Intracellular binding pocket revealed in the human bitter taste receptor TAS2R14
Bitter taste receptors (TAS2Rs), a subfamily of G-protein coupled receptors (GPCRs) expressed orally and extraorally, elicit signaling in response to a large set of ligands. Among the 25 functional TAS2Rs encoded in the human genome, TAS2R14 is the most promiscuous, and responds to hundreds of chemically diverse agonists. Here, we present the cryo-electron microscopy (cryo-EM) structure of the human TAS2R14 (hTAS2R14) in complex with its cognate signaling partner gustducin, and bound to flufenamic acid (FFA), a clinically approved nonsteroidal anti-inflammatory drug. The structure reveals an unusual binding mode for FFA, where two copies are bound at distinct binding pockets: one at the canonical GPCR site within the trans-membrane bundle, and the other in the intracellular facet, bridging the receptor with gustducin. Combined with site-directed mutagenesis and the design of a fluorescent FFA derivative for pocket-specific ligand binding BRET assays, our studies support a dual binding mode for FFA in TAS2R14. These results fill a gap in the understanding of bitter taste signaling and provide tools for guided design of TAS2R-targeted compounds.