Carbohydrate moiety of Plasmodium falciparum glycoproteins: the nature of the carbohydrate-peptide linkage in the MSP-2 glycoprotein

Glycosylation in malaria parasites: what do we know?

In malaria parasites, although post-translational modification of proteins with N-. O-, and C-glycosidic bond-linked glycans is limited, it is confined to relatively fewer proteins in which the glycans are present at significant levels and may have important functions. Furthermore, several proteins are modified with glycosylphosphatidylinositols (GPIs) which represent the predominant glycan synthesized by parasites. Modification of proteins with GPIs is obligatory for parasite survival as GPI-anchored proteins (GPI-APs) play essential roles in all life cycle stages of the parasites, including development, egress, gametogenesis, motility, and host cell adhesion and invasion. Here, we discuss the current knowledge on the structures and potential functions of the glycan moieties of parasite proteins. The knowledge has important implications for the development of drugs and vaccines for malaria.

O-GlcNAc modification of the anti-malarial vaccine candidate PfAMA1: in silico-defined structural changes and potential to generate a better vaccine

The complex life cycle of plasmodial parasites makes the selection of a single subunit protein a less than optimal strategy to generate an efficient vaccinal protection against malaria. Moreover, the full protection afforded by malarial proteins carried by intact parasites implies that immune responses against different antigens expressed in different phases of the cycle are required, but also suggests that native malarial antigens are presented to the host immune system in a manner that recombinant proteins do not achieve. The malarial apical membrane antigen 1 (AMA1) represents a suitable vaccine candidate because AMA1 is expressed on sporozoites and merozoites and allows them to invade hepatocytes and erythrocytes, respectively. Anti-AMA1 antibodies and cytotoxic T-cells are therefore expected to interfere both with the primary invasion of hepatocytes by sporozoites and with the later propagation of merozoites in erythrocytes, and thus efficiently counteract parasite development in its human host. AMA1 bears potential glycosylation sites and the human erythrocytic O-linked N-acetylglucosamine transferase (OGT) could glycosylate AMA1 through combinatorial metabolism. This hypothesis was tested in silico by developing binding models of AMA1 with human OGT complexed with UDP-GlcNc, and followed by the binding of O-GlcNAc with the hydroxyl group of AMA1 serine and threonine residues. Our results suggests that AMA1 shows potential for glycosylation at Thr517 and Ser498 and that O-GlcNAc AMA1 may constitute a conformationally more appropriate antigen for developing a protective anti-malarial immune response.