Structural and functional characterization of bitter taste receptors, T2R1 and T2R4

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2011, 2012, 2012, 2013, 2014
Pydi, Sai Prasad
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American Society for Biochemistry and Molecular Biology
John Wiley and Sons
Bentham Science Publishers Ltd
Elsevier Ltd
Elsevier Ltd
In humans, taste is one of the five senses, and helps in the recognition of nutritionally important and potentially harmful substances. It triggers innate behaviour to accept or reject food. Humans can sense five basic tastes, which are sweet, umami, bitter, salt and sour. The receptors that mediate bitter, sweet and umami tastes belong to the G protein-coupled receptor (GPCR) superfamily. A group of three receptors sense sweet and umami tastes, whereas bitter taste is sensed by 25 bitter taste receptors (referred as T2Rs). T2Rs are activated by structurally diverse natural and synthetic bitter compounds. Many common pharmaceutical compounds are bitter in taste and these are effective ligands for T2Rs. Recent finding of T2Rs in extra-oral tissues suggests these receptors are also involved in various physiological and pathophysiological processes. To understand the structure and function of these receptors, studies directed at elucidating their mechanisms of activation, and identification of novel ligands including bitter blockers (antagonists and inverse agonists), are required. To obtain mechanistic insights into the role of the highly conserved, and receptor specific residues, two bitter taste receptors (T2R1 and T2R4) were targeted. In this study, a combination of molecular, biochemical and pharmacological approaches were used to identify the amino acids and motifs, important for T2Rs to switch from inactive to active state. A hydrogen-bonding network between transmembrane (TM) helices 1-2-7 was identified as important for T2R activation. Alanine-scan mutagenesis of intracellular loops (ICLs) 2 and 3 identified T2R regions important for G protein binding, and receptor activation. A pharmacological method was developed, to screen potential bitter blockers for T2Rs. Using this method, three novel bitter blockers, which include two natural antagonists and one synthetic inverse agonist for T2R4, were discovered. The role of expression tags in enhancing T2R4 expression was also pursued. T2R4 expression on the cell surface was increased 2.5 fold, when its N-terminus was tagged with rhodopsin N-terminal 33 residues (Rho33- T2R4 chimera). In conclusion, work carried out provides novel insights into the mechanisms of T2R activation, and in the discovery of bitter blockers for T2R4.
Bitter taste receptors, GPCRs, Taste, Bitter blockers, Over expression, Constitutive activity
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