Fibronectin and its cryptic proteinases
Fibronectin (FN) is a multifunctional glycoprotein, which occurs in blood plasma (pFN, plasma fibronectin) and also on cell surfaces (cFN, cell surface fibronectin). It consists of two almost identical units subdivided into different characteristic binding domains. The variety of binding affinities to extracellular macromolecules, cells, DNA, and bacteria is important for the participation of fibronectin in a multitude of biological processes like cell adhesion, migration, wound healing, metastasis, embryonic development, thrombosis and bacterial infection. In contrast to these characteristics, various studies showed that fragments of fibronectin may perform completely different functions compared to the intact protein. Some fragments were found to be responsible for inhibition of endothelial cell growth or promotion of tissue reconstruction. Other fragments stimulate the expression of matrix metalloproteinases, tumor necrosis factor-α, TIMP-1 and urokinase-type plasminogen activator. Finally, several studies described various proteolytic activities of pFN fragments from four cryptic proteinases in human pFN designated as fibronectinase (Ser-enzyme), FN-gelatinase (Asp-enzyme), FN-lamininase (Asp-enzyme) and FN-type IV-collagenase (metallo-enzyme).