The ins and outs of fibronectin matrix assembly

Abstract

3270of assembly, where it binds to FN and colocalizes with clustered α5β1 integrin (Dzamba et al., 1994; WierzbickaPatynowski and Schwarzbauer, 2002). The existence of one essential FN-binding site enhances control of the assembly process since all interactions depend on accessibility to this single site. The fact that there are multiple partners for this site suggests that the alignment of FN dimers within fibrils varies depending on which partners are available for assembly domain binding. Variable alignment would place the other binding domains (for heparin, cells, collagen, etc.) into different molecular contexts and close to different 'near-neighbors' on adjacent dimers. In this way, dimer alignment would have a significant impact on fibril complexity. In most instances, FN assembly is initiated by integrins that recognize the RGD and synergy sequences. Surprisingly, the specific location of the cell-binding site within FN is not critical. Placement of repeats III 9-10 more N-terminal in place of III4-5 (Fig. 1) generated a recombinant FN that assembled normally (Sechler et al., 2001). An RGD-independent mechanism acts through binding of α4β1 integrin to the CS1 site † within the alternatively spliced V region near the C-terminus (Sechler et al., 2000). Clearly, the integrin-binding site does not need to be centrally located for initiation and propagation of FN fibril formation.At early stages of de novo assembly, FN fibrils are short and usually extend between adjacent cells or from the cell to nearby substrate (Fig. 2). These fibrils are soluble in buffers containing 2% deoxycholate detergent. As more FN accumulates at the cell surface, fibrils are gradually converted into a detergentinsoluble form, and a significant proportion of these exist as high-molecular-weight multimers (McKeown-Longo and Mosher, 1983). Insolubility and multimerization might involve intermolecular disulfide bonding catalyzed by the intrinsic protein disulfide isomerase activity of FN (Langenbach and Sottile, 1999) or might result from highly stable protein-protein interactions (Chen and Mosher, 1996). Partial unfolding of the III9 module of FN promotes formation of amyloid-like fibrils in vitro (Litvinovich et al., 1998); so perhaps a similar process of β-strand exchange contributes to the detergent insolubility of the FN matrix. Further investigation of this and other potential mechanisms is needed to decipher the process by which FN fibrils become insoluble. Fibronectin activation by conformational changeA key feature of the matrix assembly model is a conformational change that converts soluble FN into an activated dimer. In vitro manipulation of soluble FN has provided compelling evidence for conformational changes that take FN from a compact to an extended form. Changes in pH or ionic strength, addition of mild denaturants, as well as interactions with heparin or collagen fragments, can induce conformational changes as measured by a variety of biophysical, biochemical and microscopic approaches (Bushuev et al., 1985;Erickson an...

Differential Contributions of Conformation Extension and Domain Unfolding to Properties of Fibronectin Nanotextiles

“…For example, Fibronectin (FN) is a 450 kDa dimer composed of modular folded domains (I, II, or III) that are categorized by their sequence homology. [10][11][12][13][14] Unlike the FN I and II domains, the β-sheet rich, FN III domains do not contain internal disulfide bonds-a characteristic that enables the large extensibility of FN in the presence of tensile load.…”

Membrane-type 1 matrix metalloproteinase regulates fibronectin assembly and N-cadherin adhesion

“…FN matrix assembly requires the increased cytoskeletal tension generated by cadherin adhesions [1][2][3][4][5]. We hypothesized that N-cadherin adhesions augmented by MT1-MMP knockdown facilitates further FN matrix assembly.…”

“…Continuous FN assembly causes the formation of FN matrix. This process requires interaction between integrin cytoplasmic domain and actin-associated proteins, which is promoted by the increased cytoskeletal tension generated by cadherin adhesions [4,5].…”

MT1-MMP prevents growth inhibition by three dimensional fibronectin matrix

“…FN assembly is known to require cellular FN synthesis [20,21]. To confirm it, HT1080 cells were transfected with siRNA for MT1-MMP and/or FN.…”

“…Subsequent steps convert FN into a dense 3D meshwork of interconnected assembled FN that provides a dynamic environment for cells [20,21]. Oncogenically transformed cells are known to exhibit decreased synthesis and increased ability to degrade FN, resulting in a decrease of FN matrix [22].…”