Characterization of a novel thrombospondin-related protein in Toxoplasma gondii.

Toxoplasma gondii is an obligate intracellular protozoan parasite that invades a wide range of host cells. The parasite releases a large variety of proteins from a secretory organelle, microneme, and the secretion is essential for the parasite invasion. We cloned a secreted protein with an altered thrombospondin repeat of Toxoplasma gondii (TgSPATR), which was the homologue of Plasmodium SPATRs. Immunofluorescence double staining experiment revealed that TgSPATR was co-localized with a microneme protein, MIC2, and immuno-electron microscopic (IEM) analysis detected TgSPATR in the microneme-like structure. TgSPATR secretion was induced by ethanol, while an intracellular Ca(2+) chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA-AM), suppressed the ethanol-induced secretion, suggesting the secretion was Ca(2+)-dependent, similarly to known microneme proteins. Furthermore, TgSPATR, existed on outer surface of the parasites, was detected by incomplete membrane permeabilization by saponin and immunofluorescent antibody test (IFAT). Both TgSPATR and MIC2 were detected on outer surface of extracellular parasites, but not of intracellular single parasites, suggesting they were similarly secreted during early stages of parasite invasion. Therefore, TgSPATR is probably new member of microneme protein and maybe involved in parasite invasion.