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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.
