Conserved Residues Control the T1R3-Specific Allosteric Signaling Pathway of the Mammalian Sweet-Taste Receptor

Abstract

Mammalian sensory systems detect sweet taste through the activation of a single heteromeric T1R2/T1R3 receptor belonging to class C G-protein-coupled receptors. Allosteric ligands are known to interact within the transmembrane domain, yet a complete view of receptor activation remains elusive. By combining site-directed mutagenesis with computational modeling, we investigate the structure and dynamics of the allosteric binding pocket of the T1R3 sweet-taste receptor in its apo form, and in the presence of an allosteric ligand, cyclamate. A novel positively charged residue at the extracellular loop 2 is shown to interact with the ligand. Molecular dynamics simulations capture significant differences in the behavior of a network of conserved residues with and without cyclamate, although they do not directly interact with the allosteric ligand. Structural models show that they adopt alternate conformations, associated with a conformational change in the transmembrane region. Site-directed mutagenesis confirms that these residues are unequivocally involved in the receptor function and the allosteric signaling mechanism of the sweet-taste receptor. Similar to a large portion of the transmembrane domain, they are highly conserved among mammals, suggesting an activation mechanism that is evolutionarily conserved. This work provides a structural basis for describing the dynamics of the receptor, and for the rational design of new sweet-taste modulators.

Systematic analysis reveals novel insight into the molecular determinants of function, diversity and evolution of sweet taste receptors T1R2/T1R3 in primates

“…Therefore, it appears that the proposed residues aforesaid in the binding pocket are insufficient to account for the species-dependent taste toward cyclamate. In support of this, it has been widely accepted that other regions beyond the binding sites can also modulate the responses toward sweeteners via allosteric regulation, thus determining the sensitivity ( Chéron et al, 2019 ). Moreover, although the potential binding sites of some other sweeteners (e.g., neohesperidin dihydrochalcone and saccharin) have been identified ( Winnig et al, 2007 ; Masuda et al, 2012 ), no corresponding behavioral or physiological results are available now to interpret these mechanical findings, and an accurate elucidation of the relationships between the molecular determining residues and species-dependent sweet taste should wait for resolution of the spatial structures of sweetener-receptor complexes in the future.…”

“…A plausible explanation is that the in vitro function of sweet taste receptor of some species may not fully reflect the behavioral outcome of response toward some sweeteners, such as an averse reaction that could be a positive response in cell-based assays (van Giesen et al, 2016). Alternatively, other residues could be involved in mediating the non-sensitivity to aspartame of sweet taste receptors in prosimians species (Chéron et al, 2019). Further investigations on the sensitivities of sweet taste receptors in prosimians species toward Frontiers in Molecular Biosciences frontiersin.org aspartame as well as their determinative residues should be interesting and informative.…”

<strong></strong><strong>Anosmia and Ageusia as Initial or Unique Symptoms after SARS-COV-2 Virus Infection</strong>

“…The primary gustatory cortex is where the perception of a particular taste is processed. 64,66,71,76,[83][84][85][86][87][88][89][90][91][92]…”

Biophysical and functional characterization of the human TAS1R2 sweet taste receptor overexpressed in a HEK293S inducible cell line

“…One binding site is located in the TAS1R3-VFT module, where natural sugars (sucrose, fructose and glucose) and the chlorodeoxysugar sucralose have been found to bind 13 , 18 . Another binding site is located in TAS1R3-TMD, where the sweeteners cyclamate and neohesperidin dihydrochalcone and the sweet taste inhibitors lactisole and gymnemic acid bind 19 – 23 . Although the functional role of the CRD of TAS1R3 remains to be elucidated, it has been shown that this domain is also involved in the response to sweet tasting proteins, including brazzein and thaumatin 24 , 25 .…”