains approach one another during desensitization. Residues in the palm co-determine the pH dependence of desensitization, its kinetics, and the stability of the desensitized state. Mutations of palm residues impair desensitization by preventing the closing movement of the palm. Overexpression of desensitization-impaired channel mutants in central neurons allowed-in contrast to overexpression of wild type-a sustained signaling response to rapid pH fluctuations. We identify and describe here the function of an important regulatory domain that most likely has a conserved role in ENaC/degenerin channels.-Roy, S., Boiteux, C., Alijevic, O., Liang, C., Bernèche, S., Kellenberger, S. Molecular determinants of desensitization in an ENaC/degenerin channel.