Topological arrangement of the major structural features of fibronectin.

Using tryptic cleavage coupled with biosynthetic labeling, we have established the order along the polypeptide chain of the major tryptic fragments derived from dimeric fibronectin.The dimer is held together by interchain disulfide(s) all of which are very close to the COOH-terminal.This COOH-terminal region is very readily removed by proteolysis.The NH2-terminal region of the molecule is extremely cystine-rich and is released in a 25,000-dalton fragment.The rest of the molecule remains as a 200,000-dalton internal fragment.By the use of cyanide cleavage, we have shown that the large fragment contains a disulfide-rich region at its NHz-terminal, adjacent to the 25-kd fragment, and contains a free sulfhydryl group which is 170,000 daltons from the NHz-terminal of the intact 230,000-dalton chain.There may be another free sulilydryl group 30,000 to 40,000 daltons further toward the COOH-terminal.The free sulfhydryl groups are necessary for binding of fibronectin to the cell surface and apparently participate in the formation of high molecular weight aggregates which contain fibronectin and are held together by disulfide bonds.These results, together with others in the literature, have allowed us to locate within the molecule the sites at which fibronectin binds to collagen and fibrin and the region at which these proteins are cross-linked to fibronectin by factor XIII transglutaminase.The crosslinking region is in the NHz-terminal 25-kd fragment and the binding site is immediately adjacent in the cystine-rich region of the 200-kd fragment.Fibronectin is a large glycoprotein present on the surfaces of many cell types (1-3).A related molecule is present in plasma (4).The major role of fibronectin is probably adhesion of cells to the substratum (5-7), and cell-surface fibronectin appears to be connected to actin microfilament bundles inside cells (8,9).These properties could explain some of the changes in behavior of malignant cells which have lost fibronectin on transformation (1-3).Fibronectin also appears to be involved in cell migration in vitro (10) and conceivably also in vivo (11, 12).Fibronectin binds strongly to gelatin and less strongly to