The small GTP-binding protein Rac promotes the dissociation of gelsolin from actin filaments in neutrophils

Gelsolin is an actin filament-capping protein that has been shown to play a key role in cell migration. Here we have studied the involvement of phosphoinositide 3-kinase (PI 3-kinase) and GTP-binding proteins (G-proteins) in the regulation of gelsolin-actin interactions in neutrophils. Inhibition of PI 3-kinase activity in vivo by wortmannin did not affect the dissociation of actin-gel-solin (1:1) complexes induced by neutrophil stimulation with N-formyl-Met-Leu-Phe. Guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) indirectly promoted the dissociation of actin gelsolin complexes in a cell-free system using neutrophil cytosol, and this effect was blocked by the GDP dissociation inhibitor for Rho (Rho-GDI). The GTP gamma S-loaded (i) alpha(2) and the beta(1) gamma(2) subunits of heterotrimeric G-proteins (G(i) alpha(2) and G beta(1) gamma(2)) also triggered actin-gelsolin dissociation in a Rho-GDI-sensitive manner. GTP-loaded activated Rac, but not activated Rho, induced the dissociation of cytosolic actin-gelsolin complexes. The guanine nucleotide exchange on Rac was increased by addition of GTP gamma S-loaded G(i) alpha(2) or G beta(1) gamma(2) to neutrophil cytosol. These findings suggest that activation of Rac by G-protein-coupled receptors in neutrophils triggers uncapping of actin filaments, independently of PI 3-kinase.