Post-translational processing of concanavalin A

Summary

Concanavalin A (Con A) is a plant lectin, making up some 30% of the protein in jackbean seeds. At seed maturity, the protein is located in the protein bodies of the storage parenchyma cells within the cotyledons of the seed. The biosynthesis and processing of Con A have been studied in a number of laboratories; the resulting data indicate a complex and novel series of events.

Glycoconjugates in jackbean seeds are recognised by the endogenous lectin

At Leeds, our studies on concanavalin A (Con A) began more than ten years ago, when we were interested in determining the range of endogenous glycoconjugates in jackbeans that could interact with the lectin. Our aim was to gain insight into the nature of lectin–receptor interactions within the seed, and thereby to understand the potential function of the lectin through the types of glycoproteins and/or glycolipids available for complexing.

Using the technique of Con A overlays of jackbean polypeptides separated by SDS–PAGE, we found that only one polypeptide bound the lectin, the heavy subunit (66 kDa) of the enzyme α-mannosidase (Bowles, Andralojc & Marcus, 1982). However, the interaction between the lectin and the hydrolase could only be demonstrated if the enzyme was first denatured; in the native form of the enzyme, the N-glycan was inaccessible for binding to Con A (Bowles, Chaplin & Marcus, 1983). This suggested that in native tetrameric form, the N-glycan is sterically hindered from binding through positioning of the heavy and light subunits within the oligomer.