Abstract
Although bitter taste receptors (TAS2Rs) are important for human health, little is known of the determinants of ligand specificity. TAS2Rs such as TAS2R16 help define gustatory perception and dietary preferences that ultimately influence human health and disease. Each TAS2R must accommodate a broad diversity of chemical structures while simultaneously achieving high specificity so that diverse bitter toxins can be detected without all foods tasting bitter. However, how these G protein-coupled receptors achieve this balance is poorly understood. Here we used a comprehensive mutation library of human TAS2R16 to map its interactions with existing and novel agonists. We identified 13 TAS2R16 residues that contribute to ligand specificity and 38 residues whose mutation eliminated signal transduction by all ligands, providing a comprehensive assessment of how this GPCR binds and signals. Our data suggest a model in which hydrophobic residues on TM3 and TM7 form a broad ligand-binding pocket that can accommodate the diverse structural features of β-glycoside ligands while still achieving high specificity.
Highlights
Human bitter taste receptors have evolved the ability to detect and respond to an enormous range of chemical classes such as β-glycosides, thioureas, and sesquiterpene lactones, with specific receptors able to respond to an array of related structures[5]
The entire TAS2R16 mutation library was transfected into human HEK-293T cells in a 384-well array format and evaluated for salicin-dependent activation measured by a calcium flux assay (Fig. 1a)
Salicin is highly prevalent in plants and the best characterized natural ligand of TAS2R16, and Ca2+-flux signaling assays are commonly used to measure the function of TAS2R16 and other GPCRs, so this measurement represents the key function of the receptor
Summary
Human bitter taste receptors have evolved the ability to detect and respond to an enormous range of chemical classes such as β-glycosides, thioureas, and sesquiterpene lactones, with specific receptors able to respond to an array of related structures[5]. Our understanding of TAS2R ligand recognition has come primarily from mutational analyses of small subsets of residues or from in silico models based on crystal structures of distantly related proteins such as rhodopsin[6, 16,17,18] Such studies broadly suggest a TAS2R ligand-binding pocket formed by several transmembrane (TM) domains ( TM3, TM5, TM6, and TM719). No crystal structures of any TAS2R receptor currently exist, and co-crystal structures with such low affinity ligands present an even larger challenge It remains unclear what structural mechanisms are used by these GPCRs to detect the enormous diversity of natural bitter compounds while simultaneously achieving high selectivity for specific types of molecules. Many of the critical residues identified are conserved among TAS2R family members, suggesting that the mechanisms used by TAS2R16 may be more broadly applicable to other TAS2Rs
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