Evolutionary tuning of an auditory transduction channel.

TMC1 and TMC2 are mechanosensory ion channels of the vertebrate inner ear that mediate hearing and balance. How these channels open in response to mechanical force remains unresolved. Through comparative analyses of TMCs across eukaryote species, we find that TMC1 and TMC2 arose in vertebrates by gene duplication and evolved elaborate extracellular loops. Structural models demonstrate that the loop between transmembrane domains 1 and 2 arches over the channel pore and lies near TMIE, an auxiliary protein essential for function. In mammals, this loop shows signatures of positive selection and contains multiple sites linked to hereditary deafness, consistent with TMC1's specialization for auditory function. Electrophysiological recordings from mouse Tmc1/Tmc2-null cochlear hair cells expressing TMC1 variants demonstrate that alterations within this loop affect channel activation, identifying it as a modulatory feature that has been refined through structural adaptation.