%0 Generic
%A Beyer, Daniel
%C Heidelberg
%D 2025
%F heidok:36870
%R 10.11588/heidok.00036870
%T Molecular Dominoes: The interplay between FGF2 oligomerization and PI(4,5)P2 clustering in membrane pore formation
%U https://archiv.ub.uni-heidelberg.de/volltextserver/36870/
%X Within the fibroblast growth factor family, fibroblast growth factor 2 (FGF2), a potent mitogen,  follows an unconventional secretory pathway (UPS), as FGF2 does not contain a signal peptide  that directs the protein for classical ER/Golgi export. It is able to translocate directly across  the plasma membrane (type I UPS). This mechanism involves sequential interactions with  cellular components at the plasma membrane. The cytosolic ɑ1 domain of the Na,K-ATPase  recruits FGF2 to the membrane. FGF2 then interacts with Tec kinase, phosphorylaHng FGF2.  FGF2 binds to the phosphoinositide PI(4,5)P2 lipids in the inner leaflet of the plasma  membrane, leading to oligomerization and membrane insertion. The translocation is then  completed by membrane-proximal heparan sulfate chains of heparan sulfate proteoglycans,  which outcompete PI(4,5)P2 in the mutually exclusive binding site, making the translocation  unidirecHonal and irreversible. Bound to the cell surface, FGF2 can exert its function in  autocrine and paracrine signaling. Direct visualization of single secretion events from living  cells by total internal reflection microscopy (TIRF) revealed an average time interval of 200 ms  from FGF2 recruitment at the inner plasma membrane leaflet to full translocation to cell  surfaces. Reconstitution of FGF2 translocation in giant unilamellar vesicles identified FGF2, a  PI(4,5)P2-containing membrane, and luminal long-chain heparin as the minimal component  system for translocation. The average FGF2 oligomer size for pore formation was determined  by fluorescence brightness analysis to be four to eight monomers, both in vitro and in cells.  Using Molecular Dynamics simulations, I demonstrated that the C95-C95 disulfide-bridged  FGF2 dimer self-assembles into oligomers on flat, PI(4,5)P2-containing membranes to induce  a substantial, spatially-confined membrane remodeling event. The FGF2 oligomer gathered 4  to 5 of the negatively charged PI(4,5)P2 molecules per FGF2 monomer, thereby creating a  strong, local electrical charge gradient across the membrane. In addition, PI(4,5)P2  accumulation caused negaHve curvature deformaHons underneath the FGF2 oligomer.  Combined with a very high concentration of non-bilayer lipids such as PI(4,5)P2 and  phosphatidylethanolamine, these effects generate spatially confined stress on the membrane,  which I propose to be the primary driving force for membrane pore formation, the process  that mediates unconventional secretion of FGF2 into the extracellular space.