Protein-engineenng Studies on Unique Structures and Multi-functional Expression Mechanism of Plant-type Lectins

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Of five lectins from the roots of pokeweed, three-dimensional structures of PL-C, PL-D1, PL-D2, and a complex of PL-D2 with tri-N-acetylchitotnose have been determined by X-ray crystal structure analysis. Furthermore, the site for the mitogenic activity has been explored by comparing the structures of PL-D1 and PL-D2, the relationship between intersubunit interaction and function of PL-C has been discussed on the basis of its dimer structure, and the similarity between protein-protein and protein-carbohydrate interactions has also been discussed by a structural comparison of PL-C and the carbohydrate complex of PL-D2. PL-D1 was crystallized by a vapor diffusion method using 30% PEG8000 as precipitant, PL-D2 by a batch method using 18% PEG8000 as precipitant, PL-C by a vapor diffusion and macro-seeding method using 2.0 M ammonium sulfate as precipitant and 0.5% dioxane as additive, and the carbohydrate complex of PL-D2 by a vapor diffusion method using 20% PEG8000 as precipitant. X-ray … More diffraction data were collected up to high resolution in a SR facility. The structures of PL-D1, PL-D2 and the complex were determined by molecular replacement and that of PL-C by double isomorphous replacement. Since the structures of PL-D1 and PL-D2 are very similar to each other, it has been expected that the difference in mitogenic activity comes from the extension of two C-terminal residues in PL-D1 and that the site for interaction of PL-D with target molecules may exist near its C-terminus. The PL-C molecule adopts a unique dimmer structure in a head-to-tail manner through intersubunit interactions involved by sugar-binding residues to loose its sugar-binding activity by covering the sugar-binding sites. The carbohydrate complex of PL-D2 adopts a unique structure in which two PL-D2 molecules, which are related to each other by a crystallographic 2_1-screw axis, share one sugar molecule. The protein-protein interaction in PL-C is similar to the protein-carbohydrate interaction in the complex of PL-D2. However, the former interaction is stronger than the latter. The similarity between these two kinds of interactions suggests structural information that is important in designing peptide medicines for modulating the function of lectins. Less