Fibronectins bind cell surfaces and various compounds including collagen, fibrin, heparin, DNA, and actin. Fibronectins are involved in cell adhesion, cell motility, opsonization, wound healing, and maintenance of cell shape (By similarity). Involved in osteoblast compaction through the fibronectin fibrillogenesis cell-mediated matrix assembly process, essential for osteoblast mineralization. Participates in the regulation of type I collagen deposition by osteoblasts (By similarity). Mostly heterodimers or multimers of alternatively spliced variants, connected by 2 disulfide bonds near the carboxyl ends; to a lesser extent homodimers. Interacts with FBLN1, AMBP, TNR, LGALS3BP and COL13A1. Interacts with FBLN7 (By similarity). Interacts with COMP. Interacts (via type III repeats 9-14) with TNFAIP6 (via CUB domain); this interaction enhances fibronectin fibril assembly. TNFAIP6 may act as a bridging molecule between FN1 and THBS1 (By similarity). Interacts with TNR; the interaction inhibits cell adhesion and neurite outgrowth (By similarity). Interacts with FST3 and MYOC (By similarity). A number of isoforms are produced. The diversity of isoforms depends on the V region and either of the two extra domains which can be either included or excluded (partially or completely for the V region). Plasma FN (soluble dimeric form) is secreted by hepatocytes. Cellular FN (dimeric or cross-linked multimeric forms), made by fibroblasts, epithelial and other cell types, is deposited as fibrils in the extracellular matrix. Sulfated. Forms covalent cross-links mediated by a transglutaminase, such as F13A or TGM2, between a glutamine and the epsilon-amino group of a lysine residue, forming homopolymers and heteropolymers (e.g. fibrinogen-fibronectin, collagen-fibronectin heteropolymers). Some lysine residues are oxidized to allysine by LOXL3, promoting fibronectin activation and matrix formation. Serotonylated on Gln residues by TGM2 in response to hypoxia.