Phosphorylation of the oligosaccharide of uteroferrin by UDP-GlcNAc:glycoprotein N-acetylglucosamine-1-phosphotransferases from rat liver, Acanthamoeba castellani, and Dictyostelium discoideum requires α1,2-linked mannose residues

We have investigated the oligosaccharide requirements of the UDP-GlcNAc:glycoprotein N-acetylglucosamine-1-phosphotransferases from rat liver, Acanthamoeba castellani, and Dictyostelium discoideum. Uteroferrin, an acid hydrolase, was phosphorylated by the three N-acetylglucosaminylphosphotransferases, and the phosphorylated oligosaccharides were isolated and analyzed by ion suppression high performance liquid chromatography. In all three cases, the phosphorylated species contained 6 or more mannose residues. Phosphorylation of the Man₅GlcNAc₂ oligosaccharide could not be detected even though this was the major species on the native uteroferrin. The Man₅GlcNAc₂ oligosaccharides lack α1,2-linked mannose residues, whereas the larger oligosaccharides contain 1 or more mannose residues in this linkage. Treatment of intact uteroferrin with an α1,2-specific mannosidase-generated molecules whose oligosaccharides consisted almost entirely of species with 5 mannose residues. The N-acetylglucosaminylphosphotransferase could no longer phosphorylate such molecules. These data indicate that at least 1 α1,2-linked mannose residue must be present on uteroferrin's oligosaccharide for phosphorylation to occur.