Modeling and Structural Characterization of the Sweet Taste Receptor Heterodimer

Sweet taste receptor, a heterodimer belonging to the class C G-protein coupled receptor (GPCR) family and composed of the T1R2 and T1R3 subunits, is responsible for the perception of natural sugars, sweet proteins, various o-amino acids, as well as artificial sweeteners. Despite the critical importance of the sweet receptor not only in mediating gustation but also in its role in the food industry, the architecture of the TIR2-TIR3 complex and the mechanism by which extracellular stimuli induce conformational changes that are propagated to the intracellular milieu, i.e., the signal transduction pathway, remain largely unknown. Here, we constructed and characterized a full-length structural model of the T1R2-T1R3 receptor, including both the transmembrane (TM) and extracellular (EC) domains of the heterodimer, using comparative modeling and extensive all-atom molecular dynamics simulations. Several heterodimer interfaces were first examined for the TM domain, and conforma- tional changes occurring at the intracellular side and associated with the receptor's activation were characterized. From the analysis on the simulated data, putative allosteric binding sites for ligands, ions, and cholesterol were proposed. Also, insights into the protein interface of the TM domain upon activation are provided. The EC domain of the heterodimer, including both the Venus flytrap and cysteine-rich domains, was also investigated. Several important intersubunit interactions located at regions responsible for the receptor's proper function were observed, which resemble those recently identified in other class C GPCR members. Integration of the results from the TM and EC domains facilitates the generation of a full-length T1R2-T1R3 receptor. These findings along with the full-length structural model of the TIR2-T1R3 receptor provide a structural framework that may assist in understanding the mechanistic details associated with the receptor activation process for the sweet T1R2- T1R3 receptor as well as other members of the same family.