The trypanosome ESCRT system possesses both novel components and under-recognised pan-eukaryotic features
Abstract
Endocytosis, responsible for the turnover of surface receptors, transporters and modulation of signal transduction and other functions, is a near-universal process amongst eukaryotic cells, but with considerable diversity in roles and composition between lineages. Diverse components have been described for clathrin-mediated endocytosis in trypanosomatid flagellates, specifically Trypanosoma brucei and T. cruzi . The endosomal sorting complexes required for transport (ESCRT) system is a multisubunit protein assembly coordinating late steps in endocytosis and sorting of ubiquitylated proteins. A major evolutionary shift was the evolution of the ESCRT-0 subcomplex in animals and fungi, which is implicated in substrate recognition and membrane binding. We investigated the composition of the trypanosome ESCRT-I subcomplex together with a candidate ESCRT-0 analogue. Firstly, we identified two novel, trypanosome-specific Vps23-interacting proteins, indicating that the ESCRT-I subcomplex of trypanosomes is a heteropentameric complex. Further, we found that the pan-eukaryotic VHS-domain-containing protein Tom1 colocalises with TbVps23 and interacts with a second protein “Jerry” specific to Kinetoplastea. Cells harbouring tagged TbTom1 or TbJerry display increased flagellar pocket dimensions, indicating an impact on endocytosis. We extend the compositional divergence within the trypanosome endocytic complex with further evidence for evolutionary flexibility within the ESCRT system. Tom1 is broadly present in eukaryotes and suggested here as the ancestral VHS-GAT-domain-containing protein. Evidence for Tom1 acting in the ESCRT system of a fourth eukaryotic supergroup is presented, suggesting that Tom1, and not Vps27/STAM, is likely part of the pan-eukaryotic early ESCRT machinery. Further, we uncover additional components within the trypanosome ESCRT system suggesting ongoing specialisation within this highly plastic system.