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J Biol Chem. 2016 May 27;291(22):11706-16. doi: 10.1074/jbc.M116.716613. Epub 2016 Apr 7.

Directly Activating the Integrin αIIbβ3 Initiates Outside-In Signaling by Causing αIIbβ3 Clustering.

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From the Hematology-Oncology Division, Department of Medicine, and.
Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
the Department of Chemistry and Biochemistry and the BioFrontiers Institute, University of Colorado, Boulder, Colorado 80030, and.
the Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94158-9001.
From the Hematology-Oncology Division, Department of Medicine, and


αIIbβ3 activation in platelets is followed by activation of the tyrosine kinase c-Src associated with the carboxyl terminus of the β3 cytosolic tail. Exogenous peptides designed to interact with the αIIb transmembrane (TM) domain activate single αIIbβ3 molecules in platelets by binding to the αIIb TM domain and causing separation of the αIIbβ3 TM domain heterodimer. Here we asked whether directly activating single αIIbβ3 molecules in platelets using the designed peptide anti-αIIb TM also initiates αIIbβ3-mediated outside-in signaling by causing activation of β3-associated c-Src. Anti-αIIb TM caused activation of β3-associated c-Src and the kinase Syk, but not the kinase FAK, under conditions that precluded extracellular ligand binding to αIIbβ3. c-Src and Syk are activated by trans-autophosphorylation, suggesting that activation of individual αIIbβ3 molecules can initiate αIIbβ3 clustering in the absence of ligand binding. Consistent with this possibility, incubating platelets with anti-αIIb TM resulted in the redistribution of αIIbβ3 from a homogenous ring located at the periphery of discoid platelets into nodular densities consistent with clustered αIIbβ3. Thus, these studies indicate that not only is resting αIIbβ3 poised to undergo a conformational change that exposes its ligand-binding site, but it is poised to rapidly assemble into intracellular signal-generating complexes as well.


Src; integrin; platelet; signal transduction; transmembrane domain

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