Fibroniini

Fibroniini, known in English as fibronectin, is a high-molecular-weight glycoprotein of the extracellular matrix (ECM) and of blood plasma. In its soluble form it circulates as a dimer of two nearly identical polypeptide chains linked by disulfide bonds, with a total molecular weight of roughly 440 kDa per monomer. In tissues it forms insoluble fibrils that integrate with other ECM components to create a scaffold that supports cells.

Fibroniini is encoded by the FN1 gene in humans and related genes in other species. Its protein structure comprises repeating modules of three types: type I, type II, and type III. The molecule exhibits extensive alternative splicing, producing multiple isoforms that differ by the inclusion or exclusion of the EDA (EIIIA) and EDB (EIIIB) segments, among others. Plasma fibroniini generally lacks these inserts, whereas cellular fibroniini found in tissues can include them, allowing tissue-specific functions.

A central role of fibroniini is to mediate cell adhesion, migration, and organization of the ECM. It binds to collagen, heparan sulfate proteoglycans, fibrin, and integrins on cell surfaces, particularly integrin α5β1. The RGD (arginine-glycine-aspartic acid) motif in the FnIII10 domain is a key integrin-binding site, often with a nearby synergistic site in FnIII9 that enhances binding. Through these interactions, fibroniini acts as a cellular adhesion substrate, a reservoir for growth factors, and an organizer of cell movement during development, wound healing, and angiogenesis. Fibroniini assembly into fibrils is a force-dependent process driven by cell surface receptors and the actin cytoskeleton.

Clinical and research relevance includes its use as a coating for cell culture, its involvement in tissue remodeling and fibrosis, and altered expression patterns in cancer and inflammatory diseases.