Proximity Labeling by a Recombinant APEX2–FGF1 Fusion Protein Reveals Interaction of FGF1 with the Proteoglycans CD44 and CSPG4

Fibroblast growth factor 1 (FGF1) binds to specific FGF receptors (FGFRs) at the surface of target cells to initiate intracellular signaling. While heparan sulfate proteoglycans (HSPGs) are well-described coreceptors, it is uncertain whether there are additional binding sites for FGF1 at the cell surface. To address this, we devised and tested a method to identify novel binding sites for FGF1 at the cell surface, which may also be applicable for other protein ligands. We constructed an APEX2-FGF1 fusion protein to perform proximal biotin labeling of proteins following binding of the fusion protein to the cell surface. After functional validation of the fusion protein by a signaling assay, we used this method to identify binding sites for FGF1 on cell surfaces of living cells. We confirmed the feasibility of our approach by detection of FGFR4, a well-known and specific receptor for FGF1. We subsequently screened for novel interactors using RPE1 cells and identified the proteoglycans CSPG4 (NG2) and CD44. We found that FGF1 binds CD44 through its heparin-binding moiety. Moreover, we found that FGF1 was colocalized with both CSPG4 and CD44 at the cell surface, suggesting that these receptors act as storage molecules that create a reservoir of FGF1. Importantly, our data demonstrate that recombinant ligand-APEX2 fusion proteins can be used to identify novel receptor interactions on the cell surface.