The protozoan parasite Toxoplasma gondii is both a pathogen in its own right and an important apicomplexan model organism. Like malaria parasites, T. gondii possess an unusual organelle which contains a genome reminiscent of those found in chloroplasts. The organelle, called the apicoplast, is bounded by four membranes, suggesting it arose from a secondary endosymbiosis in which the apicomplexan ancestor engulfed a photosynthetic alga.
Most apicoplast proteins appear to be encoded in the nucleus.
To dissect the essential properties of apicoplast targeting sequences, we are transfecting GFP fusion constructs bearing putative targeting sequences. Our work has shown that the targeting sequences are structurally and functionally bipartite. The amino portion of the targeting sequence is an ER signal sequence. The downstream portion of the targeting sequence resembles a chloroplast transit peptide. Indeed, we have shown this region is capable of directing protein import into isolated pea chloroplasts.