Proton-selective ion channel (PubMed:29371428, PubMed:36266567). Biphasically modulated by acid and alkali, mediating proton influx and efflux in response to extracellular acid and base stimulation, respectively. Sour taste receptor, which carries inward currents in response to extracellular acidification (By similarity). Sensor for ammonium chloride (NH(4)Cl) in taste receptor cells (PubMed:37798269). NH(4)Cl acts by increasing the intracellular pH, thereby generating a driving force for proton entry through OTOP1 channel (PubMed:37798269). Might also participate in alkaline sensation. Plays a role in the regulation of Ca(2+) flux in response to purigenic (ATP, ADP and UDP) stimuli, leading to increase in cytosolic Ca(2+) due to influx of extracellular calcium. May play this role by inhibiting P2Y purinoceptor-mediated Ca(2+) release in a Ca(2+)- dependent manner and promote an influx of Ca(2+) in response to ATP. Through this mechanism and possibly others, plays a role in the formation and function of calcium carbonate-based structures in the vestibular system of the inner ear, called otoconia, that sense gravity and linear acceleration. In obesity, may attenuate adipose tissue inflammation, through the negative regulation of IFNG signaling, hence may play an adaptive role in the maintainance of metabolic homeostasis. Following alkali activation, may also be permeable Na(+), K(+), Cs(+) and Li(+) (By similarity). {ECO:0000250|UniProtKB:Q80VM9, ECO:0000269|PubMed:29371428, ECO:0000269|PubMed:36266567, ECO:0000269|PubMed:37798269}. This is the function of ENSG00000163982 (OTOP1, otopetrin 1).