The Fucose-binding Lectin from Ralstonia solanacearum

Abstract

Plant pathogens, like animal ones, use protein-carbohydrate interactions in their strategy for host recognition , attachment, and invasion. The bacterium Ralsto-nia solanacearum, which is distributed worldwide and causes lethal wilt in many agricultural crops, was shown to produce a potent L-fucose-binding lectin, R. so-lanacearum lectin, a small protein of 90 amino acids with a tandem repeat in its amino acid sequence. In the present study, surface plasmon resonance experiments conducted on a series of oligosaccharides show a preference for binding to Fuc1-2Gal and Fuc1-6Gal epitopes. Titration microcalorimetry demonstrates the presence of two binding sites per monomer and an unusually high affinity of the lectin for Fuc1-2Gal-containing oligosaccharides (K D 2.5 10 7 M for 2-fuco-syllactose). R. solanacearum lectin has been crystallized with a methyl derivative of fucose and with the highest affinity ligand, 2-fucosyllactose. X-ray crystal structures , the one with-methyl-fucoside being at ultrahigh resolution, reveal that each monomer consists of two small four-stranded anti-parallel-sheets. Trimeriza-tion through a 3-fold or pseudo-3-fold axis generates a six-bladed-propeller architecture, very similar to that previously described for the fungal lectin of Aleuria au-rantia. This is the first report of a-propeller formed by oligomerization and not by sequential domains. Each monomer presents two fucose binding sites, resulting in six symmetrically arranged sugar binding sites for the-propeller. Crystals were also obtained for a mutated lectin complexed with a fragment of xyloglucan, a fuco-sylated polysaccharide from the primary cell wall of plants, which may be the biological target of the lectin.