Plasma membrane transbilayer asymmetry of PI(4,5)P₂ drives unconventional secretion of Fibroblast Growth Factor 2.

Abstract

Unconventional secretion of Fibroblast Growth Factor 2 (FGF2) is mediated by direct translocation across the plasma membrane. This process is initiated by PI(4,5)P₂-dependent FGF2 oligomerization at the inner plasma membrane leaflet. PI(4,5)P₂ is a non-bilayer lipid that accumulates at sites of FGF2 oligomerization, imposing severe membrane stress that is relieved by the formation of a lipidic membrane pore. At the outer leaflet, FGF2 oligomers are captured and disassembled by the heparan sulfate proteoglycan Glypican-1 (GPC1), making available FGF2 to engage in ternary signaling complexes on cell surfaces. Using an in vitro reconstitutions system, this study provides direct evidence that transbilayer asymmetry of PI(4,5)P₂ promotes rapid kinetics of membrane pore formation. Likewise, FGF2 secretion from cells is inhibited when PI(4,5)P₂ plasma membrane transbilayer asymmetry is disrupted. We propose the asymmetric distribution of PI(4,5)P₂ to lower the energetic barrier for membrane pore formation, enabling rapid kinetics of FGF2 membrane translocation into the extracellular space.