Schistosoma mansoni synthesizes glycoproteins containing terminal O-linked N-acetylglucosamine residues

O-glycosylation of eukaryotic transcription factors: implications for mechanisms of transcriptional regulation

Glycosylation is often regarded as being restricted to proteins confined to the cell surface or within the lumen of intracellular organelles. Here we show that the human RNA polymerase II transcription factor Sp1 bears multiple O-linked N-acetylglucosamine (GlcNAc) monosaccharide residues. The lectin wheat germ agglutinin specifically inhibits the transcriptional activation but not the DNA binding function of Sp1. Furthermore, many other RNA polymerase II transcription factors also bear terminal GlcNAc residues, whereas most nuclear proteins, including RNA polymerase I and III transcription factors tested, do not. In some cases, only a subset of the polypeptide species within a particular family of closely related RNA polymerase II factors appears to be glycosylated. Our findings raise the possibility that O-linked GlcNAc residues play a role in the mechanism or regulation of transcriptional activation of RNA polymerase II.

Characterization of the high mannose asparagine-linked oligosaccharides synthesized by Schistosoma mansoni adult male worms

This report describes the structures of the high-mannose-type N-linked oligosaccharides in glycoproteins synthesized by Schistosoma mansoni adult male worms. Adult male schistosomes were incubated in vitro in media containing either [2-3H]mannose, [6-3H]glucosamine or [6-3H]galactose to allow metabolic radiolabeling of the oligosaccharide moieties of newly synthesized glycoproteins. Glycopeptides were prepared from the radiolabeled glycoproteins by digestion with Pronase and fractionation by chromatography on concanavalin A-Sepharose. Eleven percent of [3H]mannose incorporated into the schistosome glycopeptides was recovered in high mannose-type Asn-linked oligosaccharides which bound to the immobilized lectin. Upon treatment of [3H]mannose-labeled glycopeptide with endo-beta-N-acetylglucosaminidase H, the high mannose-type chains were released and their structures were determined by high performance liquid chromatography, methylation analysis, acetolysis and exoglycosidase digestion. The major species of high mannose-type chains synthesized by S. mansoni adult males have the composition Man7GlcNAc2, Man8GlcNac2 and Man9GlcNA2. Structural analyses indicate that these oligosaccharides are similar to high mannose-type chains synthesized by mammalian cells.

Molecular mimicry revisited

The host immune response is an important line of defence against parasites. Tactics to evade this response are therefore expected in host-parasite relationships, and the clearest example is the antigenic variation displayed by African trypanosomes. But while few other parasites seem to have quite this ability, many seem to display a form of antigenic convergence with the host - allowing them a degree of molecular camouflage against the host's immune system. Ideas about such antigenic convergence were developed some 30 years ago, with, for example, John Sprent's theory of 'adaptation tolerance', John Dineen's 'selection for fitness antigens' and Raymond Damian's concept of 'antigen sharing' between host and parasite which was subsequently formulated in a now classical exposition on 'molecular mimicry'. Damian's theory, that one of the mechanisms by which parasites could avoid the host immune response was by mimicking host molecules, has greatly influenced both the theoretical and practical approaches to immunoparasitology. Earlier this year, at a UCLA Symposium, Professor Damian discussed how the theory had progressed since its original exposition. This article is based on that presentation.