The study investigates the functional properties of human and rat T1R receptors, which are part of the T1R class of taste-specific G protein-coupled receptors. The authors cloned and expressed human and rat T1R2/T1R3 and T1R1/T1R3 receptors in HEK-293T cells. They found that human T1R2/T1R3 receptors respond to a wide range of sweet taste stimuli, including sugars, amino acids, sweet proteins, and synthetic sweeteners. In contrast, human T1R1/T1R3 receptors respond to the umami taste stimulus l-glutamate, which is enhanced by 5′-ribonucleotides. The ligand-specificities of rat T1R2/T1R3 and T1R1/T1R3 correspond to those of their human counterparts. These findings suggest that different combinations of T1Rs function as heterodimeric sweet and umami taste receptors, with T1R2/T1R3 being the only sweet taste receptor and T1R1/T1R3 being the only umami taste receptor. The study also discusses the potential involvement of other receptors in umami taste and the challenges in characterizing T1R surface expression.The study investigates the functional properties of human and rat T1R receptors, which are part of the T1R class of taste-specific G protein-coupled receptors. The authors cloned and expressed human and rat T1R2/T1R3 and T1R1/T1R3 receptors in HEK-293T cells. They found that human T1R2/T1R3 receptors respond to a wide range of sweet taste stimuli, including sugars, amino acids, sweet proteins, and synthetic sweeteners. In contrast, human T1R1/T1R3 receptors respond to the umami taste stimulus l-glutamate, which is enhanced by 5′-ribonucleotides. The ligand-specificities of rat T1R2/T1R3 and T1R1/T1R3 correspond to those of their human counterparts. These findings suggest that different combinations of T1Rs function as heterodimeric sweet and umami taste receptors, with T1R2/T1R3 being the only sweet taste receptor and T1R1/T1R3 being the only umami taste receptor. The study also discusses the potential involvement of other receptors in umami taste and the challenges in characterizing T1R surface expression.