The hepta-beta-glucoside elicitor-binding proteins from legumes represent a putative receptor family

The ability of legumes to recognize and respond to beta-glucan elicitors by synthesizing phytoalexins is consistent with the existence of a membrane-b ound beta-glucan-binding site. Related proteins of approximately 75 kDa and the corresponding mRNAs were detected in various species of legumes which respond to beta-glucans. The cDNAs for the beta-glucan-binding proteins of bean and soybean were cloned. The deduced 75-kDa proteins are predominantly hydrophilic and constitute a unique class of glucan-binding proteins with no currently recognizable functional domains. Heterologous expression of th e soybean beta-glucan-binding protein in tomato cells resulted in the gener ation of a high-affinity binding site for the elicitor-active hepta-beta-gl ucoside conjugate (K-d = 4.5 nM). Ligand competition experiments with the r ecombinant binding sites demonstrated similar ligand specificities when com pared with soybean. In both soybean and transgenic tomato, membrane-bound, active forms of the glucan-binding proteins coexist with immunologically de tectable, soluble but inactive forms of the proteins. Reconstitution of a s oluble protein fraction into lipid vesicles regained beta-glucoside-binding activity but with lower affinity (K-d = 130 nM). We conclude that the beta -glucan elicitor receptors of legumes are composed of the 75 kDa glucan-bin ding proteins as the critical components for ligand-recognition, and of an as yet unknown membrane anchor constituting the plasma membrane-associated receptor complex.