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A. Mithöfer, J. Fliegmann, G. Neuhaus-Url, H. Schwarz, J. Ebel

The Hepta-? -Glucoside Elicitor-Binding Proteins from Legumes Represent a Putative Receptor Family

The ability of legumes to recognize and respond to ?-glucan elicitors by synthesizing phytoalexins is consistent with the existence of a membrane-bound ?-glucan-binding site. Related proteins of approximately 75 kDa and the corresponding mRNAs were detected in various species of legumes which respond to ?-glucans. The cDNAs for the ?-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 ?-glucan-binding protein in tomato cells resulted in the generation of a high-affinity binding site for the elicitor-active hepta-?-glucoside conjugate (K_d = 4.5nM). 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 ?-glucoside-binding activity but with lower affinity (K_d = 130 nM). We conclude that the ?-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.

Biological Chemistry, Walter de Gruyter

Print ISSN: 1431-6730
Volume: 381, 09/2000
Pages: 705 - 713

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