Filamin links cell shape and cytoskeletal structure to Rho regulation by controlling accumulation of p190RhoGAP in lipid rafts.

Cytoskeleton-dependent changes in the activity of the small GTPase Rho mediate the effects of cell shape on cell function; however, little is known about how cell spreading and related distortion of the cytoskeleton regulate Rho activity. Here we show that rearrangements of the actin cytoskeleton associated with early phases of cell spreading in human microvascular endothelial (HMVE) cells suppress Rho activity by promoting accumulation of p190RhoGAP in lipid rafts where it exerts its Rho inhibitory activity. p190RhoGAP is excluded from lipid rafts and Rho activity increases when cell rounding is induced or the actin cytoskeleton is disrupted, and p190RhoGAP knockdown using siRNA prevents Rho inactivation by cell spreading. Importantly, cell rounding fails to prevent accumulation of p190RhoGAP in lipid rafts and to increase Rho activity in cells that lack the cytoskeletal protein filamin. Moreover, filamin is degraded in spread cells and cells that express a calpain-resistant form of filamin exhibit high Rho activity even when spread. Filamin may therefore represent the missing link that connects cytoskeleton-dependent changes of cell shape to Rho inactivation during the earliest phases of cell spreading by virtue of its ability to promote accumulation of p190RhoGAP in lipid rafts.