The prodomain of Toxoplasma gondii GPI-anchored subtilase TgSUB1 mediates its targeting to micronemes

Subtilisin-like proteases have been proposed to play an important role for parasite survival in Toxoplasma gondii (Tg) and Plasmodium falciparum. The T. gondii subtilase TgSUB1 is located in the microneme, an apical secretory organelle whose contents mediate adhesion to the host during invasion. TgSUB1 is predicted to contain a glycosyl-phosphatidylinositol (GPI) anchor. This is unusual as Toxoplasma GPI-anchored proteins are targeted to the parasite's surface. In this study, we report that the subtilase TgSUB1 is indeed a GPI-anchored protein but contains dominant microneme targeting signals. Accurate targeting of TgSUB1 to the micronemes is dependent upon several factors including promoter strength and timing, accurate processing and folding. We analyzed the targeting domains of TgSUB1 using TgSUB1 deletion constructs and chimeras made between TgSUB1 and reporter proteins. The TgSUB1 prodomain is responsible for trafficking to the micronemes and is sufficient for targeting a reporter protein to the micronemes. Trafficking is dependent upon correct folding or other context-dependent conformation as the prodomain expressed alone is unable to reach the micromenes. Therefore, TgSUB1 is a novel example of a GPI-anchored protein in T. gondii that bypasses the GPI-dependent surface trafficking pathway to traffic to micronemes, specialized regulated secretory organelles.