FGF1 folding is critical for its nonclassical release

Fibroblast growth factor 1 (FGF1), a ubiquitously expressed proangiogenic protein that is involved in tissue repair, carcinogenesis and maintenance of vasculature stability, is released from the cells via a stress-dependent nonclassical secretory pathway. FGF1 secretion is the result of transmembrane translocation of this protein. It correlates with FGF1 ability to permeabilize membranes composed of acidic phospholipids. Similarly to several other nonclassically exported proteins, FGF1 exhibits β-barrel folding. To assess the role of FGF1 folding in its secretion, we applied targeted mutagenesis in combination with a complex of biophysical methods and molecular dynamics studies, followed by artificial membrane permeabilization and stress-induced release experiments. It has been demonstrated that a mutation of proline 135 located in the C-terminus of FGF1 results in: (i) partial unfolding of FGF1, (ii) decrease of FGF1 ability to permeabilize bilayers composed of phosphatidylserine, and (iii) drastic inhibition of stress-induced FGF1 export. Thus, FGF1 folding is critical for its nonclassical secretion.