Substratum contacts and cytoskeletal reorganization of BALB/c 3T3 cells on a cell-binding fragment and heparin-binding fragments of plasma fibronectin.
BALB/c 3T3 cells make both close contacts and tight-focal contacts (with associated microfilament stress fibers) on plasma fibronectin (pFN)-coated substrata. To resolve the importance of the heparan sulfate-binding or cell-binding activities of the pFN molecule in these adhesive responses, a cell-binding fragment (120K) (CBF) free of any heparan sulfate-binding activity was prepared from human pFN by chymotrypic digestion and isolated as described by Pierschbacher et al. (Cell 26 (1981) 259). These adhesive responses to CBF were also compared to those of the model heparan sulfate-binding protein, platelet factor-4 (PF4), or heparin-binding fragments (HBF) of pFN. On intact pFN, greater than 70% of the cells formed tight-focal contacts and associated stress fibers by 4 h, the latter staining with NBD-phallacidin. In contrast, cells spread differently on CBF and failed to form tight-focal contacts; staining with NBD-phallacidin was localized to spiky projections at the cell margin with no detectable stress fiber formation. On PF4 or HBF, cells failed to form tight-focal contacts but did spread well and formed long microfilament bundles in peripheral lamellae. Spreading on CBF, HBF, or PF4 was paralleled by formation of close contacts. Spreading and to some extent attachment of cells on CBF was inhibited with a small peptide containing the Arg-Gly-Asp-Ser sequence; responses on HBF were unaffected by this peptide. When mixtures of CBF and PF4 were tested, cells still failed to form tight-focal contacts and stress fibers. These results demonstrate that the binding of CBF to its probable receptor under conditions routinely used to assay spreading activity results in an incomplete adhesive response compared with intact pFN. While this partial response may result from quantitative differences in the density of active cell-binding domains on the substratum, the pattern of microfilament reorganization produced by the binding of PF4 to cell surface heparan sulfate proteoglycans suggests that the ability of pFN to promote formation of tight-focal contacts and stress fibers may reside in the coordinate interaction of two or more binding activities in the intact molecule.