Fibroblast cellular and plasma fibronectins are similar but not identical

Abstract

Fibronectins are multimeric, adhesive glycoproteins present on cell surfaces and circulating in blood. Cellular fibronectin produced by fibroblasts in vitro and fibronectin isolated from plasma are very similar immunologically and biochemically. Their similarity was investigated. Purified chicken and human cell-surface fibronectins are 150-fold more active in hemagglutination of fixed erythrocytes than plasma fibronectins. Cell-surface fibronectin is also 50-fold more active in restoring a more normal morphology to transformed cells originally missing the protein. In 2 other assays that measure cell attachment to collagen and cell spreading, cell-surface and plasma fibronectins have identical specific activities. In sodium dodecyl sulfate polyacrylamide gel electrophoresis, the subunits of human and chicken plasma fibronectins have significantly smaller apparent subunit MW than cellular fibronectins on cell surfaces or secreted into culture media. These differences are also present in a characteristic large subfragment of both forms of fibronectin after limited proteolysis by trypsin. By biological and biochemical criteria, cellular and plasma fibronectins are probably similar but not identical.