Distinct roles for Rap1b protein in platelet secretion and integrin αIIbβ3 outside-in signaling

J Biol Chem. 2011 Nov 11;286(45):39466-77. doi: 10.1074/jbc.M111.239608. Epub 2011 Sep 22.


Rap1b is activated by platelet agonists and plays a critical role in integrin α(IIb)β(3) inside-out signaling and platelet aggregation. Here we show that agonist-induced Rap1b activation plays an important role in stimulating secretion of platelet granules. We also show that α(IIb)β(3) outside-in signaling can activate Rap1b, and integrin outside-in signaling-mediated Rap1b activation is important in facilitating platelet spreading on fibrinogen and clot retraction. Rap1b-deficient platelets had diminished ATP secretion and P-selectin expression induced by thrombin or collagen. Importantly, addition of low doses of ADP and/or fibrinogen restored aggregation of Rap1b-deficient platelets. Furthermore, we found that Rap1b was activated by platelet spreading on immobilized fibrinogen, a process that was not affected by P2Y(12) or TXA(2) receptor deficiency, but was inhibited by the selective Src inhibitor PP2, the PKC inhibitor Ro-31-8220, or the calcium chelator demethyl-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis. Clot retraction was abolished, and platelet spreading on fibrinogen was diminished in Rap1b-deficient platelets compared with wild-type controls. The defects in clot retraction and spreading on fibrinogen of Rap1b-deficient platelets were not rescued by addition of MnCl(2), which elicits α(IIb)β(3) outside-in signaling in the absence of inside-out signaling. Thus, our results reveal two different activation mechanisms of Rap1b as well as novel functions of Rap1b in platelet secretion and in integrin α(IIb)β(3) outside-in signaling.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphate / genetics
  • Adenosine Triphosphate / metabolism
  • Animals
  • Blood Platelets / metabolism*
  • Chelating Agents / pharmacology
  • Clot Retraction / drug effects
  • Clot Retraction / physiology*
  • Enzyme Activation / drug effects
  • Enzyme Activation / genetics
  • Enzyme Inhibitors / pharmacology
  • Fibrinogen / metabolism
  • Indoles / pharmacology
  • Mice
  • Mice, Mutant Strains
  • P-Selectin / genetics
  • P-Selectin / metabolism
  • Platelet Adhesiveness / drug effects
  • Platelet Adhesiveness / physiology*
  • Platelet Glycoprotein GPIIb-IIIa Complex / genetics
  • Platelet Glycoprotein GPIIb-IIIa Complex / metabolism*
  • Protein Kinase C / antagonists & inhibitors
  • Protein Kinase C / genetics
  • Protein Kinase C / metabolism
  • Pyrimidines / pharmacology
  • Receptors, Purinergic P2Y12 / genetics
  • Receptors, Purinergic P2Y12 / metabolism
  • Receptors, Thromboxane A2, Prostaglandin H2 / genetics
  • Receptors, Thromboxane A2, Prostaglandin H2 / metabolism
  • Signal Transduction / drug effects
  • Signal Transduction / physiology*
  • rap GTP-Binding Proteins / genetics
  • rap GTP-Binding Proteins / metabolism*


  • AG 1879
  • Chelating Agents
  • Enzyme Inhibitors
  • Indoles
  • P-Selectin
  • P2ry12 protein, mouse
  • Platelet Glycoprotein GPIIb-IIIa Complex
  • Pyrimidines
  • Receptors, Purinergic P2Y12
  • Receptors, Thromboxane A2, Prostaglandin H2
  • Adenosine Triphosphate
  • Fibrinogen
  • Protein Kinase C
  • Rap1b protein, mouse
  • rap GTP-Binding Proteins
  • Ro 31-8220