Figure 1. Cells and receptors for sour taste.

(A) A taste bud is composed of ~50–100 cells, of which ~15% are Type III TRCs (yellow). The Type III TRCs extend an apical process to the epithelial surface through a taste pore and are innervated by nerve fibers that send information to gustatory nuclei. (B) The transduction cascade from detection of acid stimuli to release of neurotransmitter is illustrated. In response to acidic stimuli, the sour receptor, OTOP1, conducts H⁺ ions (protons) into the cell cytosol. This changes the membrane potential (V_m) directly, and the change in intracellular pH blocks KIR2.1 K⁺ channels which further depolarizes the membrane potential. With sufficient depolarization, voltage-gated Na⁺ channels open causing a train of action potentials that open voltage-gated calcium channels and lead to neurotransmitter release. (C) The structure of the sour receptor, OTOP1, as solved by cryo-EM. A side view shows the 12 transmembrane helices, which are divided into two structurally similar N and C domains. A top down view shows that the channel assembles as a dimer. Possible permeation pathways for protons (arrows), are found in the N domain, the C domain, and in the intrasubunit interface between N and C domains (Based on structure reported in [25]).