Structure-Function Analyses of Human Bitter Taste Receptors—Where Do We Stand?

The finding that bitter taste receptors are expressed in numerous tissues outside the oral cavity and fulfill important roles in metabolic regulation, innate immunity and respiratory control, have made these receptors important targets for drug discovery. Efficient drug discovery depends heavily on detailed knowledge on structure-function-relationships of the target receptors. Unfortunately, experimental structures of bitter taste receptors are still lacking, and hence, the field relies mostly on structures obtained by molecular modeling combined with functional experiments and point mutageneses. The present article summarizes the current knowledge on the structure–function relationships of human bitter taste receptors. Although these receptors are difficult to express in heterologous systems and their homology with other G protein-coupled receptors is very low, detailed information are available at least for some of these receptors.

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Functional Microswitches of Mammalian G Protein-Coupled Bitter-Taste Receptors

10.1101/2020.10.23.348706 ◽

2020 ◽

Author(s):

Jérémie Topin ◽

Cédric Bouysset ◽

Yiseul Kim ◽

MeeRa Rhyu ◽

Sébastien Fiorucci ◽

...

Keyword(s):

G Protein ◽

Bitter Taste ◽

Current Knowledge ◽

Receptor Activation ◽

Amino Acid Sequences ◽

Site Directed Mutagenesis ◽

Taste Receptors ◽

Bitter Taste Receptors ◽

G Protein Coupled

AbstractBitter taste receptors (TAS2Rs) are a poorly understood subgroup of G protein-coupled receptors (GPCR). No experimental structure of these receptors is available and key-residues controlling their function remain mostly unknown. Here, we have identified the functional microswitches that encode agonist sensing and downstream signaling mechanisms within TAS2Rs sequences. We thoroughly re-aligned the amino-acid sequences of the 25 human TAS2Rs considering residue conservations and all the experimental data from the literature as constraints. As a test case, an accurate homology model of TAS2R16 was constructed and examined by site-directed mutagenesis and in vitro functional assays. Conserved motifs acting as microswitches during agonist-sensing and receptor activation were pinpointed by comparison with the current knowledge on class A GPCRs. Unravelling these sequence – function relationships is of utmost importance to streamline how TAS2Rs functions are encrypted in their sequence.

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The short third intracellular loop and cytoplasmic tail of bitter taste receptors provide functionally relevant GRK phosphorylation sites in TAS2R14

Journal of Biological Chemistry ◽

10.1074/jbc.ra120.016056 ◽

2020 ◽

pp. jbc.RA120.016056

Author(s):

Donghwa Kim ◽

Maria Castaño ◽

Lauren K Lujan ◽

Jung A. Woo ◽

Stephen B. Liggett

Keyword(s):

Bitter Taste ◽

Taste Receptors ◽

Intracellular Loop ◽

Early Endosomes ◽

Bitter Taste Receptors ◽

Novel Target ◽

Intracellular Loops ◽

G Protein Coupled ◽

Gst Fusion Proteins

For most GPCRs, the third intracellular loops (IL3) and C-terminal tails (CT) are sites for GRK-mediated phosphorylation, leading to b-arrestin binding and agonist-specific desensitization. These regions of the G protein-coupled bitter taste receptors (TAS2Rs) are short compared to the superfamily, and their functional role is unclear. TAS2R14 expressed on human airway smooth muscle (HASM) cells relax the cell, suggesting a novel target for bronchodilators. To assess IL3 and CT in agonist-promoted TAS2R14 desensitization (tachyphylaxis), we generated GST-fusion proteins of both the WT sequence and Ala substituted for Ser/Thr in the IL3 and CT sequences. In vitro, activated GRK2 phosphorylated both WT IL3 and WT CT proteins but not Ala-substituted forms. Next, TAS2R14s with mutations in IL3 (IL-5A), CT (CT-5A) and in both regions (IL/CT-10A) were expressed in HEK-293T cells. IL/CT-10A and CT-5A failed to undergo desensitization of the [Ca2+]i response compared to WT, indicating functional desensitization by GRK-phosphorylation is at residues in the CT. Short-term desensitization of TAS2R14 was blocked by GRK2 knockdown in HASM cells. Receptor:b-arrestin binding was absent with IL/CT-10A and CT-5A, but was also reduced in IL-5A, indicating a role for IL3 phosphorylation in the b-arrestin interaction for this function. Agonist-promoted internalization of IL-5A and CT-5A receptors was impaired and these receptors failed to colocalize with early endosomes. These results show that agonist-promoted functional desensitization of TAS2R14 occurs by GRK phosphorylation of CT residues and b-arrestin binding. However, b-arrestin function in the internalization and trafficking of the receptor requires cooperative GRK phosphorylation of IL3 and CT residues.

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Extraoral bitter taste receptors in health and disease

The Journal of General Physiology ◽

10.1085/jgp.201611637 ◽

2017 ◽

Vol 149 (2) ◽

pp. 181-197 ◽

Cited By ~ 75

Author(s):

Ping Lu ◽

Cheng-Hai Zhang ◽

Lawrence M. Lifshitz ◽

Ronghua ZhuGe

Keyword(s):

Innate Immunity ◽

Bitter Taste ◽

G Protein Coupled Receptors ◽

The Body ◽

Taste Perception ◽

Taste Receptors ◽

Human Disorders ◽

Health And Disease ◽

Bitter Taste Receptors ◽

G Protein Coupled

Bitter taste receptors (TAS2Rs or T2Rs) belong to the superfamily of seven-transmembrane G protein–coupled receptors, which are the targets of >50% of drugs currently on the market. Canonically, T2Rs are located in taste buds of the tongue, where they initiate bitter taste perception. However, accumulating evidence indicates that T2Rs are widely expressed throughout the body and mediate diverse nontasting roles through various specialized mechanisms. It has also become apparent that T2Rs and their polymorphisms are associated with human disorders. In this review, we summarize the physiological and pathophysiological roles that extraoral T2Rs play in processes as diverse as innate immunity and reproduction, and the major challenges in this emerging field.

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