Structural Basis of Ligand-Specific Orthosteric-Allosteric Coupling and Sensory Tuning in the Human Bitter Taste Receptor TAS2R14

The bitter taste receptor TAS2R14 recognizes hundreds of structurally diverse ligands, yet the mechanisms governing its broad promiscuity, graded agonist efficacy, and multi-site regulation remain unclear. Here, we report cryo-EM structures of TAS2R14–Gi ₁ complexes bound to 3,5-diiodosalicylic acid (DA), flufenamic acid (FA) and aristolochic acid (AA) at resolutions of 2.44 Å, 2.39 Å, and 2.69 Å, with a clear efficacy hierarchy of AA > FA > DA. Transmembrane helix 6 (TM6) acts as a core regulatory hub, with agonist binding triggering 12 Å lever-like TM6 rearrangements to reshape receptor interfaces and modulate G-protein coupling strength. We also identify cholesterol hemisuccinate (CHS) as a new TAS2R14 orthosteric agonist, selectively potentiating DA-mediated signaling potency by ∼5-fold via allosteric-orthosteric coupling without affecting FA or AA. Our findings show TM6 dynamics unify TAS2R14’s key functional features, advancing insights into bitter receptor activation and sterol regulation.