Purification of Cajanus cajan Root Lectin and Its Interaction with Rhizobial Lipopolysaccharide as Studied by Different Spectroscopic Techniques

A lectin present in roots of Cajanus cajan seedlings was isolated and purified by affinity chromatography. Sugar specificity assayed by hemagglutination-inhibition activity indicated that lectin belongs to glucose/mannose-specific group. The root lectin was found to be mannose-specific from the second day onwards as it was reconfirmed by specific elution of different daysʹ sample from mannose agarose matrix. The maximum interaction of lectin with goat IgM was obtained in 10-day-old sample, indicating the highest crude lectin content. Lectin (total amount of eluted protein) from different days soil sample showed a maximum amount in 10-day-old sample. For further studies, the lectin has been isolated from the roots of 10-day C. cajan seedlings and purified on mannose–CL agarose column by affinity chromatography. Lectin was found to be a dimer of 18.5-kDa subunit as revealed by SDS–PAGE. Tryptophan quenching fluorescence was studied for C. cajan root lectin. Secondary structure of C. cajan root lectin as studied by circular dichroism was found to be a typical β-pleated sheet structure. The interaction of purified root lectin with C. cajan-specific rhizobial lipopolysaccharide and its inhibition by specific and nonspecific sugars was demonstrated by fluorescence and circular dichroism. Results discussed in this paper were studied for the first time by different spectroscopic methods, suggesting that C. cajan root lectin–lipopolysaccharide interaction is specific.