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First published online July 10, 2003
doi: 10.1242/10.1242/jcs.00670

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The ins and outs of fibronectin matrix assembly

Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014 USA

View larger version (12K): [in a new window]   Fig. 1. Domain structure of fibronectin (FN). FN consists of type I (rectangles), type II (ovals) and type III (circles) repeats. Sets of repeats constitute binding domains for fibrin, FN, collagen, cells and heparin, as indicated. The three alternatively spliced segments, EIIIA, EIIIB and V (or IIICS), are in yellow. The assembly domain and FN-binding sites are highlighted in orange. SS indicates the C-terminal cysteines that form the dimer.

View larger version (99K): [in a new window]   Fig. 2. Progression of fibronectin (FN) fibril formation. CHO5 cells were incubated with rat plasma FN (25 µg ml–1) for 1 hour or 8 hours. Matrix was visualized by staining with IC3 anti-rat FN antibody followed by fluorescein-labeled secondary antibody. FN fibrils accumulate over time to form a dense network around the cells.

View larger version (30K): [in a new window]   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.