Isolation and characterization of glycosylphosphatidylinositol-anchored, mucin-like surface glycoproteins from bloodstream forms of the freshwater-fish parasite Trypanosoma carassii

Wild and farmed freshwater fishes are widely and heavily parasitized by the haemoflagellate Trypanosoma carassii. In contrast, common carp, a natural host, can effectively control experimental infections by the production of specific anti-parasite antibodies. In this study we have identified and partially characterized mucin-like glycoproteins which are expressed in high abundance [(6.0+/-1.7) x 10(6) molecules . cell(-1)] at the surface of the bloodstream trypomastigote stage of the parasite. The polypeptide backbone of these glycoproteins is dominated by threonine, glycine, serine, alanine, valine and proline residues, and is modified at its C-terminus by a glycosylphosphatidylinositol membrane anchor. On average, each polypeptide carries carbohydrate chains composed of about 200 monosaccharide units (galactose, N-acetylglucosamine, xylose, sialic acid, fucose, mannose and arabinose), which are most probably O-linked to hydroxy amino acids. The mucin-like molecules are the target of the fish's humoral immune response, but do not undergo antigenic variation akin to that observed for the variant surface glycoprotein in salivarian trypanosomes. The results are discussed with reference to the differences between natural and experimental infections, and in relation to the recently delineated molecular phylogeny of trypanosomes.