Two mutants of Dictyostelium discoideum that lack a sulfated carbohydrate antigenic determinant synthesize a truncated lipid-linked precursor of N-linked oligosaccharides.

Dictyostelium discoideum glycoproteins contain mannose-6-SO4 in highly immunogenic N-linked oligosaccharides. To more precisely define the structural requirements of the antigenic determinant, we have analyzed the oligosaccharides synthesized by two mutant strains (HL241 and HL243) that lack it. Both mutant strains synthesize N-linked oligosaccharides which are very similar to each other but are smaller and less charged than those derived from the wild-type. Both mutants contain substantial amounts of Man-6-SO4, and only a single residue of Man-6-P-OCH3 per chain, in contrast to the wild-type which may have 1 or 2 such residues. Neutral species are similar to the wild-type in that they can still be modified by the addition of residues of fucose and N-acetylglucosamine. Both mutant strains synthesize a truncated lipid-linked oligosaccharide, Man6GlcNAc2, with the most probable structure being: (sequence; see text) based on Jack bean alpha-mannosidase, alpha-1,2-specific mannosidase digestions and methylation analysis. The presence of this small oligosaccharide appears to result from the loss of the mannosyltransferase(s) needed to synthesize structures larger than Man6GlcNAc2 and not from the absence of dolichol phosphate or dolichol-P-mannose synthetase. These data along with the analysis of another mutant strain suggest that the expression of the antigenic determinant requires a specific arrangement of Man-6-SO4 on the alpha-1,6 branch of the oligosaccharide linked to the beta-mannose.