Structural Studies of Phosphorylated High Mannose-type Oligosaccharides

Phosphomannosyl residues on acid hydrolases serve as recognition markers which target these enzymes to lysosomes. We have found that the oligosaccharide units of newly synthesized beta-glucuronidase contain phosphate residues in diester linkage between mannose and alpha-linked N-acetylglucosamine residues (Tabas, I., and Kornfeld, S. (1980) J. Biol. Chem. 255, 6633--6639). To obtain larger amounts of these molecules for structural studies, total cellular glycopeptides were isolated from [2-3H]mannose-labeled mouse lymphoma cells, and the phosphorylated oligosaccharides were released by endo-beta-N-acetylglucosaminidase CII and H and isolated by gel filtration and ion exchange chromatography. The fractions were characterized by alpha-mannosidase digestion before and after removal of the phosphate residues and by acetolysis. We also determined whether the phosphate was present as a phosphomonoester or as a diester. The phosphorylated oligosaccharides consisted of a family or related molecules, all of which contained a high mannose-type oligosaccharide core. The major class consisted of isomers containing a single phosphate in diester linkage to 1 of 3 mannose residues of the underlying oligosaccharide. The second class contained isomers with two phosphodiester groups located at five different positions of the oligosaccharide. In both of these classes, the phosphodiester group could be converted to a phosphomonoester by pig liver alpha-N-acetylglucosaminidase, indicating that the cover is alpha-linked N-acetylglucosamine. The third class was similar to the second except that the 2 phosphate residues were present as monoester groups. The last class contained molecules with a single phosphomonoester group. These molecules differed from those with single phosphodiester groups in that the core oligosaccharides were smaller. This is consistent with their being more mature species which have undergone partial processing. These data demonstrate that phosphorylation can occur at 5 separate mannose residues on the high mannose-type oligosaccharides. Individual molecules can have 1, 2, and perhaps even 3 phosphate residues. The majority of the newly synthesized phosphorylated oligosaccharides contain phosphate groups in diester linkage.