Abnormal processing of the modified oligosaccharide side chains of phytohemagglutinin in the presence of swainsonine and deoxynojirimycin

Phytohemagglutinin, the glycoprotein lectin of the common bean, Phaseolus vulgaris, has both high-mannose (Man_8-9GlcNAc₂) and modified oligosaccharide side chains. The modified side chains have glucosamine, mannose, fucose, and xylose in the molar ratios 2:3.8:0.6:0.5, and are resistant to hydrolysis by endoglycosidase H. Synthesis and processing of side chains in the presence of 1-deoxynojirimycin, an inhibitor of α-glucosidase, results in the formation of chains which are all alike. They are sensitive to endoglycosidase H, do not contain fucose, and are largely resistant to α-mannosidase. This indicates that they are probably high-mannose chains blocked by terminal glucose residues. Synthesis and processing of side chains in the presence of swainsonine, an inhibitor of α-mannosidase II, results in the formation of normal high-mannose chains, and of modified chains which contain fucose residues, are resistant to endoglycosidase H, and can be distinguished from normal modified chains only by the presence of extra mannose residues.

Processing of the phytohemagglutinin modified chains of PHA under normal conditions involves the attachment of peripheral N-acetylglucosamine residues in the Golgi complex and their subsequent removal in the protein bodies. The attachment of the N-acetylglucosamine residues is largely inhibited by deoxynojirimycin but still occurs in the presence of swainsonine. The results presented in this work show that processing of the asparagine-linked oligosaccharides is under the control of several glycosidases and glycosyltransferases and involves the formation of intermediate products.