Nanoscale PI(4,5)P2 partitioning driven by FGF2 oligomerization triggers membrane pore formation

Abstract

Fibroblast Growth Factor 2 (FGF2) is a potent tumor cell survival factor. Unlike most secreted proteins, FGF2 lacks a signal peptide and is exported by an unconventional secretory pathway, bypassing the ER/Golgi system. A key event of this unusual secretory mechanism is FGF2 recruitment at the inner plasma membrane leaflet mediated by the phosphoinositide PI(4,5)P2. This interaction induces FGF2 oligomerization which, in turn triggers the formation of a transient lipidic membrane pore with a toroidal structure. FGF2 oligomers populating such membrane pores are captured and disassembled at the outer plasma membrane leaflet through interactions with heparan sulfate chains of the cell surface proteoglycan GPC1, resulting in full translocation of FGF2 across the plasma membrane. Once FGF2 appears on cell surfaces, it engages in signaling complexes initiating autocrine or paracrine signaling. Here, using a multidisciplinary and multiscale approach, we show that FGF2 self-assembles into oligomers that induce lateral PI(4,5)P2 partitioning and membrane remodeling. This process depends on PI(4,5)P2 reaching a critical local threshold concentration that, due to its non-bilayer properties destabilizes the bilayer, facilitating pore formation and FGF2 translocation across the plasma membrane. Our findings highlight a fundamental principle of how protein oligomerization can induce lateral partitioning of membrane lipids that in turn leads to a drastic membrane remodeling event: the transformation of the lipid bilayer into a toroidal membrane pore. The underlying mechanism is likely to be applicable to a wide range of membrane lipid interacting proteins that induce membrane remodeling in other cellular processes.