Predicted Structure of Fully Activated Tas1R3/1R3′ Homodimer Bound to G Protein and Natural Sugars: Structural Insights into G Protein Activation by a Class C Sweet Taste Homodimer with Natural Sugars

Abstract

The Tas1R3 G protein-coupled receptor constitutes the main component of sweet taste sensory response in humans via forming a heterodimer with Tas1R2 or a homodimer with Tas1R3. The Tas1R3/1R3′ homodimer serves as a low-affinity sweet taste receptor, stimulating gustducin G protein (G Gust ) signaling in the presence of a high concentration of natural sugars. This provides an additional means to detect the taste of natural sugars, thereby differentiating the flavors between natural sugars and artificial sweeteners. We report here the predicted 3D structure of active state Tas1R3/1R3′ homodimer complexed with heterotrimeric G Gust and sucrose. We discovered that the G Gust makes ionic anchors to intracellular loops 1 and 2 of Tas1R3 while the Gα−α5 helix engages the cytoplasmic region extensively through salt bridge and hydrophobic interactions. We show that in the activation of this complex the Venus flytrap domains of the homodimer undergo a remarkable twist up to ∼100°r otation around the vertical axis to adopt a closed−closed conformation while the intracellular region relaxes to an open−open conformation. We find that binding of sucrose to the homodimer stabilizes a preactivated conformation with a largely open intracellular region that recruits and activates the G Gust . Upon activation, the Gα subunit spontaneously opens up the nucleotidebinding site, making nucleotide exchange facile for signaling. This activation of G Gust promotes the interdomain twist of the Venus flytrap domains. These structures and transformations could potentially be a basis for the design of new sweeteners with higher activity and less unpleasant flavors.

The mechanism for ligand activation of the GPCR–G protein complex

“…This opening of Gαq expedites GDP release, a critical event in activation of GP and GP signaling (45,46). Our metaD simulation finds that although opening the Gαq protein provides an exit path for GDP dissociation, the GDP remains bound to the Ras-like domain in our simulations, consistent with previous experimental (47,48) and computational (49,50) studies showing that GDP still remained only bound to the Ras-like domain even when the Gα subunit opens up.…”

Cholesterol in Class C GPCRs: Role, Relevance, and Localization

“…Due to these distinct structural features and mandatory dimerization, the class C GPCRs have been the most complex of the GPCRs in terms of understanding their activation mechanism [ 31 , 32 , 33 , 34 , 35 ]. Using several methods such as crystallization [ 30 ], lipid cubic phase [ 36 ], and most commonly single particle Cryo-EM, structures of over 20 human class C GPCRs have been solved to date [ 37 ], comprising metabotropic glutamate receptors (mGluR1–5,mGluR7) [ 36 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ], gamma-aminobutyric acid receptors (GABA 1 and GABA 2) [ 23 , 24 , 47 ], calcium-sensing receptors (CaS) [ 48 , 49 , 50 ], the extra-cellular domain of taste receptors (TAS1R1–TAS1R3) [ 51 , 52 , 53 , 54 , 55 ], and orphan receptors (GPR158, GPR179, GPR156) [ 56 , 57 , 58 , 59 , 60 ]. Similarly to other GPCR structures, class C GPCR structures are solved with inclusion of cholesterol or cholesteryl hemisuccinate (CHS) to the detergent mix during crystallization and recently, Cryo-EM ( Table 1 ).…”

Exploring the potential for an evolutionarily conserved role of the taste 1 receptor gene family in gut sensing mechanisms of fish

“…Paradoxically, the sa T1R3 gene encoding the shared subunit of receptors signaling both sugar (T1R2/T1R3) and protein (T1R1/T1R3) rich foods in mammals ( Roper, 1989 ; Hoon et al., 1999 ; Adler et al., 2000 ; Yarmolinsky et al., 2009 ), and responding to a wide spectrum of L-amino acids (T1R1/T1R3 and T1R2 n /T1R3) in fish ( Oike et al., 2007 ), was among the lowest expressed gene throughout the larval stages analyzed. It is believed that alternative dimeric arrangements among T1R subunits might also potentially occur in cell membranes, including homodimerization among the highest expressed sa T1R subunits ( Damak et al, 2003 ; Herness, 2018 ), albeit these types of protein combinations have mainly been reported for the T1R3 homodimer ( Masubuchi et al., 2013 ; Kojima et al., 2014 ; Lee and Cohen 2015 ; Mafi et al., 2021 ). Therefore, the reason behind these unexpected differences in gene expression levels remains elusive at present.…”