Giardia intestinalis is a protozoan parasite that causes the gastrointestinal disease giardiasis, one of the most frequent parasitic infections worldwide. Giardia was for many years considered to be a primitive or early diverging eukaryote because it lacks organelles typically present in eukaryotes such as classical mitochondria, nucleoli and peroxisomes. On the other hand, it has a cytoskeleton, an endomembrane system and subcellular protein sorting functions of higher cells, which suggest it is a complex and highly developed cell. It has recently been shown that two giardial mitochondrial- related proteins, the IscU and IscS, eo-localize inside a double-membrane organelle of mitochondrial origin, the mitosome. This thesis presents evidence, from in vivo and in vitro protein import studies, that Giardia mitosomes also harbor another mitochondrion-related protein, ferredoxin, and that despite their advanced state of reductive evolution, mitosomes have retained saturable presequence-dependent and presequence-independent protein import pathways analogous to those that operate in mammalian mitochondria. Giardia also encodes a Fe-hydrogenase gene, even though it lacks recognizable hydrogenosomes. The cytosolic localization of the Fe- hydrogenase observed in this study allows the conclusion that the remnant organelles of Giardia are not hydrogenosomes, but mitosomes. The development of an in vitro protein import assay using highly enriched intact mitosomes from Giardia trophozoites and the radiolabeled precursor Fd protein is also reported. The purification method devised in this study yielded a highly enriched, intact mitosomal population competent for protein import. The mitosomal protein import assay demonstrated the proteolytic removal of the targeting peptide of Fd upon organelle import, which is a functional feature of the mitochondrial and hydrogenosomal protein import systems. These results are strong evidence that G. intestinalis possess a mitosomal protein import machinery similar to that of mitochondria and hydrogenosomes, providing further evidence that these three organelles arose from a common endosymbiont.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:588756 |
| Date | January 2012 |
| Creators | Zourmpanou, Danai Maura |
| Publisher | University of London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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