Fibronectin Fibrillogenesis Involves the Heparin II Binding Domain of Fibronectin

Abstract

1 repeats may also promote binding of the I 1-5 repeats. Neither mAb IST-2 nor mAb 9D2, alone or in combination, inhibited binding of 125 I-labeled 70-kDa fragments to cycloheximide-treated cells plated on the 160-kDa substrate, suggesting that additional I 1-5 binding sites, independent of the III 1 and III 12-14 repeats, may be involved in fibrillogenesis.Fibronectin is required for normal growth and development (1) and plays an important role in regulating cell attachment and movement, wound healing, and tumorigenesis (for review, see Refs. 2 and 3). It is a 500-kDa disulfide-bonded dimer consisting of similar subunits and is found as a soluble glycoprotein in blood and other body fluids and as an insoluble fibrous matrix component in tissues. Each subunit of fibronectin consists of three different types of repeating sequences, called types I, II, and III, which are arranged into discrete structural and functional modules.Assembly of dimeric fibronectin into the extracellular matrix involves multiple consecutive binding interactions with integrin receptors, with itself, and with matrix components such as type I collagen (for review, see Refs. 4 -6). Although the ␣ 5 ␤ 1 integrin appears to be the primary fibronectin receptor involved in matrix assembly (7-12), at least two other integrins, ␣ IIb ␤ 3 and ␣ v ␤ 3 , can also support fibronectin fibrillogenesis (13,14). High affinity binding interactions between these integrins and the RGD site in the 10th type III repeat (III 10 ) of fibronectin are thought to promote fibrillogenesis by exposing appropriate self-assembly sites in fibronectin. Such sites may become exposed through local integrin-induced conformational changes in III 10 repeats (15) or through integrin-mediated stretching (reversible unfolding) of one or a whole array of type III repeats in fibronectin in response to cell movements (13, 16).Self-assembly of fibronectin dimers into fibrils is currently thought to involve primarily interactions between the first five type I repeats (I 1-5 ) and the first type III (III 1 ) repeat (17, 18). The I 1-5 repeats are critical for matrix assembly, i.e. peptides including these repeats block assembly of fibronectin into fibrils, and fibronectin dimers lacking these repeats will not be incorporated into fibrils (19 -22). The III 1 repeats are also important for fibril formation, and either anti-III 1 monoclonal antibodies or peptides derived from III 1 repeats can block assembly of fibronectin into matrix (23, 24). The mechanism, however, by which I 1-5 -III 1 interactions affect matrix assembly remains controversial. For example, Hocking et al. (25) have shown that III 1 repeats will interact not only with I 1-5 repeats but also with heat-denatured III 10 repeats. They have proposed that the latter interaction activates the III 1 repeat thereby allowing it to function as a receptor for the amino termini of a second fibronectin dimer. In contrast, Sechler et al. (26) have shown that fibronectin dimers lacking III 1-7 repeats are readily polymerized into fib...

Domain Unfolding Plays a Role in Superfibronectin Formation

“…Interestingly, in this experiment III 12-14 did not interfere with FN matrix formation (Fig. 10E), in contrast to a previous report (41). The addition of anastellin did not block matrix formation but made the FN fibrils thicker and clumpy (Fig.…”

The N-terminal 70-kDa fragment of fibronectin binds to cell surface fibronectin assembly sites in the absence of intact fibronectin

“…The 70K N-terminal fragment of FN, however, binds to cells with the same affinity and distribution as FN and is a potent inhibitor of FN assembly (Bultmann et al, 1998;Magnusson and Mosher, 1998;McKeown-Longo and Mosher, 1985). 70K binding to fibroblasts thus mimics the characteristics of initial FN binding.…”

“…These latter interactions drive fibril progression and formation of detergent-insoluble FN-FN complexes. The FN-FN interactions may involve the 70K region in FN binding to type III domains (Aguirre et al, 1994;Bultmann et al, 1998;Hocking et al, 1994Hocking et al, ,1996. Thus, this model assigns roles to the 70K region in both initiation and progression of FN assembly.…”

“…This proposed interaction is incorporated into a model in which FN fibrillogenesis begins by binding of an integrin recognition sequence in FN, e.g., RGD in repeat III 10 , to an integrin, e.g., α 5β 1 (Mao and Schwarzbauer, 2005;Wierzbicka-Patynowski and Schwarzbauer, 2003). Interaction with integrins is hypothesized to promote extension of FN and thereby facilitate interactions of the N-terminal 70K region with other parts of FN (Aguirre et al, 1994;Bultmann et al, 1998;Hocking et al, 1994Hocking et al, , 1996.…”

Retrovirally Mediated Expression of Decorin by Macrovascular Endothelial Cells

“…There are several domains within fibronectin that are required for matrix assembly, with the RGDcontaining cell binding domain required for fibrillogenesis initiation, and the N-terminal and C-terminal heparin-binding domains required for de novo assembly of fibronectin into the pericellular matrix (62,72,73). Because chondroitin/dermatan sulfates compete for binding to fibronectin with heparin (74), decorin GAG chains may influence matrix assembly by altering the involvement of the heparin-binding domains of fibronectin that are required for fibrillogenesis.…”