Abstract
Insects sense the taste of foods and toxic compounds in their environment through the gustatory system. Genetic studies using fruit flies have suggested that putative seven-transmembrane gustatory receptors (Grs) expressed in gustatory sensory neurons are required for responses to specific tastants. We reconstituted sugar responses of Bombyx mori Gr-9 (BmGr-9), a silkworm Gr, in two heterologous expression systems. Xenopus oocytes or HEK293T cells expressing BmGr-9 selectively responded to D-fructose with an influx of extracellular Ca(2+) and a nonselective cation current conductance in a G protein-independent manner. Outside-out patch-clamp recording of BmGr-9-expressing cell membranes provides evidence supporting the hypothesis that BmGr-9 constitutes a ligand-gated ion channel. The fructose-activated current associated with BmGr-9 was suppressed by other hexoses, including glucose and sorbose. The activation and inhibition of insect Gr ion channels may be the molecular basis for the decoding system that discriminates subtle differences in sweet taste. Finally, Drosophila melanogaster Gr43a (DmGr43a), a BmGr-9 ortholog, also responded to D-fructose, suggesting that DmGr43a relatives appear to compose the family of fructose receptors.
