The human sweet receptor, composed of the monomers T1R2 + T1R3, appears to be the main (and perhaps the only) receptor underlying sweet taste in humans. When co-expressed with a reporter G-protein in heterologous systems, this heterodimeric receptor responds to the full range of sweet-tasting compounds sensed by humans at appropriate concentrations. The sweet receptor responds to a surprisingly diverse set of ligands, from small amino acids to moderately sized sweet-tasting plant proteins. No common structure accounts for the sweetness of all of these compounds. Studies from our lab and others indicate that the sweet receptor can be activated by means of a variety of domains and distinct binding sites on the receptor. Using heterologous expression, calcium imaging, BRET, mutagenesis and computational modeling, my lab and those of my colleagues have described at least 4 binding regions of the sweet receptor: the venus fly trap module (VFTM) of hT1R2, the cysteine-rich domain (CRD) of hT1R3 and the transmembrane domain (TMD) of hT1R3. The current proposal focuses and builds on our recent discovery of overlapping binding pockets within the TMD of hT1R3 for agonists (cyclamate and analogs) and antagonists (lactisole and analogs). This domain is of critical importance for sweet receptor activation since all of the diverse ligands that humans perceive as sweet can be blocked by binding lactisole in the T1R3 TMD pocket. This suggests that this domain is the key element in the final conformational change leading to receptor activation. Since the sweet receptor can also be activated by ligands that bind in the TMD of T1R3 and these binding pockets share common residues, characterization of these binding pockets at a molecular level will provide insight into the differences between the ground and active state requirements oft the sweet receptor. We propose here to characterize the environment of the hT1R3 TMD responsible for both active and inactive conformations of the receptor using mutagenesis, heterologous expression and activity assays and computational modeling of ligand docking sites. In addition, through the collaborative effort outlined in the proposal, we will directly monitor the binding environment and binding of ligands to the TMD of T1R3 using STD-NMR, a powerful tool recently adapted to monitor the taste system. Our long-term goal is to elucidate the molecular events that underlie ligand binding and ligand induced activity (or stabilization of the ground state) and the conformational changes of the receptor required for G-protein activation.
There is today in the affluent countries of the world an epidemic of obesity, insulin-resistant diabetes and diet-related disorders. This is only made worse by our species evolutionary love affair with high- carbohydrate/energy-rich sweet foods. Taste perception and taste preference undoubtedly contribute to sweet-seeking behavior and food consumption. The identification of T1R2+T1R3 as the sweet receptor provides the target for intercession in modifying a behavior, which is maladaptive because of the plentiful food in affluent countries. Understanding the sweet receptor at a molecular level will enable the design of better low calorie sweeteners.
