Cytoskeletal Innovations in Giardia lamblia

Abstract

The single-celled eukaryote Giardia lamblia is one of the most common gastrointestinal parasites worldwide; infection causes diarrhea and vomiting, which can lead to dehydration and malnutrition. As an early divergent eukaryotic parasite, its cell biology is highly unusual compared to model organisms. Giardia lamblia’s actin (GlActin) is only 58% identical to the average eukaryotic actin and its genome lacks genes encoding actin interactors in other organisms. Despite this, its actin cytoskeleton retains conserved functions in cell polarity, membrane trafficking, and cytokinesis. To understand how GlActin carries out these activities, I used mass spectrometry to identify candidate GlActin filament interactors. I categorized the general structure and used fluorescent protein fusions to examine subcellular localizations of 46 putative interactors. The proteins were enriched in the cell cortex, internal membranes, marginal plate, nuclei, flagella, or ventral disc, a microtubule-based organelle that the parasite uses to attach to its host’s intestine. One interactor, denoted Disc and Actin Associated Protein 1 (DAAP1), was enriched in the two areas of the ventral disc that are key for the fluid flow that mediates attachment. I confirmed complex formation between GlActin and DAAP1. I found that GlActin depletion results in severely impaired attachment and defects in disc morphology, while DAAP depletion results in decreased ability to attach and impaired disc seal formation. These studies revealed a role for actin and DAAP1 in ventral disc function. Since attachment is required for infection, targeting attachment could be an avenue for developing new treatments for giardiasis.