Dictyostelium discoideum: mutants in the biosynthesis of the lipid-linked precursor of N-linked oligosaccharides

The lysosomal enzymes of Dictyostelium discoideum share highly immunogenic oligosaccharides which contain multiple Man-6-SO4 residues. Two mutant strains which lack the shared antigenic determinant were analyzed in an attempt to identify the primary defect in each. [3H]Man labelled N-linked oligosaccharides of secreted glycoproteins were released by Endo/PNGaseF digestion and analyzed. Both of the mutant strains produced smaller, less sulfated oligosaccharides compared to the wild-type, yet both still contained considerable amounts of Man-6-SO4. The size of the precursor lipid-linked oligosaccharide from the wild-type is consistent with a Glc3Man9GlcNAc2 structure, while those from both of the mutants have an oligosaccharide the size of Man5GlcNAc2. The authors conclude that both of the mutants are defective in the biosynthesis of the precursor oligosaccharide. Both oligosaccharides from the mutants contain a tri-mannosyl core and are not glucosylated. Two of the five Man residues are released by a 1,2 specific α mannosidase. Based on the size and mannosidase digestions the authors conclude that 4/5 of the Man residues on the α1,6 branch of the β-linked Man residues are missing. Thus, these residues must be required to define the shared antigenic determinant