Imaging and Mapping Heparin-Binding Sites on Single Fibronectin Molecules with Atomic Force Microscopy

Fibronectin is composed of multiple homologous repeats and contains many functional domains. Two major heparin-binding domains have previously been identified: the Hep I site near the amino terminus and the Hep II site near the carboxyl terminus. The Hep II site has been considered the high-affinity heparin-binding site based on studies of fibronectin fragments. However, few studies have been carried out on heparin binding by intact fibronectin. We imaged single fibronectin molecules as well as heparin-coated gold particles bound to whole dimeric plasma fibronectin molecules with tapping mode atomic force microscopy. We observed heparin-gold particles preferentially bound at two locations that correspond to the Hep I and Hep II sites. Quantitative analysis of images of individual fibronectin-heparin-gold complexes showed that almost twice as many heparin-gold particles bound to the N-terminal Hep I site compared to the Hep II site. In contrast to previous findings with fibronectin fragments, these results suggest that the Hep I site has a binding affinity higher than or comparable to the Hep II site in the intact fibronectin molecule.