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Abstract

The ability of legumes to recognize and respond to beta-glucan elicitors by synthesizing phytoalexins is consistent with the existence of a membrane-bound 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 the soybean beta-glucan-binding protein in tomato cells resulted in the generation of a high-affinity binding site for the elicitor-active hepta-beta-glucoside conjugate (Kd = 4.5 nM). Ligand competition experiments with the recombinant binding sites demonstrated similar ligand specificities when compared with soybean. In both soybean and transgenic tomato, membrane-bound, active forms of the glucan-binding proteins coexist with immunologically detectable, soluble but inactive forms of the proteins. Reconstitution of a soluble protein fraction into lipid vesicles regained beta-glucoside-binding activity but with lower affinity (Kd = 130 nM). We conclude that the beta-glucan elicitor receptors of legumes are composed of the 75 kDa glucan-binding proteins as the critical components for ligand-recognition, and of an as yet unknown membrane anchor constituting the plasma membrane-associated receptor complex.