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Abstract

Fibroblast growth factor 2 (FGF2) is exported from mammalian cells via an unconventional secretory pathway that depends on the ability of FGF2 to self-translocate across plasma membranes into the extracellular space. This process is triggered by phosphatidylinositol 4,5- bisphosphate (PI(4,5)P2)-dependent FGF2 oligomerization, inducing the formation of a lipidic membrane pore. The sodium-potassium adenosine triphosphatase (Na+,K+ -ATPase) participates in the transport of FGF2 into the extracellular space, representing the entry point of this unusual pathway of protein secretion. In this thesis, I contributed evidence for a direct role of the FGF2 residue cysteine 95 (C95) as an essential determinant for membrane-dependent oxidative FGF2 dimerization in a cellular context. This process is further shown to represent a prerequisite for PI(4,5)P₂-dependent formation of high-order oligomers, a requirement for FGF2 membrane translocation into the extracellular space. I further show that the Na+,K+ -ATPase acts as a critical facilitator of this process in a cellular context, as high-order oligomers FGF2 could not be observed under experimental conditions that impair the interaction between FGF2 and the cytoplasmic domain of the ⍺1 subunit of the Na+,K+ -ATPase. Moreover, I observed that overexpression of a membrane-attached variant of the cytosolic domain of the ⍺1 subunit of the Na+,K+ -ATPase causes both an increase of FGF2 recruitment at the inner leaflet of the plasma membrane and FGF2 secretion into the extracellular space. My findings suggest that the Na+,K+ -ATPase enhances the local assembly of FGF2 oligomers and thereby promotes efficient FGF2 membrane translocation to cell surfaces. Together, these findings corroborate the proposed FGF2 secretion mechanism proposed by the group of Prof. Nickel in the recent years. First, FGF2 secretion requires the C95- and PI(4,5)P₂-dependent formation of high-order FGF2 oligomers. Second, the Na+,K+ -ATPase consolidates as the starting point of this unconventional secretory pathway by recruiting and accumulating FGF2 at the inner leaflet of the plasma membrane.