Plasma membrane transbilayer asymmetry of PI(4,5)P ₂ drives unconventional secretion of Fibroblast Growth Factor 2

Unconventional secretion of Fibroblast Growth Factor 2 (FGF2) is based upon direct self-translocation across the plasma membrane, a process that involves the transient formation of a lipidic membrane pore. The opening of this pore is triggered by PI(4,5)P ₂ -dependent oligomerization of FGF2 at the inner plasma membrane leaflet. Subsequently, FGF2 oligomers populating the pore are captured by membrane-proximal heparan sulfate chains of Glypican-1 (GPC1), resulting in translocation of FGF2 to cell surfaces. PI(4,5)P ₂ is a highly negatively charged membrane lipid that is exclusively localized at the inner plasma membrane leaflet. Therefore, local accumulation of PI(4,5)P ₂ triggered by FGF2 oligomerization at the inner plasma membrane leaflet produces a steep and spatially restricted electrochemical gradient across the plasma membrane. Furthermore, PI(4,5)P ₂ has a wedge-like shape, turning it into a non-bilayer lipid that destabilizes membranes at high local concentrations. Here we demonstrate that an asymmetric distribution of PI(4,5)P ₂ across the leaflets of synthetic lipid bilayers accelerates the opening of FGF2-induced membrane pores. Consistently, we find unconventional secretion of FGF2 from cells to be inhibited under conditions compromising the native transbilayer asymmetry of PI(4,5)P ₂ of plasma membranes. We propose the asymmetric distribution of PI(4,5)P ₂ to lower the free energy required to transform the lipid bilayer into a lipidic membrane pore. Furthermore, in the proximity of FGF2 membrane translocation sites, PI(4,5)P ₂ in the outer plasma membrane leaflet could potentially repel negatively charged heparan sulfates chains compromising the function of GPC1 in FGF2 translocation into the extracellular space. Thus, transbilayer asymmetry of PI(4,5)P ₂ is a key parameter enabling fast kinetics of FGF2 membrane translocation into the extracellular space.