Differential activation of focal adhesion kinase, Rho and Rac by the ninth and tenth FIII domains of fibronectin.

Fibronectins are widely expressed extracellular matrix ligands that are essential for many biological processes. Fibronectin-induced signaling pathways are elicited in diverse cell types when specific integrin receptors bind to the ninth and tenth FIII domains, FIII9-10. Integrin-mediated signal transduction involves activation of signaling pathways of the growth factor-dependent Ras-related small GTP-binding proteins Rho and Rac, and phosphorylation of focal adhesion kinase. We have dissected the requirement of FIII9 and FIII10 for Rho and Rac activity and phosphorylation of focal adhesion kinase in BHK fibroblasts and Swiss 3T3 cells. We demonstrate that FIII10 supports cell attachment but does not induce phosphorylation of focal adhesion kinase. In Swiss 3T3 cells, growth factor-independent phosphorylation of focal adhesion kinase and downstream adhesion events are dependent upon the presence of FIII9 in the intact FIII9-10 pair, whereas FIII10-mediated focal adhesion kinase phosphorylation requires a synergistic signal from growth factors. Furthermore, FIII10 is able to elicit cellular responses mediated by Rho, but not Rac, whereas FIII9-10 can elicit both Rho- and Rac-mediated responses. We propose that activation of specific integrin subunits by the FIII10 and FIII9-10 ligands elicits distinct signaling events. This may represent a general molecular mechanism for activation of receptor-specific signaling pathways by a multi-domain ligand.