O- and N-Glycosylation of the Leishmania mexicana-secreted acid phosphatase: characterization of a new class of phosphoserine-linked glycans

The protozoan parasite Leishmania mexicana secretes a heavily glycosylated 100-kDa acid phosphatase (sAP) which is associated with one or more polydisperse proteophosphoglycans. Most of the glycans in this complex were released using mild acid hydrolysis conditions that preferentially cleave phosphodiester linkages. The released saccharides were shown to consist of monomeric mannose and a series of neutral and phosphorylated glycans by Dionex high performance liquid chromatography, methylation analysis, exoglycosidase digestions, and one-dimensional H NMR spectroscopy. The neutral species comprised a linear series of oligosaccharides with the structures [Mana1-2]Man. The phosphorylated oligosaccharides were characterized as PO-6Galß1-4Man and PO-6[Glcß1-3]Galß1-4Man. The attachment of these glycans to the polypeptide backbone via the linkage, Mana1-PO-Ser, is suggested by: 1) the finding that more than 60% of the serine residues in the polypeptide are phosphorylated and 2) the resistance of the phosphoserine residues to alkaline phosphatase digestion unless the sAP was first treated with either mild acid (to release all glycans) or jack bean a-mannosidase (to release neutral mannose glycans). Analysis of the partially resolved components of the complex indicated that the most of the O-linked glycans on the 100-kDa phosphoglycoprotein comprised mannose and the mannose-oligosaccharides. In contrast the major O-linked glycans on the proteophosphoglycan were short phosphoglycan chains, containing on average two repeat units per chain. In addition to the O-linked glycans, both components in the sAP complex contained N-linked glycans. The N-glycanase F- released glycans were characterized by Bio-Gel P4 chromatography and exoglycosidase digestions to be the biantennary oligomannose type with the structures GlcManGlcNAc and ManGlcNAc. The O-linked glycans of the sAP complex are similar to those found in the phosphoglycan chains of the abundant surface lipophosphoglycan, but differ in having much shorter phosphoglycan chains and a more diverse series of mannose cap oligosaccharides. These data suggest that there are marked differences in the ability of different glycosyltransferases to utilize peptide-linked versus glycolipid-linked acceptors.