## 2 Fn as a Pertinent Key Actor in the ECM
### Fn's General Properties
This is also confirmed by the RH/Rg ratio of 0.75 \(\pm\) 0.02, consistent with a Gaussian chain, whereas the RH/Rg ratio of 1.3 was identified for spherical-shaped molecules. However, adding a denaturing agent (8 M urea) increases Rg by a factor of two. This means that the Fn \"native\" chain is not completely unfolded. The average shape of the Fn conformation was also probed by small-angle neutron scattering performed for a reverse scattering vector q-1 smaller than Rg (0.2 \(<\) q-1 \(<\) 15 nm). The measured form factor is in complete agreement with the form factor of a random string of 56 beads with 5 nm in diameter (Figure 1B). It rules out the possibility of an unfolded chain as well as globular structures [18]. This study demonstrates not only the complexity, but also the versatility offered by Fn in modulating and adapting its structural and conformational organization. In other words, considering the nature of Fn, it is crucial to make a distinction between structure and conformation and to deal with the two definitions to clearly explain and discuss some specificity. For instance, conformation is used more to refer to the overall global whole-chain organization, whereas the structural level concerns the intrinsic remarkable structural elements in the modules. For example, without compromising the presence of the \(\beta\)-sheet in the individual modules, which provides the structure of the modules, the overall Fn molecule can transition from a highly extended and almost linear conformation to a globular and compact state. In other words, the molecule appears as a highly flexible and extendable string of beads capable of large conformational modulations. Moreover, the global molecular organization of the whole Fn, at interfaces versus in solutions, is under control and mediated by the specificity and the nature (hydrophobic, charged, etc.) of the elements found along the chain. First, the beta structures found in the three types of modules are composed of hydrophobic and large amino acids (Figure 1A). The structures of these apolar and hydrophobic elements are strongly influenced by polar or apolar environments, which, in turn, strongly impact the Fn's structure and its subsequent organization. The nature of certain solvents and the physicochemical properties of materials strongly affect the overall conformation and structure of the Fn [19]. Second, the charges of the different modules and the subsequent electrostatic effects also play a crucial role in the structuration and conformation of the molecule. The complete amino acid sequence of human Fn has been known for decades [20]. The protein data bank contains a specific amino acid sequence for the 32 modules of each chain. The attribution of one positive charge for arginine and lysine residues and one negative charge for aspartic acid and glutamic acid residues gives 192 positive charges and 259 negative charges, forming an overall net charge per chain equal to \(-259+192=-67\). With more than 450 negative and positive residues charged among 2500, Fn is a true polyelectrolyte. Moreover, with 134 (67 \(\times\) 2) more negative charges than positive ones, Fn, owing to this excess of negative charges, can be globally considered a polyanion. At a more detailed level, a fine analysis of the contained positively charged residues in each module, compared to the quantity of negatively charges, permits the calculation of the net charge for each module. The differences range between \(-11\) (for the III-2 and the EDA modules with \(-14/+3\) and \(-17/+6\), respectively) and \(+6\) (for the III-14 with \(-7/+13\)) (see Figure 2A). This heterogeneous repartition of charges toward the different modules, precisely in the cell-binding region (the middle of the chain that contains the type III modules), gives rise, led by electrostatic interactions, to the specific folding of the chain in solution (Figure 2A). Moreover, the other part of the chains also adopts a specific orientation.