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 p roteins that are critical for parasite motility and host cell invasion. One apicomplexan-wide family of microneme proteins has been identified with me mbers that are characterised by the possession of thrombospondin type I rep eats, conserved adhesive motifs which are implicated in binding to glycosam inoglycan chains. In this paper We describe a micronemal glycoprotein, EtMI C5, from Eimeria tenella which contains eleven cysteine-rich motifs that ha ve striking similarity to the adhesive Apple (A-) domains of blood coagulat ion factor XI and plasma pre-kallikrein. EtMIC5 is confined to an intracell ular location in resting sporozoites but is translocated to the parasite su rface and secreted into the culture supernatant during parasite infection o f MDBK cells. During intracellular replication, the protein is switched off in early schizogony and is then re-expressed within the apical tips of new ly formed merozoites. A-domain sequences were also found in microneme prote ins from Sarcocystis muris and Toxoplasma gondii and in a protein of unknow n localisation from Eimeria acervulina. These studies suggest that A-domain containing proteins may comprise a novel apicomplexan-wide family of micro neme adhesins. (C) 2000 Elsevier Science B.V. All rights reserved.