Characterization of atypical BAR domain-containing proteins coded by Toxoplasma gondii

Toxoplasma gondii, the causative agent of toxoplasmosis, infects cells and replicates inside via the secretion of factors stored in specialized organelles (rhoptries, micronemes, and dense granules) and the capture of host materials. The genesis of the secretory organelles and the processes of secretion and endocytosis depend on vesicular trafficking events whose molecular bases remain poorly known. Notably, there is no characterization of the BAR (Bin/Amphiphysin/Rvs) domain-containing proteins expressed by T. gondii and other apicomplexans, although such proteins are known to play critical roles in vesicular trafficking in other eukaryotes. Here, by combining structural analyses with in vitro assays and cellular observations, we have characterized TgREMIND (regulators of membrane interacting domains), involved in the genesis of rhoptries and dense granules, and TgBAR2 found at the parasite cortex. We establish that TgREMIND comprises an F-BAR domain that can bind curved neutral membranes with no strict phosphoinositide requirement and exert a membrane remodeling activity. Next, we establish that TgREMIND contains a new structural domain called REMIND, which negatively regulates the membrane-binding capacities of the F-BAR domain. In parallel, we report that TgBAR2 contains a BAR domain with an extremely basic membrane-binding interface able to deform anionic membranes into very narrow tubules. Our data show that T. gondii codes for two atypical BAR domain-containing proteins with very contrasting membrane-binding properties, allowing them to function in two distinct regions of the parasite trafficking system.