Thrombin receptor ligation and activated Rac uncap actin filament barbed ends through phosphoinositide synthesis in permeabilized human platelets

Cell. 1995 Aug 25;82(4):643-53. doi: 10.1016/0092-8674(95)90036-5.


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, Pl(4)P, Pl(4,5)P2, Pl(3,4)P2, and Pl(3,4,5)P3, uncap F-actin in resting permeabilized platelets. The thrombin receptor-activating peptide (TRAP), GTP, and GTP gamma S, but not GDP beta S, also uncap F-actin in permeabilized platelets. GDP beta S inhibits TRAP-induced F-actin uncapping, and Pl(4,5)P2 overcomes this inhibition. Constitutively active mutant Rac, but not Rho, activates uncapping of F-actin. Pl(4,5)P2-binding peptides derived from gelsolin inhibit F-actin uncapping by TRAP, Rac, and GTP gamma S. TRAP and Rac induce rapid Pl(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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Actins / blood*
  • Actins / chemistry
  • Amino Acid Sequence
  • Blood Platelets / drug effects
  • Blood Platelets / metabolism
  • Cell Membrane Permeability
  • GTP-Binding Proteins / metabolism*
  • Guanosine Diphosphate / pharmacology
  • Guanosine Triphosphate / pharmacology
  • Humans
  • In Vitro Techniques
  • Molecular Sequence Data
  • Peptides / chemistry
  • Peptides / metabolism
  • Phosphatidylinositol Phosphates / pharmacology*
  • Platelet Activation
  • Receptors, Thrombin / metabolism*
  • Signal Transduction
  • rac GTP-Binding Proteins


  • Actins
  • Peptides
  • Phosphatidylinositol Phosphates
  • Receptors, Thrombin
  • Guanosine Diphosphate
  • Guanosine Triphosphate
  • GTP-Binding Proteins
  • rac GTP-Binding Proteins