Dimeric character of fibronectin, a major cell surface-associated glycoprotein

Exposed proteins of cultured chick and human fibroblasts were labeled by lactoperoxidase-catalyzed iodination and analyzed by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Extracts from both cell types contained the characteristic, heavily labeled band of fibronectin (molecular weight = 2.2 × 10⁵) when analyzed after reduction with 2-mercaptoethanol. Without prior reduction, however, the 2.2×10⁵ molecular weight band was missing and replaced by labeled bands of 4.4×10⁵ and of very high molecular weight. This finding indicates that fibroblast cell-surface fibronectin, like the fibronectin purified from plasma, is composed of two high molecular weight polypeptides hed together by disulfide bonds, and suggests that the dimer may in addition form disulfide-bonded multimers.