Fig. 3. Fibronectin (FN) matrix assembly model. (A) Binding of compactly folded, inactive FN to diffusely distributed integrins induces receptor clustering and co-localization of talin (white ovals) and focal adhesion kinase (FAK) (red rectangles). FAK autophosphorylation (P) recruits Src (pink circles). (B) Clustered integrins with co-localized syndecan (gray and black bars) organize the actin cytoskeleton (green lines) and activate signaling molecules including Ras/MAP kinase (orange), Rho GTPase (violet) and protein kinase C (PKC) (blue). Signals downstream of these pathways further reinforce organization of actin and focal complexes. Contractile forces aid in converting inactive FN into the active extended form. (C) Concentration of active FN dimers at integrin clusters promotes FN–FN interactions and fibril assembly. Movement of 5ß1 integrins and associated proteins along stress fibers towards the cell center redistributes intracellular components into paxillin-rich focal adhesions (pink oval) and tensin-rich fibrillar adhesions (yellow rectangle). This movement may facilitate fibril formation.