Toxoplasma gondii homologue of plasmodium apical membrane antigen 1 is involved in invasion of host cells

Proteins with constitutive or transient localization on the surface of Apic omplexa parasites are of particular interest for their potential role in th e invasion of host cells. We describe the identification and characterizati on of TgAMA1, the Toxoplasma gondii homolog of the Plasmodium apical membra ne antigen 1 (AMA1), which has been shown to elicit a protective immune res ponse against merozoites dependent on the correct pairing of its numerous d isulfide bonds. TgAMA1 shows between 19% (Plasmodium berghei) and 26% (Plas modium yoelii) overall identity to the different Plasmodium AMA1 homologs a nd has a conserved arrangement of 16 cysteine residues and a putative trans membrane domain, indicating a similar architecture. The single copy TgAMA1 gene is interrupted by seven introns and is transcribed into an mRNA of sim ilar to3.3 kb. The TgAMA1 protein is produced during intracellular tachyzoi te replication and initially localizes to the micronemes, as determined by immunofluorescence assay and immunoelectron microscopy. Upon release of mat ure tachyzoites, TgAMA1 is found distributed predominantly on the apical en d of the parasite surface. A similar to 54-kDa cleavage product of the larg e ectodomain is continuously released into the medium by extracellular para sites. Mouse antiserum against recombinant TgAMA1 blocked invasion of new h ost cells by approximately 40%. This and our inability to produce a viable TgAMA1 knock-out mutant indicate that this phylogenetically conserved prote in fulfills a key function in the invasion of host cells by extracellular T . gondii tachyzoites.