ASSEMBLY OF AMINO-TERMINAL FIBRONECTIN DIMERS INTO THE EXTRACELLULAR-MATRIX

Fibronectin is a dimeric adhesion molecule that consists of three type s of repeating modules. Adherent cells bind soluble fibronectin and in corporate it into insoluble fibrils in the extracellular matrix. The a minoterminal 70-kDa portion of fibronectin mediates binding to the cel l surface, but amino terminal fragments do not accumulate in the extra cellular matrix. The ninth type I and first type III modules, the cell adhesion region, and the cysteines that form the interchain disulfide bonds have also been implicated in matrix assembly. To further define which regions of fibronectin are essential for matrix assembly, we ge nerated a dimeric protein (d70 kDa) in which the 70-kDa amino terminus is directly linked to the last 51 amino acids of fibronectin, which c ontain the cysteines involved in interchain disulfide bonding. d70 kDa bound to cells and accumulated in the extracellular matrix. Incorpora tion of d70 kDa into the extracellular matrix was dependent upon prote in synthesis; in cycloheximide-treated cultures that lacked a pre-exis ting matrix, d70 kDa accumulated in the extracellular matrix only in t he presence of intact fibronectin. Monomeric 70-kDa protein was not in corporated into the matrix in the presence or absence of cycloheximide . These data indicate that fibronectin molecules containing only the a mino-terminal 70-kDa region and the carboxyl-terminal 51 amino acids c an become assembled into the extracellular matrix.