Adhesion activity in fibronectin's alternatively spliced domain EDa (EIIIA): complementarity to plasma fibronectin functions.

The EDa (EIIIA) domain is one of two alternatively spliced type III homology repeats in fibronectins which differentiate cellular (cFN) from plasma isoforms (pFN). A bacteria-expressed recombinant polypeptide encoding the EDa type III repeat promotes adhesion of some cell types and its activity is synergistic with neighboring repeats III11 and III12. We show in this study that co-coating substrata with the EDa-only polypeptide and a suboptimal concentration of pFN leads to increased attachment and extensive spreading of v-src-transformed 3T3 cells relative to that found on substrata of suboptimal pFN or EDa polypeptide alone. This complementarity of activities requires as little as 1 microgram/ml EDa polypeptide in the adsorbing mixture and displays sequence specificity for only EDa (recombinant polypeptides of neighboring repeats III11 or III12 were without effect). Furthermore, stress fibers and focal contacts are inducible on the EDa:pFN mixture, suggesting that the EDa sequence and its receptor participate in signal transduction. The codistribution of phosphotyrosine proteins, including pp125FAK, along with vinculin and talin into focal contacts supports this hypothesis. Therefore, an alternatively spliced domain EDa which is expressed in various proportions in cells and tissues may have special functions related to adhesion processes by complementing the functions of pFN circulating in blood.