Phosphatidylserine externalization and membrane blebbing are involved in the nonclassical export of FGF1

Abstract

The mechanisms of nonclassical export of signal peptide-less proteins remain insufficiently understood. Here we demonstrate that stress-induced unconventional export of FGF1, a potent and ubiquitously expressed mitogenic and proangiogenic protein, is associated with and dependent on the formation of membrane blebs and localized cell surface exposure of phosphatidylserine. In addition, we found that the differentiation of promonocytic cells results in massive FGF1 release, which also correlates with membrane blebbing and exposure of phosphatidylserine. These findings indicate that the externalization of acidic phospholipids could be used as a pharmacological target to regulate the availability of FGF1 in the organism.

Transgenic Expression of Nonclassically Secreted FGF Suppresses Kidney Repair

“…C-terminal HA tag, which does not interfere with FGF1 release [24], was used for immunohistochemical FGF1 detection. To ensure cell type specific FGF1 expression, we chose a Tet-based system.…”

Evaluating membrane affinity by integrating protein orientations

“…Indeed, it was suggested that the protein complex needs to undergo conformational changes such as partial denaturation before the release [39,40]. Furthermore, the FGF1 release may involve destabilization of the lipid bilayer [8,41], which cannot be described by our model here.…”

Protein-Phospholipid Interactions in Nonclassical Protein Secretion: Problem and Methods of Study

“…The peripheral translocation of FGF1 is stimulated also by thrombin [48]. The staining of nonpermeabilized stressed cells for externalized FGF1 showed that it is not evenly distributed across the cell membrane but concentrated in distinct oval-shaped domains with the diameters from 0.5 to 5 μm [40]. Most of these FGF1-positive domains are also stained for the acidic phospholipid (PL) phosphatidylserine (PS).…”

“…However, cell stress induces the translocation of FGF1 to the outer leaflet of the PL bilayer [80]. The simultaneous externalization of PS and FGF1 was observed also in U937 promonocytic leukemia cells induced to differentiate to macrophages [40]. Interestingly, the knockdown of PL scramblase 1 (PLSCR1), an enzyme involved in PS transmembrane translocation [81], influenced neither the secretion of FGF1 nor PS externalization [40].…”

“…Instead, they are diffusely distributed in the cells, usually both in nuclei and cytoplasm. The released FGF1 and FGF2, which strongly bind heparan sulfate proteoglycans, are associated with the extracellular matrix [39,40]. In this review, we will focus on FGF1 export to discuss the structural aspects of nonclassical protein export, particularly protein-lipid interactions, and approaches to their study.…”