Fibronectin conformation switch induced by coadsorption with human serum albumin

The dynamic adsorption of human serum albumin (HSA) and plasma fibronectin (Fn) onto hydrophobic poly(hydroxymethylsiloxane) (PHMS) and the structures of adsorbed protein layers from single and binary protein solutions were studied. Spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation monitoring (QCM-D) together with atomic force microscopy (AFM) were used to measure the effective mass, thickness, viscoelastic properties, and morphology of the adsorbed protein films. Adsorbed HSA formed a rigid, tightly bound monolayer of deformed protein, and Fn adsorption yielded a thick, very viscoelastic layer that was firmly bound to the substrate. The mixed protein layers obtained from the coadsorption of binary equimolecular HSA-Fn solutions were found to be almost exclusively dominated by Fn molecules. Further sequential adsorption experiments showed little evidence of HSA adsorbed onto the predeposited Fn layer (denoted as Fn ≫ HSA), and Fn was not adsorbed onto predeposited HSA (HSA ≫ Fn). The conformational arrangement of the adsorbed Fn was analyzed in terms of the relative availability of two Fn domains. In particular, (4)F(1)·(5)F(1) binding domains in the Hep I fragment, close to the amino terminal of Fn, were targeted using a polyclonal antifibronectin antibody (anti-Fn), and the RGD sequence in the 10th segment, in the central region of the molecule, was tested by cell culture experiments. The results suggested that coadsorption with HSA induced the Fn switch from an open conformation, with the amino terminal subunit oriented toward the solution, to a close conformation, with the Fn central region oriented toward the solution.