The protozoan parasite Toxoplasma gondii is both an important pathogen in its own right and a useful model to study apicomplexan cell biology. Like the malaria parasites, T. gondii possess an unusual organelle, the apicoplast, which contains a genome reminiscent of those found in chloroplasts. Two points make the apicoplast of interest as a potential drug target: 1) the human host lacks organelles related to the apicoplast, and 2) the apicoplast appears to be essential in T. gondii and Plasmodium. 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 epitope or GFP-tagged versions of proteins destined for the apicoplast.  Early in this project, our studies, as well as those of others, demonstrated that proteins that reside in the apicoplast lumen contain a bipartite N-terminal targeting sequence that routes them first into the ER and from there to the apicoplast.  More recently, we have focused on membrane proteins, identifying the first apicoplast membrane proteins in T. gondii. These proteins lack the canonical targeting sequences typical of luminal proteins.  We have shown that the proteins traffic in vesicles and that this trafficking occurs primarily during the stage of the cell cycle when the apicoplast is enlarging prior to division.  Current studies are aimed at dissecting the sequences and mechanisms responsible for routing membrane proteins to the apicoplast.  

This work is a collaboration the Coppens lab at Johns Hopkins University.  It has been funded by a grant from the National Institutes of Health: R01 AI 50506 Marilyn Parsons, Principal Investigator.

 
This immunofluorescence image shows a host cell infected with multiple T. gondii parasites.  Distribution of Toxoplasma apicoplast phosphate translocator APT1 (green) varies during the cell cycle as revealed by plastid shape and location (red), inner membrane complex formation (magenta) and nuclear position (blue).   The large blue object is the host cell nucleus.  (Karnataki et al., PMID:  17367386.

Mitochondrial Function in Bloodstream Trypanosoma brucei

The mitochondrion of T. brucei changes dramatically during the parasite’s life cycle.  Many functions are repressed in the pathogenic bloodstream form, but some functions remain essential.  This project is aimed at understanding the role of mitochondrial complex I (NADH dehydrogenase) and complex V (ATP synthase) in the bloodstream stage.  The project uses genetic approaches to knock down or overexpress proteins in the parasite to examine their function.  It also examines how these functions change in parasites that lack mitochondrial DNA, (“dyskinetoplastid” strains).  This project is a collaboration with Dr. Achim Schnaufer at the University of Edinburgh and is funded by a grant from the National Institutes of Heal R01 AI069057.

Localization of an epitope-tagged subunit of complex I (NdhK, green) to the tubular mitochondrion of bloodstream form T. brucei.  Cells were also stained with mitotracker (red) and DAPI (blue).