## 2 Fn as a Pertinent Key Actor in the ECM
### Fn's General Properties  
Fn is a multifunctional glycoprotein expressed by many different cell types (hepatocytes, fibroblasts, macrophages, and leukocytes, among others). It is found in the plasma and the ECM. Its main biological functions are based on its affinity for different biological compounds such as cell-surface receptors, ECM proteins (collagens, heparin, etc. ), DNA, gangliosides, and immunoglobulins [5]. In vivo, Fn mediates cell adhesion to the ECM and contributes to phagocytosis regulation, wound healing, cell proliferation, differentiation, and locomotion [6]. At the structural level, Fn is composed of two identical polypeptide subunits covalently linked via two disulfide bonds near their carboxy termini. Each subunit contains nearly 2500 amino acids (with a molecular weight of approximately 250,000 Da). It consists of a repetition of 56 modules of three types having a well-defined structure and a high degree of internal homology. Modules of types I, II, and III consist of 45, 60, and 90 amino acids, respectively. Type I modules make up the NH\({}_{2}\)-terminal and COOH-terminal ends of the polypeptide. Two type II modules interrupt a row of nine type I modules at the NH\({}_{2}\)-terminus. Fifteen type III modules make up the middle of the polypeptide [7] (Figure 1A). Type I, II, and III modules are also found in many other proteins, with type III even being ubiquitous in animal proteins [8]. These modules are connected by short polypeptide segments sensitive to proteinases and are grouped into functional domains that express specific binding activities [9]. The above structure of Fn varies, depending on the cell source of the protein. These variations are mainly due to the alternative splicing of the Fn mRNA [10]. The three types of modules are globular. Separately, their structures are known as being mainly \(\beta\)-sheet based on X-ray and NMR studies on isolated modules. Figure 1A provides the detailed information of this organization with several illustrations of the representative structural secondary elements found in the different modules [11; 12; 13]. The global conformation of Fn cannot be assumed from the folding of the individual isolated modules. No success in the crystallization of the whole protein has been reported. To understand the relationship between the Fn's structure and its various adhesive biological functions, several studies have been performed in solution. Fluorescence studies on the overall protein have shown the existence of hydrophobic domains [14; 15]. Circular dichroism and infrared spectroscopy studies have only detected \(\beta\)-sheet secondary structures [15; 16; 17]. These results suggest that the modules within the overall protein retain the same structure as those that are isolated. The conformation of the whole Fn in physiological conditions is debated and controversial. In 2000, in \"native\" conditions, through quasi-elastic light and small-angle neutron scattering studies, the radius of gyration (Rg) of 15.3 \(\pm\) 0.3 nm and the hydrodynamic radius (RH) of 11.5 \(\pm\) 0.1 nm were determined. These results imply a low internal concen  
Figure 1: (**A**) Schematic representation of the modular composition of a fibronectin (Fn) chain. Type I, II, and III domains are depicted in light blue, purple, and pink, respectively. Upper: representative molecular model view of the \(\beta\)–sheet’s secondary structural elements involved in the three different domains. The variable domain and extra domains A (EDA) and B (EDB) are indicated in green and red, respectively. Above: from left to the right, the brackets show the Fn assembly domain at the N–terminus, the integrin- and heparin–binding site in the central cell–binding domain, and sulfhydryl groups for covalent Fn dimerization at the C-terminus. (**B**) Representative view of the volume occupied by the dimeric soluble compact Fn in a solution. tration (M/R\({}^{3}\)g/H) compared to those of the usual globular proteins.