Multiple receptor proteins for sweet taste in Drosophila discriminated by papain treatment

Treatment of the labellar chemosensory setae of the fruit fly, Drosophila melanogaster, with 0.1 % papain for 3 min induced a complete elimination of the taste nerve response to fructose (Fig. 1). Responses to other sugars examined were not affected (Table 1). Responses to 20 mM LiCl and 0.1 M NaCl also remained unchanged by the treatment. The experiment on the time-dependency of the papain treatment showed a clear difference in the proteasesensitivity between the response to fructose and to glucose and sucrose (Fig. 2). The treatment with 0.005% trypsin for 3 min produced the same results. The response to fructose which was eliminated with the papain treatment, was restored after 3 hrs. These findings reveal the presence of a specific receptor site for fructose and its protease-sensitive nature and suggest the involvement of multiple receptor proteins in the sugar receptor of Drosophila.