27.6:
Fibronectins Connect Cells with ECM
Fibronectins are adhesive glycoproteins present in the extracellular matrix. Their primary role is to connect cells with the matrix.
Fibronectins consist of two large subunits that are joined at their C-terminal by a couple of disulfide bonds, forming a dimer. Both the subunits contain various repeating domains that can bind to collagen fibrils, heparin of proteoglycans, and integrins – the cell-adhesion receptors. In this way, fibronectins connect cells with the matrix components.
Binding with integrin is greatly influenced by the presence of arginine, glycine, and aspartate residues, or RGD repeats, as seen in type III domains – one of the major repeating domains in fibronectins.
Once bound to fibronectin, integrins can bind to actin filaments, forming a bridge between the extracellular components and the cell's cytoskeleton. This binding transmits the tension, stretching the fibronectins and exposing their cryptic binding sites to which multiple fibronectin dimers can then bind, forming fibronectin fibrils.
These fibronectin fibrils help in epidermal cell migration and proliferation, facilitating various biological processes like wound healing.
27.6:
Fibronectins Connect Cells with ECM
Fibronectin is an adhesive glycoprotein present in the extracellular matrix of embryogenic and adult tissue. These molecules primarily aid in regulating cell motility and attachment. A fibronectin molecule is composed of two identical polypeptide chains attached to each other by a pair of disulfide bonds at the C-terminal.
Both proteoglycans and collagen are attached to fibronectin proteins, which, in turn, are attached to integrin proteins. These integrin proteins interact with transmembrane proteins in the plasma membranes of eukaryotic cells that lack cell walls. In addition to associating with integrin, fibronectins also have an affinity for other host and non-host molecules.
Vertebrates have two types of fibronectins. The soluble plasma fibronectins found in blood plasma produced by hepatocytes are one type, and the other type is insoluble cellular fibronectin found in the extracellular matrix.
Fibronectin molecules also play an important role in blood clotting, as the plasma fibronectin is also deposited along with fibrin at the site of injury to stop bleeding. The plasma fibronectin is later digested by proteases secreted by fibroblasts. Further, the fibronectin produced by fibroblasts is insoluble and forms an insoluble matrix.
Suggested Reading
- Alberts, Bruce, et al. Molecular Biology of the Cell. 6th ed. Garland Science, 2017. Pp 1066.
- Karp et al., Cell and Molecular Biology. John Wiley & Sons, 6th edn. Pg.46, 236.
- Lodish, Harvey, et al. Molecular Cell Biology. 8th ed. W.H. Freeman and Company, 2016. Pp 956.