2 - Understanding Phosphoinositide functions in vesicular trafficking to organelles

maryse lebrun

Maryse Lebrun (Project leader)

Wassim Daher (permanent researcher)
Juliette Morlon-Guyot (permanent researcher)

Apicomplexan parasites have acquired a plastid (apicoplast) from secondary endosymbiosis ofa red algae that is home to numerous essential metabolic functions. Much of the symbiont genetic information has been transferred to the parasite nucleus and the corresponding proteins now have to be postranslationally imported in the plastid. Therefore, they need to cross the four membranes surrounding this organelle in Apicomplexa. It has been shown that the protein-import machinery of each apicoplast membrane reflects the origin of that membrane. How the proteins reach the first membrane is presently a mystery. We have recently discovered the implication of a vesicular PI3P-dependent machinery involved in this first step in the complex journey of nuclear encoded proteins to the apicoplast. PI3P was found to be present at the apicoplast in T. gondii and on small electron-dense vesicles, which were also shown to carry proteins destined to the apicoplast outer membrane. Interference with PI3P-function either by drug treatment or by sequestration of PI3P through over-expression of a PI3P-binding module led to vesicle accumulation and to severe plastid biogenesis defects. These results revealed a function of PI3P in apicomplexan parasites that is different from classical endosomal traffic observed in other organisms including the human host. We propose that PI3P serves as a marker to recruit PI3P-binding proteins that promote fusion of these vesicles with the apicoplast. As the outermost membrane of the apicoplast is originally derived from the phagocytic compartment during the secondary endosymbiosis event, the fascinating question arises whether apicomplexan parasites have reshaped the classical PI3P-dependent endocytic machinery found in other eukaryotes to target proteins to the apicoplast.

In eukaryotic cells in general, PI3P exerts its function in vesicle trafficking and endocytic fusion events through the binding of effector proteins that contain specific PI3P-binding domains (FYVE- and PX-domains). Some of these effector proteins in turn influence endocytic membrane fusion through the action of recruited SNARE proteins. A single putative Vps34–type PI3-kinase, as generally expected for a unicellular eukaryotic organism, and six PI3P-binding proteins are predicted in the genome of Toxoplasma, and are likely candidates for proteins involved in the traffic and fusion steps of endoplasmic reticulum-derived vesicles with the outermost apicoplast membrane.

Our program is geared towards

► the functional characterization of PI3P effectors proteins and PI3-kinase ► isolating and identifying the content of PI3P-containing vesicles that carry apicoplast membrane proteins.

Expected outcomes :

► elucidate how PI3P exerts its function on apicoplast biogenesis ► gain new insights into the machinery that allows protein delivery to the outermost membrane. ► expand the repertoire of apicoplast proteins, some of which might not contain the typical bipartite target sequence and thus have not yet been identified by bioinformatic approaches

This project is conducted in collaboration with Kai Wengelnik, who is working on Plasmodium. Working together in the same laboratory and on the same subject on these two important parasites has already proved to be scientifically extremely powerful, since the results obtained in one system can stimulate and inspire research strategies in the other system.

Project financed through a grant by the French National Research Agency ANR.


Recent references related to this project :

Daher W, Morlon-Guyot J, Alayi TD, Tomavo S, Wengelnik K, Lebrun M. Identification of Toxoplasma TgPH1, a pleckstrin homology domain-containing protein that binds to the phosphoinositide. Mol Biochem Parasitol. 2016 May;207(1):39-44.

Tawk, L., Dubremetz, J.F., Montcourrier, P., Chicanne, G., Merezegue, F., Richard, V., Payrastre, B., Meissner, M., Vial, H.J., Roy, C., Wengelnik, K., and Lebrun, M. (2011) Phosphatidylinositol 3-monophosphate is involved in Toxoplasma apicoplast biogenesis. PLoS Pathog 7 : e1001286.