Seemann, R. | Search bioRxiv

bioRxiv2022

Phospholipids Diffusion on the Surface of Model Lipid Droplets

Lipid droplets (LD) are organelles localized in the membrane of the Endoplasmic Reticulum (ER) that play an important role in metabolic functions. They consist of a core of neutral lipids surrounded by a monolayer of phosphoplipids and proteins resembling an oil-in-water emulsion droplet. Many studies have focused on the biophysical properties of these LDs. However, despite numerous efforts, we are lacking information on the mobility of phospholipids on the LDs surface, although they may play a key role in the protein distribution. In this article, we developed a microfluidic setup that allows the formation of a triolein-buffer interface decorated with a phospholipid monolayer. Using this setup, we measured the motility of phospholipid molecules by performing Fluo-rescent Recovery After Photobleaching (FRAP) experiments for different lipidic compositions. The results of the FRAP measurements reveal that the motility of phospholipids is controlled by the monolayer packing decorating the interface.

biophysics↗

bioRxiv2021

Cholesterol promotes both head group visibility and clustering of PI(4,5)P2 driving unconventional secretion of Fibroblast Growth Factor 2

Fibroblast Growth Factor 2 (FGF2) is a cell survival factor involved in tumor-induced angiogenesis. FGF2 is secreted through an unconventional secretory pathway based upon direct protein translocation across the plasma membrane. Here we demonstrate that both PI(4,5)P2-dependent FGF2 recruitment at the inner plasma membrane leaflet and FGF2 membrane translocation into the extracellular space are positively modulated by cholesterol in living cells. We further reveal cholesterol to enhance FGF2 binding to PI(4,5)P2-containing lipid bilayers in a fully reconstituted system. Based on extensive atomistic molecular dynamics simulations and membrane tension experiments, we propose cholesterol to modulate FGF2 binding to PI(4,5)P2 by (i) increasing head group visibility of PI(4,5)P2 on the membrane surface, (ii) increasing avidity by cholesterol-induced clustering of PI(4,5)P2 molecules triggering FGF2 oligomerization and (iii) increasing membrane tension facilitating the formation of lipidic membrane pores. Our findings have general implications for phosphoinositide-dependent protein recruitment to membranes and explain the highly selective targeting of FGF2 towards the plasma membrane, the subcellular site of FGF2 membrane translocation during unconventional secretion of FGF2.

biophysics↗