Distinct roles for Rap1b protein in platelet secretion and integrin α IIbβ 3 outside-in signaling

Guoying Zhang, Binggang Xiang, Shaojing Ye, Magdalena Chrzanowska-Wodnicka, Andrew J. Morris, T. Kent Gartner, Sidney W. Whiteheart, Gilbert C. White, Susan S. Smyth, Zhenyu Li

Research output: Contribution to journalArticlepeer-review

58 Scopus citations


Rap1b is activated by platelet agonists and plays a critical role in integrin α IIbβ 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 IIbβ 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 IIbβ 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 IIbβ 3 outside-in signaling.

Original languageEnglish
Pages (from-to)39466-39477
Number of pages12
JournalJournal of Biological Chemistry
Issue number45
StatePublished - Nov 11 2011

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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