Mechanisms of protein production in the parasite Giardia Iamblia

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SUMMARY Giardia lamblia is a unicellular protist whose parasitic infections of humans is a significant threat to health worldwide. Treatments are limited and of variable efficacy: in the developing world Giardia infection can lead to severe disease and death, while in the developed world outbreaks have occurred even from treated water sources. Although the basic molecular biology in Giardia is similar to that in traditional model systems, it is also a deeply branching eukaryote, and so many aspects of its gene regulatory mechanisms have diverged substantially. The fundamental molecular differences between Giardia and its hosts present an opportunity for therapeutic intervention. However, little is understood about these differences, and thus this treatment opportunity has not yet been exploited. In particular, many aspects of how Giardia initiates protein production seem different from what has been studied in traditional model systems, but, currently, the mechanistic implications of these differences remains mysterious. Giardia mRNAs have remarkably short untranslated regions, their ribosomes do not appear to scan, and our analysis suggests that they lack some of the core initiation factors shared almost universally in eukaryotes. These results suggest fundamental differences in the mechanism of initiation, which we will explore through a biochemical and structural approach. We propose to partially address this gap through an initial exploration of fundamental features of the protein synthesis machinery of Giardia. We will take advantage of our ability to culture and genetically manipulate Giardia, combined with our expertise in mechanistic studies of translation initiation and structural biology. We will pursue two aims: (1) We will solve the structure of the Giardia ribosome and its subunits using cryo-electron microscopy. Preliminary analysis shows unexpected differences in these ribosomes compared to other eukaryotes; observing the conformational implications of these differences will provide an essential foundation for detailed mechanistic studies and could suggest therapeutic targets. (2) We will investigate the composition and architecture of Giardia translation initiation complexes. Overall, these exploratory studies promise to yield discoveries related to the basic molecular machinery used by Giardia to generate protein, laying the foundation for future studies that will more broadly and deeply interrogate gene regulation mechanisms in Giardia.