Eukaryotic signature proteins : guides to pathogenic eukaryotic parasites : a thesis presented in partial fulfilment of the requirements of the degree of PhD in Genetics at Massey University, Palmerston North, New Zealand
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Date
2012
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Massey University
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Abstract
Eukaryotic Signature Proteins (ESPs) are proteins that delineate the eukaryotes from the archaea and bacteria. They have no homologues in any prokaryotic genome, but their homologues are present in all main branches of eukaryotes. ESPs are thus likely to have descended from ancient proteins that have existed since the first eukaryotic cell. This project looks at ESPs of some eukaryotic parasites and human (Homo sapiens) as their host organism and focuses on Giardia lamblia, a fresh water pathogenic basal eukaryote. The ESP datasets from Giardia and two other parasites, Trichomonas vaginalis and Plasmodium falciparum, as well as the host human were calculated in light of available genomic data and the datasets contained a range of proteins associated with membrane, cytoskeleton, nucleus and protein synthesis.
ESPs have great potential in phylogenetic studies since these proteins are present in all eukaryotes and are expected to have a slow and constant rate of evolution. Phylogenetic analyses were performed on the 18 eukaryotic organisms including some basal eukaryotes, and also for mammals, using orthologues of the all ESPs from these organisms. Strategies such as concatenating sequences and constructing consensus networks were tested to evaluate their potential with large numbers of ESP alignments. The results were promising, and ESPs hold great potential for their use in future phylogenetic analyses of eukaryotes.
RNA interference is hypothesised to be an ancient mechanism for gene regulation and like the ESPs, it is typically found in all main branches of eukaryotes. High throughput sequencing data from Giardia and Trichomonas small RNAs (15-29mers) were re-analysed showing two length peaks for Giardia RNAs: a “larger peak” and an “ultra small peak”, the former of which is likely to be the product of the enzyme Dicer, which processes miRNA. The “ultra small peak” but not the “larger peak” was also found in Trichomonas. The two peaks possibly represent two different mechanisms of RNA interference (RNAi) in these parasites, but analysis of potential target sites from the Dicer-processed RNAs has not yet shown any indication that ESPs are regulated any differently from other parasite proteins.
Sugar metabolic pathways including glycolysis and citric acid cycle were searched for ESPs, this was done to determine the relationship between the conservation of
eukaryotic metabolic pathways and conservation of individual proteins. However no ESPs were identified from these pathways because Giardia has enzymes that show more similarity to those from prokaryotes than eukaryotes. These enzymes are significantly different from that of the host‟s, and these alternative enzymes offer potential as novel drug targets. In addition, ESPs that are present from host but lost in some parasites were analysed, and these ESPs are involved in many understudied pathways. It is these differences which can provide a guide in determining which pathways we should examine when designing drug targets.
Overall, numerous proteomic similarities and differences in ESPs were identified between host and parasite. These proteins show potential for future evolutionary studies, and will guide future directions in ancestral eukaryotic regulation and metabolism.
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Eukaryotic Signature Proteins, Eukaryotic parasites, Giardia lamblia, Trichomonas vaginalis, Plasmodium falciparum, Phylogeny, Protein evolution, Eukaryotes, Gene regulation