Abstract
Apicomplexan parasites are single eukaryotic cells with a highly polarised secretory system that contains unique secretory organelles (micronemes and rhoptries) that are required for host cell invasion. In contrast, the role of the endosomal system is poorly understood in these parasites. With many typical endocytic factors missing, we speculated that endocytosis depends exclusively on a clathrin-mediated mechanism. Intriguingly, in Toxoplasma gondii we were only able to observe the endogenous clathrin heavy chain 1 (CHC1) at the Golgi, but not at the parasite surface. For the functional characterisation of Toxoplasma gondii CHC1 we generated parasite mutants conditionally expressing the dominant negative clathrin Hub fragment and demonstrate that CHC1 is essential for vesicle formation at the trans-Golgi network. Consequently, the functional ablation of CHC1 results in Golgi aberrations, a block in the biogenesis of the unique secretory microneme and rhoptry organelles, and of the pellicle. However, we found no morphological evidence for clathrin mediating endocytosis in these parasites and speculate that they remodelled their vesicular trafficking system to adapt to an intracellular lifestyle.
Highlights
Apicomplexan parasites are obligate intracellular parasites that usually reside within a non-fusogenic parasitophorous vacuole [1], where they replicate
We identified a single gene for clathrin heavy chain 1 (CHC1) in the genome of apicomplexan parasites that is highly conserved (Figure S1)
The function and interaction partners of CHC1 have been best studied in ophistokonts, such as yeast, and complex CHC1 interactomes have been established that can be accessed via the Saccharomyces Genome Database (SGD; www.yeastgenome.org)
Summary
Apicomplexan parasites are obligate intracellular parasites that usually reside within a non-fusogenic parasitophorous vacuole [1], where they replicate. In the case of Toxoplasma gondii (T. gondii), this parasitophorous vacuole is highly permeable for molecules up to 1.9 kDa [2]. Clathrin is the prototype vesicle coat protein functioning in protein sorting at endosomal membranes and at the trans-Golgi network (TGN) [6,7,8,9]. It forms a three-legged hexameric protein complex in the cytoplasm termed triskelion which represents its functional unit
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