Cells respond to diverse external stimuli by polymerizing cytoplasmic actin, and recent evidence indicates that GTPases can specify where this polymerization takes place. Actin assembly in stimulated blood platelets occurs where sequestered monomers add onto the fast-growing (barbed) ends of actin filaments (F-actin), which are capped in the resting cells. We report that D3 and D4 polyphosphoinositides, PI(4)P, PI(4,5)P2, PI(3,4)P2, and PI(3,4,5)P3, uncap F-actin in resting permeabilized platelets. The thrombin receptor-activating peptide (TRAP), GTP, and GTPγS, but not GDPβS, also uncap F-actin in permeabilized platelets. GDPβS inhibits TRAP-induced F-actin uncapping, and PI(4,5)P2 overcomes this inhibition. Constitutively active mutant Rac, but not Rho, activates uncapping of F-actin. PI(4,5)P2-binding peptides derived from gelsolin inhibit F-actin uncapping by TRAP, Rac, and GTPγS. TRAP and Rac induce rapid PI(4,5)P2 synthesis in permeabilized platelets. The findings establish a signaling pathway for actin assembly involving Rac in which the final message is phosphoinositide-mediated F-actin uncapping. © 1995.
Hartwig, J. H., Bokoch, G. M., Carpenter, C. L., Janmey, P. A., Taylor, L. A., Toker, A., & Stossel, T. P. (1995). Thrombin receptor ligation and activated rac uncap actin filament barbed ends through phosphoinositide synthesis in permeabilized human platelets. Cell, 82(4), 643–653. https://doi.org/10.1016/0092-8674(95)90036-5