Detection of surface-associated and intracellular glycoconjugates and glycoproteins in Neospora caninum tachyzoites

The surface-associated molecules of the invasive stages of apicomplexan parasites such as Neospora caninum and Toxoplasma gondii are most likely crucially involved in mediating the interaction between the parasite and its host cell. In N. caninum, several antigens have recently been identified which could participate in host cell adhesion and/or invasion. These are antigens which are either constitutively expressed on the outer plasma membrane, or antigens which are only transiently localised on the surface as they are expulsed from the secretory vesicles either prior, or after host cell invasion. Some of these proteins have been characterised at the molecular level, and it has been shown that they are, with respect to protein sequences, closely related to homologous counterparts in T. gondii. Nevertheless, there is only a low degree of cross-antigenicity between the two species. In microbial interactions it has been shown that carbohydrates could also play a crucial role in host cell recognition and immunological host parasite interactions. In this study we present data which strongly suggest that the surface of N. caninum tachyzoites is glycosylated. In SDS-PAGE, glycoproteins comigrated largely with glycosylphosphatidylinositol-anchored proteins which were identified using in vivo 3H]ethanolamine labelling followed by autoradiography. The lectin Con A reacted strongly with the surface of these parasites, binding of which is indicative for the presence of N-glycans. Additional surface binding was observed, although only in a subpopulation of all tachyzoites, for wheat germ agglutinin and Jacalin. Intracellular binding sites for Con A were mainly associated with the parasite dense granules. By lectin labelling of Western blots of N. caninum protein extracts, glycoproteins were identified which reacted specifically with the lectins Con A, wheat germ agglutinin, Jacalin and soy bean agglutinin.