The minimal ESCRT machinery of Giardia lamblia has altered inter-subunit interactions within the ESCRT-II and ESCRT-III complexes

The ESCRT pathway functions at different subcellular membranes to induce their negative curvature, and it has been largely characterized in model eukaryotes belonging to Opisthokonta. But searches of the genomes of many nonopisthokonts belonging to various supergroups indicate that some of them may harbour fewer ESCRT components. Of the genomes explored thus far, one of the most minimal set of ESCRT components was identified in the human pathogen Giardia lamblia, which belongs to Excavata. Here we report that an ESCRT-mediated pathway most likely operates at the peripheral vesicles, which are located at the cell periphery and the bare zone of this protist. Functional comparison of all the identified putative giardial ESCRT components, with the corresponding well-characterized orthologues from Saccharomyces cerevisiae, indicated that only some of the ESCRT components could functionally substitute for the corresponding yeast proteins. While GlVps25, GlVps2, and all three paralogues of GlVps4, tested positive in functional complementation assays, GlVps22, GlVps20, and GlVps24 did not. Binary interactions of either GlVps22 or GlVps25, with other ESCRT-II components from Giardia or yeast indicate that the giardial Vps36 orthologue is either completely missing or highly diverged. Interactions within the giardial ESCRT-III components also differ from those in yeast; while GlVps46a interacts preferentially with Vps24 compared to Vps2, GlVps46b, like the yeast orthologue, interacts with both.