Bitter taste receptors (TAS2Rs) belong to the G protein-coupled receptor (GPCR) superfamily, which generate signaling through coupling to both the G protein and an agonist. Interestingly, TAS2Rs have been reported recently to be expressed in many extraoral tissues and are known to be involved in health and disease, presenting attractive therapeutic targets. Indeed, TAS2Rs-associated disorders and diseases include cancer, asthma, and cardiovascular diseases. Despite these important roles for TAS2Rs in biological functions and diseases, no X-ray or Cryo-electron microscopy structure is available to provide an understanding of the signal transduction mechanism or to identify the agonist or antagonist binding sites for the 25 TAS2R subtypes. Thus, there is no basis for in silico discovering the endogenous ligands or developing selective ligands with high affinity as new ligands to discover extraoral function or new therapeutic drugs. We report here the three-dimensional structures of the fully activated human TAS2R4 and TAS2R5 predicted using the GEnSeMBLE complete sampling method. We also report the coupling of these GPCRs to cognate G proteins and to several agonists. We discovered that the G protein couples to TAS2Rs through strong salt bridges to each of the three intracellular loops, orienting the Gα5 helix of the Gα subunit to interact extensively with the cytoplasmic region of the GPCR. We show that TAS2Rs exhibit unique motifs compared to typical class A GPCRs, leading to a distinct activation mechanism and a less stable inactive state. We identify several residues in the binding site that play a critical role in ligand binding and activation processes. For TAS2R4, D92 provides a favorable binding energy through polar interactions with an agonist, while triggering rotation of Y239 by changing the hydrogen bond network. For TAS2R5, S89 and Q90 form polar interactions with the 1,10-phenanthroline agonist, which is known to bind specifically to TAS2R5 among the 25 subtypes. Our predicted results explain the subtype selectivity of the agonist and correlate well with experimental results of signaling coefficients against 19 agonists (R2~0.85). These predicted fully activated bitter taste receptor complex structures provide insights into the signal transduction mechanism, a guide to structure-based drug design, and mutation sites for definitive experimental validation of the predicted structures.

Structures and binding sites of human bitter taste receptors complexed with G protein and agonists

ACS Spring 2022

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