Actin regulation in Plasmodium falciparum: towards understanding the elusive nature of malarial actin filaments
Document TypePhD thesis
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© 2015 Dr. Maya Avital Olshina
Malaria disease, caused by the unicellular parasites from the genus Plasmodium, is a major cause of morbidity and mortality in many developing countries throughout the world. While there have been many improvements in intervention strategies in recent years, parasite resistance to front-line therapeutics is on the rise, highlighting the need for new and improved treatments and vaccines. To this end, a greater understanding of the biological mechanisms underpinning the disease will be crucial in the push towards malaria eradication. Across the malaria life cycle the parasite must traverse tissues and invade host cells in order to establish an infection and replicate. A conserved acto-myosin motor, anchored at the parasite periphery, generates the requisite force to drive the parasite forward, facilitating both invasion and motility. The actin at the heart of this motor is extremely divergent, forming filaments that are highly dynamic and unstable. Tightly controlled regulation of malaria actin is therefore necessary to direct the formation and disassembly of filaments in an appropriate spatio-temporal manner. However, malaria parasites possess a markedly reduced repertoire of actin regulators, of which coronin is one of the only predicted filament regulators. Much of the current literature surrounding Plasmodium actin biology relies on the production of actin from recombinant sources. In this study I investigate the various published methods for purifying recombinant malaria actin, and determine that the unusual characteristics previously reported for this actin are likely artifacts driven by incomplete protein folding in heterologous expression systems. This finding lead to the identification of the key actin folding chaperonin CCT in the Plasmodium genome, an essential protein complex required for producing native, functional actin in the cell. In parallel, characterization of the filament regulator, coronin, revealed its critical role in the organization of actin filaments. Using in vitro observations from recombinant Plasmodium falciparum coronin (PfCoronin), I have demonstrated that PfCoronin binds to actin filaments and bundles them together in parallel arrays. Furthermore, in vivo observations revealed PfCoronin to be located at the periphery of the parasite, consistent with the pellicular space in which the actin-myosin motor is housed. This localization is likely mediated by peripheral interactions with PI(4,5)P2 at the plasma membrane. These data identify PfCoronin as a potentially key regulator of actin filament recruitment and bundling at the cell cortex of motile Plasmodium parasites. Taken together, the identification of Plasmodium CCT and the characterization of PfCoronin have opened up new avenues for further development of these as potential drug targets, with the eventual aim of potentially crippling the motile malaria parasite and halting the progression of disease.
KeywordsMalaria; actin; actin regulation; actin dynamics; coronin; CCT complex
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