Carbohydrate chains on yeast carboxypeptidase Y are phosphorylated

Carboxypeptidase Y, a vacuolar enzyme from Saccharomyces cerevisiae, was digested with endo-beta-N-acetyl-D-glucosaminidase H to release the four oligosaccharide chains that are linked to asparagine in the glycoprotein. The oligosaccharides were fractionated into a neutral and acidic component, and the latter proved to phosphorylated. From its gel filtration pattern, the neutral fraction was shown to be a mixture of at least four homologs, the smallest of which had a proton NMR spectrum almost identical to that given by an IgM oligosaccharide with eight mannoses and one N-acetylglucosamine [Cohen, R. E. & Ballou, C. E. (1980) Biochemistry 19, 4345--4358]. The yeast oligosaccharide has one additional mannose unit in an alpha 1 leads to 3 or alpha 1 leads to 6 linkage, whereas the larger homologs appear to have two, three, and four more mannose units. One phosphorylated oligosaccharides with a mannose/phosphate ratio of 12.5 was reduced with NaB3H4 and then subjected to mild acid hydrolysis. This released mannose and mannobiose that were glycosidically linked to the phosphate group, whereas complete acid hydrolysis yielded D-mannose 6-phosphate. The recovered oligosaccharide phosphomonoester, which contained 11 or 12 mannose units, was digested exhaustively with alpha-mannosidase, and the product of this reaction was treated with alkaline phosphatase, which yielded radioactive Man3GlcNAcH2. These results suggest that the mannosidase-resistant phosphorylated oligosaccharide has the structure Man leads to P leads to 6 alpha Man leads to alpha Man leads to 6 beta Man leads to 4GlcNAcH2, in which some of the phosphate groups are substituted with mannobiose instead of mannose. A second phosphorylated oligosaccharide with a mannose/phosphate ratio of 6.5 probably contains two phosphodiester groups, but its structure has not been investigated in detail.