The major yeast exoglucanase: an extracellular glycoprotein lacking the carbohydrate outer chain

The major exoglucanase (1,3-beta-D-glucan glucanohydrolase, EC 3.2.1.39) secreted by Saccharomyces cerevisiae contains protein, mannose and phosphate in a molar ratio of 1:27:1. When digested with endo-beta-N-acetylglucosaminidase H (EC 3.2.1.96) it sequentially released two asparagine-linked oligosaccharide chains. Oligosaccharides were fractionated into a neutral and acidic component, each one accounting for 50% of the total carbohydrate. The neutral oligosaccharide consisted of a mixture of three homologues ranging from GlcNAc-(Man)12 to GlcNAc-(Man)14. The acidic carbohydrate was, in turn, split into two components. The major one (45% of the initial material) contained a phosphodiester bond and released only mannose when subjected to mild acid hydrolysis. From the filtration pattern, it was shown to be a mixture of oligosaccharides ranging from GlcNAc-(Man)11-P to GlcNAC-(Man)13-P. The minor phosphorylated component, which represented the residual carbohydrate (5%), contained a phosphomonoester bond. It was also heterogeneous in size, the several homologues having one mannose less than their counterparts from the phosphodiester oligosaccharide. These results clearly indicate that the addition of an outer chain of carbohydrate is not a requirement for the externalization of yeast glycoproteins.