Metabolic Labeling and Structural Analysis of Glycosylphosphatidylinositols from Parasitic Protozoa

Glycosylphosphatidylinositol (GPI) is a complex glycolipid structure that acts as a membrane anchor for many cell-surface proteins of eukaryotes. GPI-anchored proteins are particularly abundant in protozoa and represent the major carbohydrate modification of many cell-surface parasite proteins. A minimal GPI-anchor precursor consists of core glycan (ethanolamine-PO₄-Manα1-2Manα1-6Manα1-4GlcNH₂) linked to the 6-position of the D-myo-inositol ring of phosphatidylinositol. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. The preassembled GPI-anchor precursor is post-translationally transferred to a variety of membrane proteins in the lumen of the endoplasmic reticulum in a transamidase-like reaction during which a C-terminal GPI attachment signal is released. Increasing evidence shows that a significant proportion of the synthesized GPIs are not used for protein anchoring, particularly in protozoa in which a large amount of free GPIs are being displayed at the cell surface. The characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. Especially this pathway, at least for Trypanosoma brucei, has been validated as a drug target. Furthermore, thanks to an increase of new innovative strategies to produce pure synthetic carbohydrates, a novel era in the use of GPIs in diagnostic, anti-GPI antibody production, as well as parasitic protozoa GPI-based vaccine approach is developing fast.