Oligosaccharide Side Chains of Glycoproteins that Remain in the High-Mannose Form Are Not Accessible to Glycosidases

Glycoproteins present in the soluble and organelle fractions of developing bean (Phaseolus vulgaris) cotyledons were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, affinoblotting, fractionation on immobilized concanavalin A (ConA), and digestion of the oligosaccharide side chains with specific glycosidases before and after protein denaturation. These studies led to the following observations. (a) Bean cotyledons contain a large variety of glycoproteins that bind to ConA. Binding to ConA can be eliminated by prior digestion of denatured proteins with α-mannosidase or endoglycosidase H, indicating that binding to ConA is mediated by high-mannose oligosaccharide side chains. (b) Bean cotyledons contain a large variety of fucosylated glycoproteins which bind to ConA. Because fucose-containing oligosaccharide side chains do not bind to ConA, such proteins must have both high-mannose and modified oligosaccharides. (c) For all the glycoproteins examined except one, the high-mannose oligosaccharides on the undenatured proteins are accessible to ConA and partially accessible to jack bean α-mannosidase. (d) Treatment of the native proteins with α-mannosidase removes only 1 or 2 mannose residues from the high-mannose oligosaccharides. Similar treatments of sodium dodecyl sulfate-denatured or pronase-digested glycoproteins removes all α-mannose residues. The results support the following conclusions: certain side chains remain unmodified as high-mannose oligosaccharides even though the proteins to which they are attached pass through the Golgi apparatus, where other oligosaccharide chains are modified. The chains remain unmodified because they are not accessible to processing enzymes such as the Golgilocalized α-mannosidase.