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There phosphatidylserine (PS) is an essential anionic lipid of eukaryotic cells. It is asymmetrically distributed in the cell: the PS concentration is multiplied by five between the endoplasmic reticulum (ER), its site of synthesis, and the plasma membrane, where this lipid, very abundant, brings negative charges and plays a key role in the activation of signaling proteins. We have revealed in yeast that the protein Osh6 is a PS/PI4P exchanger that exploits a PI4P gradient at ER/plasma membrane contact sites to transport PS to the plasma membrane. Using biochemistry and cell biology approaches, we seek to decipher the factors enabling the rapidity, specificity and precision of transfer. We also want to see if this process is coupled to another PS transfer process, this time across the ER membrane, via an interaction between Osh6 and Ist2This project relies on sophisticated real-time fluorescence measurements to quantify protein activity. in vitro, membrane-protein and protein-protein binding tests, microscopy observations in yeast, analyses in silico and structural. ANR Mix&Move, collaboration with Drs A. Copic (CRBM, Montpellier) and G. Lenoir (I2BC, Gif/Yvette).

Figure 1 – Structure of the Osh6 protein in complex with a PS molecule (pink) or PI4P (orange)

Figure 2 – Current model of how Osh6 protein works. The Osh6 protein uses a PI4P gradient at the ER/plasma membrane interface to transport phosphatidylserine (PS) vectorially via PS/PI4P exchange cycles: this allows the accumulation of PS in the plasma membrane. Osh6 interacts with the cytosolic and disordered region of Ist2 to localize to the ER-plasma membrane contact site. An exciting hypothesis is that Ist2 functions as a scramblase, capable of distributing PS, once synthesized, between the two layers of the endoplasmic reticulum and that this activity is coupled with that of Osh6 to ensure a correct intracellular distribution of PS in yeast.
Figure 3 – Protocols for measuring Osh6 activity in vitro And in vivo. (HAS) We have designed protocols to measure in real time by FRET the transport of PS and PI4P by recombinant Osh6p protein along opposite directions between artificial lipid vesicles (liposomes). (B) The PS transport activity of Osh6p from the ER to the plasma membrane can also be measured in yeast (coll. A. Copic, CRBM, Montpellier)

Publications

2025
Fabbre A, Syska C, Sebinelli HG, Cardinal A, Magdeleine M, Al-Qatabi N, D'Ambrosio JM, Montigny C, Lenoir G, Čopič A, Drin G, Ist2 promotes lipid transfer by Osh6 via its membrane tethering and lipid scramblase activities., Sci Adv 2025 Nov; 11(46): eadz2217.
2025
Delfosse V, Drin G, Determining the Relative Affinity of ORPs for Lipid Ligands Using Fluorescence and Thermal Shift Assays. Methods Mol Biol 2025; 2888(): 259-280.
2021
Ikhlef S, Lipp NF, Delfosse V, Fuggetta N, Bourguet W, Magdeleine M, Drin G, Functional analyzes of phosphatidylserine/PI(4)P exchangers with diverse lipid species and membrane contexts reveal unanticipated rules on lipid transfer. BMC Biol 2021 Nov; 19(1): 248.
2021
Ikhlef S, Lipp NF, Magdeleine M, Drin G, Fluorescence-Based Measurements of Phosphatidylserine/Phosphatidylinositol 4-Phosphate Exchange Between Membranes. J Vis Exp 2021 Mar; (169): .
2020
D'Ambrosio JM, Albanese V, Lipp NF, Fleuriot L, Debayle D, Drin G, Čopič A, Osh6 requires Ist2 for localization to ER-PM contacts and efficient phosphatidylserine transport in budding yeast. J Cell Sci 2020 Jun; 133(11): .
2019
Lipp NF, Gautier R, Magdeleine M, Renard M, Albanèse V, Čopič A, Drin G, An electrostatic switching mechanism to control the lipid transfer activity of Osh6p., Nat Commun 2019 Sep;10(1):3926.
2015
Moser von Filseck J, Čopič A, Delfosse V, Vanni S, Jackson CL, Bourguet W, Drin G, INTRACELLULAR TRANSPORT. Phosphatidylserine transport by ORP/Osh proteins is driven by phosphatidylinositol 4-phosphate. Science 2015 Jul; 349(6246): 432-6.