What are the metabolic functions of the apicoplast and mitochondrion in apicomplexan parasites?
The apicoplast and mitochondrial organelles of apicomplexan parasites such as Plasmodium and Toxoplasma serve as essential biosynthetic hubs and are validated drug targets. For the metabolism of these organelles to be integrated with the rest of the cell, the organelle must:
- have access to building blocks necessary to make metabolites;
- have the necessary enzymes to catalyse the synthesis of metabolites;
- be able to share these metabolites with the rest of the cell.
The questions we are trying to answer include:
- What metabolites do the apicoplast and mitochondrion provide for the parasite?
- Are these metabolites essential for parasite survival?
- What membrane transport proteins traffic building blocks and metabolic products across the membranes that surround the apicoplast and mitochondrial organelles?
Our approach in addressing these questions is:
- to generate genetic mutants in selected enzymes or membrane transporters of the metabolic pathways we are interested in. To do this we typically generate inducible mutants in Toxoplasma where we can rapidly turn off a gene- or protein-of-interest;
- to develop biochemical and other assays to measure the effect of losing a particular protein on that pathway. We do this through a range of techniques. For example, we can measure the synthesis of haem using liquid chromatography coupled to mass spectrometry;
- to determine whether the metabolic pathway in question is essential for parasite survival, a critical question in determining whether the pathway may serve as a drug target. We have several ways to measure parasite growth, such as by introducing a super-bright fluorescent protein into our mutant parasites and measuring growth over time as a function of fluorescence intensity in a population of parasites;
- where appropriate, to use the mutants we have generated to learn more about the function of the enzyme or transporter. For example, we can attempt to complement the mutants we generate with a second copy of the protein-of-interest that is missing domains or residues that we hypothesise may be important for its function.
Recent papers from this project that summarise the sorts of questions we are interested in:
- van Dooren GG, Kennedy AT, McFadden GI (2012) The use and abuse of heme in apicomplexan parasites Antioxid Redox Signal, in press.
- Brooks CF*, Johnsen H*, van Dooren GG*, Muthalagi M, Lin SS, Bohne W, Fischer K and Striepen B (2010) The Toxoplasma apicoplast phosphate translocator links cytosolic and apicoplast metabolism and is essential for parasite survival. Cell Host Microbe 7(1): 62-73. *these authors contributed equally
- van Dooren GG, Stimmler LM, McFadden GI (2006) Metabolic maps and functions of the Plasmodium mitochondrion. FEMS Microbiol Rev. 30(4): 596-630.
- alph SA, van Dooren GG, Waller RF, Crawford MJ, Fraunholz MJ, Foth BJ, Tonkin CJ, Roos DS, McFadden GI. (2004) Metabolic maps and functions of the Plasmodium falciparum apicoplast. Nat Rev Microbiol. 2(3): 203-16.