Isolamento e caracterização de um par lectina - receptor endógeno de semente de Mucuna sloanei (Fawcet & Rendle)

Abstract

A specific lectin-xyloglucan pair was characterized in Mucuna sloanei seed cotyledons. The xyloglucan was isolated from the seeds by water extraction, followed by ethanol precipitation (yield 22% of dry seed). The isolated polysaccharide showed 25 Glc:Xyl:Gal in molar ratio of 1.8:1.7:1.0 and [a-1D + 60, molar mass (Mw) of 1.6 106 g mol- 1, radium gyration from 99.9 nm and dn/dc of 1.4. Composition and linkage analysis, supported by methylation and NMR spectroscopic measurements, showed that the polysaccharide has a main chain of (3-1.4-linked glucopyranosyl residues substituted at 0-6 by a-xylopyranosyl, 2-0-0-galactopyranosyl-a-xylopyranosyl side chains, typical of those storage xyloglucans from other plant sources. These results were observed too by the analyses of xyloglucan oligosaccharides by HPAEC-PAD and eletronspray (8,98% of XXXG, 1,92% of XLXG, 27,96% of XXLG and 61,14% of XLLG.). The lectin (sloanin), isolated from the same seed by affinity chromatography on cross-linked Adenanthera pavonina galactomannan, showed two protein bands in SDS-PAGE (36 and 34 kDa) and one peak on gel filtration (63.6 kDa). The N-terminal sequence showed homology to leguminous lectins with 69% to lectin orecursor of soybean. CD spectra revealed that sloanin contains predominantly [3-sheet structures. Sloanin presents 5.5% of carbohydrate and showed hemagglutinating activity against rabbit and enzyme treated human erythrocytes, inhibited only by D-galactose containing sugars. The affinity between sloanin and the storage cell wall galactoxyloglucan was demonstrated by affinity chromatography and fluorescence spectroscopy. This work was demonstrated, by the first time, that a lectin and a specific ligant were found in the same plant organ (seed cotyledon). Although our experiments were performed in vitro, we can extrapolate them to in vivo situation. These results may, thus, suggest that the lectin and the galactoxyloglucan, found in M sloanei seed cotyledon, can interact in vivo. Since the legume lectins actively exudate from the germinating seeds, crossing the cell wall barrier, such interactions could be an internal control of both cell wall degradation and/or lectin exudation.