Activation mechanism of the G protein-coupled sweet receptor heterodimer with sweeteners and allosteric agonists

Abstract

The sweet taste in humans is mediated by the TAS1R2/TAS1R3 G protein-coupled receptor (GPCR), which belongs to the class C family that also includes the metabotropic glutamate and γ-aminobutyric acid receptors. We report here the predicted 3D structure of the full-length TAS1R2/TAS1R3 heterodimer, including the Venus Flytrap Domains (VFDs) [in the closed-open (co) active conformation], the cysteine-rich domains (CRDs), and the transmembrane domains (TMDs) at the TM56/TM56 interface. We observe that binding of agonists to VFD2 of TAS1R2 leads to major conformational changes to form a TM6/TM6 interface between TMDs of TAS1R2 and TAS1R3, which is consistent with the activation process observed biophysically on the metabotropic glutamate receptor 2 homodimer. We find that the initial effect of the agonist is to pull the bottom part of VFD3/TAS1R3 toward the bottom part of VFD2/TAS1R2 by ∼6 Å and that these changes get transmitted from VFD2 of TAS1R2 (where agonists bind) through the VFD3 and the CRD3 to the TMD3 of TAS1R3 (which couples to the G protein). These structural transformations provide a detailed atomistic mechanism for the activation process in GPCR, providing insights and structural details that can now be validated through mutation experiments.

Keywords: GPCR activation; class C GPCR; molecular dynamics; noncaloric sweetener.

Fig. 1.

Predicted best binding modes for Stev and MogV bound to the VFD (VFD2) of the human sweet taste receptor (TAS1R2/1R3). The predicted pharmacophore is at the bottom. The unified cavity binding components are in SI Appendix, Table S11.

Fig. 2.

Side views of the 3D structure of the (A) apo- and (B) MogV-bound TAS1R2 (red)/1R3 (blue) heterodimer. The MogV agonist is shown in VFD2 as a yellow space-filling model, whereas the S819 agonist modulator is the yellow structure at the EC part of TMD2. SI Appendix, Fig. S2 shows a more detailed binding site for the S819 allosteric agonist. The yellow arrows between VFD2 and VFD3 show the separation (Å) between the geometric center of lower VFD2 and lower VFD3 (VFD2–VFD3 in Table 1), whereas yellow arrows between TMD2 and TMD3 show the distance (Å) between the closest Cα of TM6/TMD2 with a Cα of TM6’/TMD3 (Dist TM6-6' Cα in Table 1). These numbers are in SI Appendix, Table S13 for all 11 cases.

Fig. 3.

The conformational rearrangement at the TMD interface of heterodimers of sweet taste receptor TAS1R2 (red)/TAS1R3 (blue) from the start (Left) to the end (Right) after 20 ns of MD. Viewed from the EC view (Top) and the cytoplasmic view (Bottom). The original TM56/TM56 interface changes by moving the upper TM6 helices closer, leading to a TM6/TM6 interface mainly at the EC side. This motion is observed experimentally for the mGlu2 homodimer. The EC views for all 11 cases are shown in SI Appendix, Fig. S15.

Fig. 4.

Trajectory analysis of the shortest Cα distance (Å) between TM6 of TAS1R2 and TAS1R3 TMD after 10 ns of MD. We see that the fix CRD2 case is the same as no fix, indicating no effect on activation, whereas the fix-VFD3 and fix-CRD3 cases show no activation (similar to apo protein) The fix-TMD2 and fix-TMD3 cases are intermediate. The total number of Cα between the two TM6s within 9 Å is shown in SI Appendix, Fig. S16.