Molecular characterisation of a novel family of cysteine-rich proteins of Toxoplasma gondii and ultrastructural evidence of oocyst wall localisation

Abstract

Among apicomplexan parasites, the coccidia and Cryptosporidium spp. are important pathogens of livestock and humans, and the environmentally resistant stage (oocyst) is essential for their transmission. Little is known of the chemical and molecular composition of the oocyst wall. Currently, the only parasite molecules shown to be involved in oocyst wall formation are the tyrosine-rich proteins gam56, gam82 and gam230 of Eimeria spp. and the cysteine-rich proteins COWP1 and COWP8 of Cryptosporidium parvum. In the present study, we searched the ToxoDB database for the presence of putative Toxoplasma gondii oocyst wall proteins (OWPs) and identified seven candidates, herein named TgOWP1 through TgOWP7, showing homology to the Cryptosporidium COWPs. We analysed a cDNA library from partially sporulated oocysts of T. gondii and cloned the full-length cDNAs encoding TgOWP1, TgOWP2 and TgOWP3, which consist of 499, 462 and 640 amino acids, respectively. The three proteins share 24% sequence identity with each other and a markedly similar overall structure, based on the presence of an N-terminal leader peptide followed by tandem duplications of a six-cysteine amino acid motif closely related to the Type I repeat of COWPs. Using antisera to recombinant TgOWP1, TgOWP2 and TgOWP3, we showed by Western blot that these molecules are expressed in T. gondii oocysts but are not detectable in tachyzoites. The solubilisation of TgOWP1–3 strictly depended on the presence of reducing agents, consistent with a likely involvement of these proteins in multimeric complexes mediated by disulphide bridges. Immunofluorescence analysis allowed the localisation of TgOWP1, TgOWP2 and TgOWP3 to the oocyst wall. Additionally, using immunoelectron microscopy and the 1G12 monoclonal antibody, TgOWP3 was specifically detected in the outer layer of the oocyst wall, thus representing the first validated molecular marker of this structure in T. gondii.