Abstract : Some mammalian cells show striking differences in the acyl chain composition of their membrane phospholipids. In most cases, the majority of phospholipids bear one saturated and one monounsaturated acyl chains at positions 1 and 2 or the glycerol, respectively. However, some cells and notably neurons contain large amounts of phospholipids with a polyunsaturated fatty acyl chain, generally at position 2. The aim of this project is to compare the impact of the phospholipid polyunsaturation vs monounsaturation on the mechanical and functional properties of the plasma membrane. For this, Immortalized Retinal Pigmental Epithelial Cells (RPE1) were cultured with various BSA-fatty acid complexes. Gas chromatography and mass spectrometry show that these cells incorporate most fatty acids in their phospholipids, preferentially in phosphatidylcholine. Notably, docosahexaenic acid (C22:6) levels in phospholipids increase from trace amounts to 5-10 mol % after a few hours. We then compared the plasma membrane elastic properties during incubation with C18:1 or C22:6. The cells were cultured on L-shaped fibronectin micropatterns and a membrane tubule was pulled from the cell hypothenus using optical tweezers. C22:6 caused a gradual 2-fold decrease within an hour of both the pulling force (F) and the tube radius (R) whereas C18:1 caused only modest changes. From these measurements, we conclude that the bending modulus (which depends on the FxR product) strongly decreases with phospholipid polyunsaturation, whereas the tension of the plasma membrane (which depends on the F/R ratio) is mostly unaffected. These results are in good agreement with studies performed on model phospholipid membranes (Biophys J 79, 328-39). We currently investigate how phospholipid insaturation affects the ability of specialized protein machineries involved in transport vesicle formation in RPE1 cells, focusing on the impact of C22:6 compared to C18:1 on the rates of transferrin and EGF endocytosis.