A microneme protein from Eimeria tenella with homology to the Apple domains of coagulation factor XI and plasma pre-kallikrein

Microneme organelles are present in all apicomplexan protozoa and contain proteins that are critical for parasite motility and host cell invasion. One apicomplexan-wide family of microneme proteins has been identified with members that are characterised by the possession of thrombospondin type I repeats, conserved adhesive motifs which are implicated in binding to glycosaminoglycan chains. In this paper we describe a micronemal glycoprotein, EtMIC 5, from Eimeria tenella which contains eleven cysteine-rich motifs that have striking similarity to the adhesive Apple (A-) domains of blood coagulation factor XI and plasma pre-kallikrein. EtMIC 5 is confined to an intracellular location in resting sporozoites but is translocated to the parasite surface and secreted into the culture supernatant during parasite infection of MDBK cells. During intracellular replication, the protein is switched off in early schizogony and is then re-expressed within the apical tips of newly formed merozoites. A-domain sequences were also found in microneme proteins from Sarcocystis muris and Toxoplasma gondii and in a protein of unknown localisation from Eimeria acervulina. These studies suggest that A-domain containing proteins may comprise a novel apicomplexan-wide family of microneme adhesins.