Fibronectins are made of two subunits linked by disulfide bonds at the C terminal. In the extracellular matrix fibrils, fibronectins are further disulfide bonded into high molecular weight polymers. Fibronectin subunits vary in size between approximately 235 and 270 kDa depending on tissue and species. Each subunit is made of repeating modules of three types: I, II, and III. There are 12 type I repeats, approximately 45 amino acids long, clustered in three groups, two adjacent type II repeats each 60 amino acids long, and 15-17 type III repeats each about 90 amino acids long. Type I and type II each contains two disulfide bonds, while type III lacks disulfide bonds. There are two free sulfhydryl groups per subunit at the type III repeat. Recombinant fragment III₁-C, modeled after the C-terminal two-thirds of the III₁ repeat, was found to bind to fibronectin and induce spontaneous disulfide crosslinking of the fibronectin molecules into multimers, which resemble matrix fibrils.(Moria, A., et al., Nature, 367, 193 (1994).

Secreted protein; extracellular space; extracellular matrix