Gustatory receptor required for response to the sugar trehalose in taste neurons. Gr5a neurons selectively respond to sugars, in contrast to Gr66a cells which respond to bitter compounds. Flies are attracted to sugars and avoid bitter substances, suggesting that Gr5a neuron activity is sufficient to mediate acceptance behavior. Sugar signal transduction occurs through coupling with G-proteins such as Galpha49B and G-salpha60A. Expressed in labellar chemosensory neurons. Flies exhibit trehalose-specific physiological and behavioral defects (reduced response by chemosensory neurons of the labellar taste hairs). Variant Thr-218 was found to be the ancestral form in D.melanogaster, suggesting that low trehalose sensitivity was an ancestral form with respect to the receptor function. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr5a subfamily.