Stereospecificity of the receptor site for glycerol, a new sweetener, in a labellar sugar receptor cell of Drosophila.

In 2001 there were remarkable developments in the research of sweet taste receptors of mammals and insects. In mammals, T1R2 and T1R3 are G protein-coupled receptors (GPCRs) with very long Nterminal extracellular domains, as shown in Figure 1. They have been shown to associate into a heterodimer and to function as a broadly tuned sweet receptor for various sugars, artificial sweeteners and D-amino acids (Nelson et al., 2001; Li et al., 2002). In Drosophila, on the other hand, TRE (trehalose sensitivity/Gr5a) is a GPCR with a short N-terminal extracellular domain. It was functionally identified and proved to respond specifically to the disaccharide trehalose (Clyne et al., 2000; Dahanuker et al., 2001; Dunipace et al., 2001; Scott et al., 2001; Ueno et al., 2001; Chyb et al., 2003). Is such a rigid specificity as that of TRE, which contrasts with mammalian T1R2 and T1R3, a general feature of the sweet taste receptors of insects? This question may be answered through the study of a specific receptor for glycerol, a new sweet tastant for Drosophila (Koseki et al., 2004).