Post‐translational modifications of the Dictyostelium discoideum glycoprotein PsA

Prespore‐specific antigen (PsA) is a cell‐surface glycoprotein isolated from Dictyostelium discoideum , which is post‐translationally modified by addition of carbohydrate to threonine residues of the carboxy‐terminal peptide domain, and a glycosylphosphatidylinositol (GPI) anchor which attaches the glycoprotein to the cell membrane. The GPI anchor was isolated by proteolytic cleavage of the protein, and the structure of the lipid and glycan portions of the anchor were determined. The lipid moiety of the anchor is an inositolphosphoceramide which contains C18:0 phytosphingosine as a long chain base, and a mixture of fatty acids with a C18:1 mono‐unsaturated fatty acid as the major component. The purified GPI anchor was susceptible to digestion by a bacterial phosphatidylinositol‐specific phospholipase‐C enzyme. The glycan of the GPI anchor consisted of two molecular species present in the ratio 55:45, the structures of which were determined by exoglycosidase sequencing and found to be Manα1‐2Manα1‐6Manα1‐4GlcNH 2 and Manα1‐2Manα1‐2Manα1‐6Manα1‐4GlcNH 2 . The glucosamine in both structures is glycosidically linked to the inositol ring of the inositolphosphoceramide. The GPI glycan structures are consistent with the conserved core structure of all characterised GPI anchors, and the structure of the D. discoideum GPI moiety has features in common with structures from yeast, protozoa and higher eukaryotes. Compositional analysis of the carbohydrate attached to threonine residues in the carboxy‐terminal peptide domain is also presented. The oligosaccharides bind to wheat germ agglutinin, and contain glucosamine and fucose as the major constituents.