Inhibition of processing of plant N-linked oligosaccharides by castanospermine

Abstract

Castanospermine (1,6,7,8-tetrahydroxyoctahydroindolizine) is a plant alkaloid that inhibits lysosomal α- and β-glucosidase. It also inhibits processing of influenza viral glycoproteins by inhibiting glucosidase I and leads to altered glycoproteins with Glc 3Man 7GlcNAc 2 structures. Castanospermine was tested as an inhibitor of glycoprotein processing in suspension-cultured soybean cells. Soybean cells were pulse-labeled with [2- 3H]mannose and chased for varying periods in unlabeled medium. In normal cells, the initial glycopeptides contained oligosaccharides having Glc 3Man 9GlcNAc 2 to Glc 1Man 9GlcNAc 2 structures and these were trimmed during the chase to Man 9GlcNac 2 to Man 2GlcNAc 2 structures. In the presence of castanospermine, no trimming of glucose residues occurred although some mannose residues were apparently still removed. Thus, the major oligosaccharide in the glycopeptides of castanospermine-incubated cells after a 90-min chase was a Glc 3Man 7GlcNAc 2 structure. Smaller amounts of Glc 3Man 6GlcNAc 2 and Glc 3Man 5GlcNAc 2 were also identified. Thus, in plant cells, castanospermine also prevents the removal of the outermost glucose residue.