Plasma membrane transbilayer asymmetry of PI(4,5)P2 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)P2-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)P2 is a highly negatively charged membrane lipid that is exclusively localized at the inner plasma membrane leaflet. Therefore, local accumulation of PI(4,5)P2 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)P2 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)P2 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)P2 of plasma membranes. We propose the asymmetric distribution of PI(4,5)P2 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)P2 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)P2 is a key parameter enabling fast kinetics of FGF2 membrane translocation into the extracellular space.