The Rac1-and RhoG-specific GEF domain of Trio targets filamin to remodel cytoskeletal actin

Rho GTPases control actin reorganization and many other cellular functions. Guanine nucleotide-exchange factors (GEFs) activate Rho GTPases by promoti ng their exchange of GDP for GTP. Trio is a unique Rho GEF, because it has separate GEF domains, GEFD1 and GEFD2, that control the GTPases RhoG/Rac1 a nd RhoA, respectively, Dbl-homology (DH) domains that are common to GEFs ca talyse nucleotide exchange, and pleckstrin-homology (PH) domains localize R ho GEFs near their downstream targets, Here we show that Trio GEFD1 interac ts through its PH domain with the actin-filament-crosslinking protein filam in, and localizes with endogenous filamin in HeLa cells. Trio GEFD1 induces actin-based ruffling in filamin-expressing, but not filamin-deficient, cel ls and in cells transfected with a filamin construct that lacks the Trio-bi nding domain. In addition, Trio GEFD1 exchange activity is not affected by filamin binding. Our results indicate that filamin, as a molecular target o f Trio, may be a scaffold for the spatial organization of Rho-GTPase-mediat ed signalling pathways.