Close and focal contact adhesions of fibroblasts to a fibronectin-containing matrix.

The fibroblast cell line Balb/c 3T3 makes both close and tight-focal adhesive contacts with the plasma fibronectin (pFn)-coated tissue culture substratum. Detachment of these cells mediated by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid leaves tight-focal contacts and a subset of close contacts as substratum-attached material (SAM). The enrichment of heparan sulfate (HS) proteoglycan (HS-PG) in SAM under specific attachment conditions, as well as the recent demonstration of HS binding to pFn or cellular Fn, has evolved a series of experiments with the selective HS-binding protein platelet factor-4 to analyze the requirement of the HS-binding activity of pFn in the formation of these two types of adhesive contacts. In addition, the cell-binding domain (CBD) of pFn, which recognizes an unidentified cell surface receptor, has been isolated free of HS-binding domains after proteolytic cleavage of pFn. These functional studies indicate that the binding of pFn on the substratum to cell surface HS-PG is necessary and sufficient to generate close contacts with transmembrane signaling of this proteoglycan to reorganize lengthy microfilament bundles in the cytoplasm. Cells spread incompletely on CBD alone, form only close contacts, and reorganize highly concentrated actin filaments only in their spiky projections. Furthermore, the formation of tight-focal contacts with associated stress fibers appears to require both reactions of pFn, binding to cell surface HS-PG and to its unidentified receptor. The importance of HS-PG in these functional studies has led to its better biochemical characterization in SAM. Initially formed close contacts of these cells contain principally a large HS-PG that aggregates into high-molecular-weight complexes by some unknown mechanism. With time, there are two different mechanisms of catabolism of HS-PG, one of which includes the liberation of single-chain HS. The importance of these changes in the HS-PG in SAM are now being analyzed with regard to the formation and disappearance of close and tight-focal adhesions.