Molecular switches in the spatio-temporal coordination of the FGF2 membrane translocation machinery.
Fibroblast Growth Factor 2 (FGF2) is an unconventionally secreted protein transported into the extracellular
space by direct translocation across plasma membranes. As part of the first step of this pathway, FGF2 gets
recruited at the inner plasma membrane leaflet mediated by the α1 subunit of the Na,K-ATPase, followed by
tyrosine phosphorylation of FGF2 by Tec kinase. Subsequently, FGF2 is recruited by the phosphoinositide
PI(4,5)P2, the key molecular switch of this pathway that triggers oligomerization of FGF2. This process drives
a highly dynamic membrane remodeling event, forming lipidic membrane pores with a toroidal architecture. At
this stage, FGF2 oligomers become accommodated within the central hydrophilic space of these pores,
representing membrane translocation intermediates. At the outer plasma membrane leaflet, FGF2 oligomers
get captured and disassembled by heparan sulfate chains associated with cell surface proteoglycans, resulting
in FGF2 translocation into the extracellular space. Several recent discoveries in this SFB/TRR 186 project
pointed at a spatial organization of the FGF2 translocation machinery in liquid-ordered nanodomains. In this
context, we found (i) the Na,K-ATPase to be palmitoylated and to interact with a sphingolipid, along with a subpopulation
found in detergent-resistant membrane domains, (ii) PI(4,5)P2-dependent membrane recruitment
to be tuned by cholesterol, (iii) FGF2 secretion efficiencies to be modulated by plasma membrane levels of
sphingomyelin and cholesterol and (iv) the predominant heparan sulfate proteoglycan driving FGF2 secretion
to be Glypican-1 that localizes in liquid-ordered domains based on a GPI anchor and could be retrieved from
detergent-resistant membranes alongside the Na,K-ATPase. In the next funding period, we aim at challenging
the hypothesis of the FGF2 membrane translocation machinery to be organized in dedicated liquid-ordered
membrane nanodomains. Using the full range of advanced biochemical and biophysical techniques of this
project, complemented by molecular dynamics simulations and MinFlux super-resolution nanoscopy, we will
reveal the nano-organization of this machinery, building the molecular basis for fast FGF2 membrane
translocation within 200 ms time intervals. Building on our findings from the preceding funding periods, this
project will provide a detailed and comprehensive understanding of the molecular mechanism by which FGF2
is secreted from cells in an ER/Golgi-independent manner.
Prof. Dr. Walter Nickel (BZH Heidelberg)
Dr. Fabio Lolicato (BZH Heidelberg)