Numerous intracellular pathogens exploit cell surface glycoconjugates for host cell recognition and entry. Unlike bacteria and viruses, Toxoplasma gondii and other parasites of the phylum Apicomplexa actively invade host cells, and this process critically depends on adhesins (microneme proteins) released onto the parasite surface from intracellular organelles called micronemes (MIC). The microneme adhesive repeat (MAR) domain of T. gondii MIC1 (TgMIC1) recognizes sialic acid (Sia), a key determinant on the host cell surface for invasion by this pathogen. By complementation and invasion assays, we demonstrate that TgMIC1 is one important player in Sia-dependent invasion and that another novel Sia-binding lectin, designated TgMIC13, is also involved. Using BLAST searches, we identify a family of MAR-containing proteins in enteroparasitic coccidians, a subclass of apicomplexans, including T. gondii, suggesting that all these parasites exploit sialylated glycoconjugates on host cells as determinants for enteric invasion. Furthermore, this protein family might provide a basis for the broad host cell range observed for coccidians that form tissue cysts during chronic infection. Carbohydrate microarray analyses, corroborated by structural considerations, show that TgMIC13, TgMIC1, and its homologue Neospora caninum MIC1 (NcMIC1) share a preference for ␣2-3-over ␣2-6-linked sialyl-N-acetyllactosamine sequences. However, the three lectins also display differences in binding preferences. Intense binding of TgMIC13 to ␣2-9-linked disialyl sequence reported on embryonal cells and relatively strong binding to 4-O-acetylated-Sia found on gut epithelium and binding of NcMIC1 to 6sulfo-sialyl Lewis x might have implications for tissue tropism. Sialic acids (Sias)6 occur abundantly in glycoproteins and glycolipids on the cell surface and are exploited by many viruses and bacteria for attachment and host cell entry. Recognition of carbohydrates and in particular sialylated glycoconjugates is important also for host cell invasion by the Apicomplexa (1-4), a phylum that includes several thousand species of obligate intracellular parasites, among them the Plasmodium spp. causing malaria. Enteroparasitic coccidians are a subclass of Apicomplexa comprising Eimeria spp. responsible for coccidiosis in poultry, Neospora spp. causing neosporosis in cattle, and Toxoplasma, the causative agent of toxoplasmosis in warmblooded animals and humans.The host range and cell type targeted by these parasites vary widely across the phylum. Whereas Plasmodium falciparum merozoites exclusively invade erythrocytes of humans and great apes (5), Toxoplasma gondii tachyzoites (the form of the parasite associated with acute infection) invade an extremely broad range of cell types in humans and virtually all warmblooded animals, enabling rapid establishment of infection in the host and dissemination into deep tissues (6). Information is emerging on the involvement of carbohydrate-protein interactions in this broad host cell recognition (1).Many intracellular...

Unnatural, NMR- and MRI-active fluorinated sugar probes, designed and synthesised to bind to the pathogenic protein TgMIC1 from Toxoplasma gondii, were found to display binding potency equal to and above that of the natural ligand. Dissection of the binding mechanism and modes, including the first X-ray crystal structures of a fluoro-oligosaccharide bound to a lectin, demonstrate that it is possible to create effective fluorinated probe ligands for the study of, and perhaps intervention in, sugar-protein binding events.

The intracellular protozoan Toxoplasma gondii is among the most widespread parasites. The broad host cell range of the parasite can be explained by carbohydrate microarray screening analyses that have demonstrated the ability of the T. gondii adhesive protein, TgMIC1, to bind to a wide spectrum of sialyl oligosaccharide ligands. Here, we investigate by further microarray analyses in a dose-response format the differential binding of TgMIC1 to 2-3-and 2-6-linked sialyl carbohydrates. Interestingly, two novel synthetic fluorinated analogs of 3 0 SiaLacNAc 1-4 and 3 0 SiaLacNAc 1-3 were identified as highly potent ligands. To understand the structural basis of the carbohydrate binding specificity of TgMIC1, we have determined the crystal structures of TgMIC1 micronemal adhesive repeat (MAR)-region (TgMIC1-MARR) in complex with five sialyl-Nacetyllactosamine analogs. These crystal structures have revealed a specific, water-mediated hydrogen bond network that accounts for the preferential binding of TgMIC1-MARR to arrayed 2-3-linked sialyl oligosaccharides and the high potency of the fluorinated analogs. Furthermore, we provide strong evidence for the first observation of a CAFÁÁÁHAO hydrogen bond within a lectin-carbohydrate complex. Finally, detailed comparison with other oligosaccharide-protein complexes in the Protein Data Bank (PDB) reveals a new family of sialic-acid binding sites from lectins in parasites, bacteria, and viruses.