EtMIC4: a microneme protein from Eimeria tenella that contains tandem arrays of epidermal growth factor-like repeats and thrombospondin type-1 repeats

Micronemes are specialised secretory organelles that release their proteins by a stimulus-coupled exocytosis that occurs when apicomplexan parasites m ake contact with target host cells. These proteins play crucial roles in mo tility and invasion, most likely by mediating adhesion between parasite and host cell surfaces and facilitating the transmission of dynamic forces gen erated by the parasite actinomyosin cytoskeleton. Members of the TRAP famil y of microneme proteins are characterised by having extracellular domains c ontaining one or more types of cysteine-rich, adhesive modules, highly-cons erved transmembrane regions and cytosolic tails that contain one or more ty rosines, stretches of acidic residues and a single tryptophan. In this pape r, we describe a novel member of the TRAP family, EtMIC4, a 218 kDa microne me protein from Eimeria tenella. EtMIC4 contains 31 epidermal growth factor (EGF) modules, 12 thrombospondin type-1 (TSP-1) modules and a highly acidi c, proline and glycine-rich region in its extracellular region, plus the co nserved transmembrane and cytosolic tail. Like EtMIC1, another TRAP family member from E. tenella, EtMIC4 is expressed in sporozoites and all the mero zoite stages of the parasite, suggesting that this parasite has a strong re quirement for TSP-1 modules. Unlike the other microneme proteins so far stu died in E. tenella, EtMIC4 appears to be found constitutively on the sporoz oite surface as well as within the micronemes. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserve d.