Evidence for involvement of more than one class of glycoprotein in cell interactions with fibronectin.

The receptor mechanism by which cells attach to fibronectin has been investigated by a combined immunologic and electrophoretic approach. One particular antiserum directed against 3T3 cell plasma membranes was found to contain antibodies that blocked spreading of these murine cells on fibronectin but not on laminin or serum spreading factor (vitronectin). Proteolysis experiments confirmed that this cell line has calcium-protected polypeptides necessary for cell spreading on fibronectin. Consequently, protein antigens were fractionated according to size by SDS gel electrophoresis, and antigens that could block the inhibitory activity of the polyclonal antibody were identified. One class of blocking antigen appeared to correspond to the 140,000-dalton complexes favored by several laboratories as fibronectin receptor candidates, but a second class of 45,000 daltons was also apparent. This 45,000-dalton antigen was a major absorbing activity from 3T3 cell membranes and the predominant activity from L929 membranes. By isoelectric focusing and two-dimensional gel electrophoresis, it was found to exist as a set of isoelectric point variants with pK = 5.3 to 6.2. Our results indicate that current models postulating a simple, unimolecular receptor mechanism for fibronectin may be oversimplified and that fibronectin may instead interact with more than one protein receptor component on the fibroblast cell surface.