Selective interactions of lectins with amino acid taste receptor sites in the channel catfish

Five lectins of varying carbohydrate specificities, Dolichosbiflorus (DBA), jacalin, Phaseolus vulgaris (PHA), Pisum sativum(PSA) and Ricinus communis (RCA I), were used to extend characterizationof the glycoprotein nature of taste plasma membranes and todifferentially affect the binding of two taste stimuli, L-alanineand L-arginine, to their respective taste receptor sites inthe cutaneous taste system of the channel catfish (Ictaluruspunctatus). The binding of the taste stimulus L-arginine toa partial membrane fraction (P_–) from taste epitheliumwas inhibited by 68 and 74% by preincubation in the presenceof the unconjugated lectins PHA and RCA I respectively. A correspondinglevel of inhibition of L-alanine binding was seen in the presenceof RCA I (76%); however, PHA had little effect upon L-alaninebinding. DBA appeared to selectively inhibit L-alanine but notL-arginine binding (60 versus 8% respectively) while jacalinmoderately inhibited the binding of both stimuli to fractionP2. PSA had little effect upon the binding of either L-alanineor L-arginine (4 and 5% inhibition respectively). Inhibitionof taste receptor binding by all lectins was time- and dose-dependent,and was fully abolished by incubation in the presence of theappropriate hapten sugar. The biotinylated lectins DBA, jacalin,PHA, RCA I and concanavalin A (Con A) were used to identifythe glycoprotein components of the chemosensory plasma membranesafter polyacrylamide gel electrophoresis. As previously shown,numerous protein components were labeled by Con A. In contrast,only a few minor protein components were labeled by PHA, DBAand RCA I. This differential labeling of the taste membranesand the differential inhibition of receptor binding by lectinssuggest that they may prove useful as tools in the isolationand purification of taste receptor proteins.