2 TYPES OF SUGAR-BINDING PROTEIN IN THE LABELLUM OF THE FLY - PUTATIVE TASTE RECEPTOR MOLECULES FOR SWEETNESS

Flies have taste cells specifically sensitive to sweetness. It has bee n suggested that the cells possess two types of receptor sites coverin g the receptive field of sweetness. By affinity electrophoresis with t he site-specific inhibitory polysaccharides, two types of sugar-bindin g protein were isolated from the labellar extract of the blowfly. Thes e proteins showed consistent sugar-binding specificities and affinitie s with the two types of receptor sites for sweetness, respectively. Th e dissociation constant of the protein-sugar complex varies 100-400 mM and the molecular weight of one type of the protein is 27,000, while that of the other is 31,000 or 32,000. Both proteins were water insolu ble and were also detected in the isolated chemosensilla. Thus they ar e probably located on the taste receptor membrane, and the proteins ar e likely to act as the taste receptor molecules for sweetness in the f ly.