Figure 5

Speculative models for the Gur binding sites of T1r2/T1r3 sweet receptor and GS and GI receptor and neural systems in 3 strains of mice (C57BL, BALB and dpa CG). Upper panel: a major binding site of Gur may be the extracellular domain of the T1r3. Gur may also possess minor binding sites at the extracellular and transmembrane domains of T1r2. Lower panels: the GS system of C57BL mice may possess Gur binding sites at both extracellular and transmembrane domains of T1r2 in addition to its major binding site for T1r3, whereas the Gur-weakly-sensitive (GWS) system of BALB may possess a Gur binding site at only the transmembrane domain of T1r2 in addition to the extracellular domain of T1r3. It remains unclear whether dpa CG may possess the GS system or the GWS system, although in this figure we tentatively label it as the GS system. In all three strains, the GI system may have Gα proteins other than Gα-gustducin in the anterior tongue innervated by the chorda tympani (CT) and posterior tongue innervated by the glossopharyngeal (GL) nerves. The proportion of GS vs GI components in the anterior tongue may be dependent on co-expression rates of Gα-gustducin and T1r2/T1r3; C57BL and dpa CG mice possess almost identical sizes of GS and GI population, whereas BALB mice possess a much larger size of GI population.