Structural and functional studies of Microneme Protein 3 (MIC3) from Apicomplexan parasite Eimeria tenella

Abstract

Eimeria spp. are a highly successful group of intracellular protozoan parasites that develop within intestinal epithelial cells of poultry, causing coccidiosis. As a result of resistance against anticoccidial drugs and the expense of manufacturing live vaccines, it is necessary to understand the relationship between Eimeria and its host more deeply, with a view to developing recombinant vaccines. Like other members of the Apicomplexan parasites, Eimeria possesses a number of microneme proteins (MICs) which are deployed at the parasite-host interface during the early stages of invasion. One of these proteins, EtMIC3, is a lectin consisting of seven tandem microneme adhesive repeats (MAR), which possess a high affinity for sialylated glycans as shown by cell-based assays and carbohydrate microarray analyses. To provide a structural basis for the sialic acid recognition of EtMIC3, the second MAR domain of EtMIC3 was expressed recombinantly and its structure was determined by Nuclear Magnetic Resonance (NMR) spectroscopy. NMR titration and chemical shift mapping was carried out to localise the carbohydrate binding site and inter-molecular NOEs were measured using filtered NOESY experiments. Structural models of the carbohydrate complexes were subsequently calculated invoking both intermolecular NOEs and chemical shift-derived distance restraints using the HADDOCK approach.