Epitope tagging with genome editing in mice reveals that the proton channel OTOP1 is apically localized and not restricted to Type III "sour" taste receptor cells.

The gustatory system allows animals to assess the nutritive value and safety of foods prior to ingestion. The first step in gustation is the interaction of taste stimuli with one or more specific sensory receptors, that are generally believed to be present on the apical surface of the taste receptor cells. However, this assertion is rarely tested.

The proton channel OTOP1 is a sensor for the taste of ammonium chloride.

Ammonium (NH), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NHCl).

Zinc activation of OTOP proton channels identifies structural elements of the gating apparatus.

Otopetrin proteins (OTOPs) form proton-selective ion channels that are expressed in diverse cell types where they mediate detection of acids or regulation of pH. In vertebrates there are three family members: OTOP1 is required for formation of otoconia in the vestibular system and it forms the receptor for sour taste, while the functions of OTOP2 and OTOP3 are not yet known. Importantly, the gating mechanisms of any of the OTOP channels are not well understood.

Structural motifs for subtype-specific pH-sensitive gating of vertebrate otopetrin proton channels.

Otopetrin (OTOP) channels are proton-selective ion channels conserved among vertebrates and invertebrates, with no structural similarity to other ion channels. There are three vertebrate OTOP channels (OTOP1, OTOP2, and OTOP3), of which one (OTOP1) functions as a sour taste receptor. Whether extracellular protons gate OTOP channels, in addition to permeating them, was not known.

The evolution of sour taste.

The evolutionary history of sour taste has been little studied. Through a combination of literature review and trait mapping on the vertebrate phylogenetic tree, we consider the origin of sour taste, potential cases of the loss of sour taste, and those factors that might have favoured changes in the valence of sour taste-from aversive to appealing. We reconstruct sour taste as having evolved in ancient fish.