Integrins are transmembrane proteins that interact with the extracellular matrix (ECM) to transduce mechanical signals to the cell cytoskeleton. Integrin dimers αvβ3 and α5β1 are known to bind to the ECM component fibronectin, which induces the formation of stress fibres and clustering of integrins into focal adhesions (FAs). Although formation of these structures is known to require RhoA GTPase signalling, the guanine nucleotide exchange factor (GEF) that couples integrin-fibronectin binding to RhoA activation is not known. In this study (Coló et al., 2023), Raquel Haga and colleagues use a nucleotide-free mutant of RhoA to trap and identify active GEFs in genetically modified mouse fibroblasts expressing only αvβ3 and/or α5β1 integrins. Here, the authors find that the RhoA GEF Lfc binds more abundantly to RhoA in αvβ3- than in α5β1-expressing cells. Mechanistically, αvβ3-fibronectin binding promotes dissociation of Lfc from microtubules, and the authors show that this is mediated by phosphorylation of Lfc at Ser151 by the MARK2/3 kinases. Subsequent activation of RhoA by Lfc results in increased stress fibre formation and maturation of FAs. Consistent with this, CRISPR-mediated knockout of Lfc in fibroblasts expressing only αvβ3 resulted in reduced RhoA activity, stress fibre formation and FA size. Together, these findings suggest that Lfc is the GEF required for RhoA activation downstream of αvβ3-fibronectin binding, and that this occurs in an integrin-specific manner.
