A direct role for phosphatidylinositol-4,5-bisphosphate in unconventional secretion of fibroblast growth factor 2.

Fibroblast growth factor 2 (FGF-2) is a mitogen that is exported from cells by an endoplasmic reticulum/Golgi-independent secretory pathway. Recent findings have shown that FGF-2 export occurs by direct translocation from the cytoplasm across the plasma membrane into the extracellular space. Here, we report that FGF-2 contains a binding site for phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)], the principal phosphoinositide species associated with plasma membranes. Intriguingly, in the context of a lipid bilayer, the interaction between FGF-2 and PI(4,5)P(2) is shown to depend on a lipid background that resembles plasma membranes. We show that the interaction with PI(4,5)P(2) is critically important for FGF-2 secretion as experimental conditions reducing cellular levels of PI(4,5)P(2) resulted in a substantial drop in FGF-2 export efficiency. Likewise, we have identified FGF-2 variant forms deficient for binding to PI(4,5)P(2) that were found to be severely impaired with regard to export efficiency. These data show that a transient interaction with PI(4,5)P(2) associated with the inner leaflet of plasma membranes represents the initial step of the unconventional secretory pathway of FGF-2.