Purification and characterization of an extracellular lectin (Lectin I) from Agrobacterium radiobacter NCIM 2443

A mannose-binding protein from the cell surface of flocculent Saccharomyces cerevisiae (NCIM 3528): its role in flocculation

A cell surface lectin was isolated and purified to homogeneity from the cell walls of a highly flocculent strain of Saccharomyces cerevisiae (NCIM 3528) by chromatography on DEAE-cellulose, phenyl Sepharose and Sephacryl S-300. It showed a molecular mass of 40 kDa on SDS-PAGE. It is an acidic protein with a pI of 4.0 and contains 44% hydrophobic amino acids. The N-terminal sequence up to 10 amino acid residues showed at least 70% homology with the predicted N-terminal sequence of the putative FLO1 as well as FLO5 gene products. The mannose-binding nature of the lectin was indicated by its high affinity and specificity towards the branched trisaccharide of mannose, a ligand which also inhibits the flocculation of yeast cells. Immunofluorescence studies confirmed the presence of lectin on the yeast cell surface and lectin-specific IgGs prevented flocculation of the cells. This cell surface mannose-specific lectin probably plays an important role in flocculation, with the branched trimannoside on the cell wall being the apparent carbohydrate receptor. The N-terminal sequence data gives a primary indication that the lectin could be a product of one of the FLO genes.

Purification of two lectins from a nopalin Agrobacterium tumefaciens strain

Lectins were evidenced on the surface of one Agrobacterium tumefaciens wild strain (82,139) by agglutination test and neoglycoprotein labelling. Bacteria were incubated in the presence of various fluorescein-labelled neoglycoproteins and the binding was assessed by a fluorimetric method. Among the fluorescein-labelled neoglycoproteins tested, the one bearing alpha-D-galactosyl residues was the most efficient. The labelling was optimal at pH 5.0 and naught at pH above 7. The binding was specifically inhibited by homologous fluorescein-free neoglycoproteins. A galactoside-specific lectin was purified to homogeneity by affinity chromatography on agarose-A4 substituted with alpha-D-galactopyranosyl residues. Upon polyacrylamide gel electrophoresis, a single band (M(r) 58,000) was detected. This alpha-D-galactoside-specific lectin agglutinated preferentially human B red blood cells at pH 5.0. Another lectin specific for alpha-L-rhamnoside (M(r) 40,000) not retained on the immobilised galactose was purified by affinity chromatography on alpha-L-rhamnosyl substituted agarose-A4. This L-rhamnoside-specific lectin preferentially agglutinated horse erythrocytes. On the basis of their M(r) and on their sugar specificity, these two lectins are novel lectins with regard to the known sugar-binding proteins present in the Rhizobiaceae family: Agrobacterium, Rhizobium or Bradyrhizobium strains.