Directly Activating the Integrin αIIbß3 Initiates Outside-In Signaling by Causing αIIbß3 Clustering.

ABSTRACT

αIIbß3 activation in platelets is followed by activation of the tyrosine kinase c-Src associated with the carboxyl terminus of the ß3 cytosolic tail. Exogenous peptides designed to interact with the αIIb transmembrane (TM) domain activate single αIIbß3 molecules in platelets by binding to the αIIb TM domain and causing separation of the αIIbß3 TM domain heterodimer. Here we asked whether directly activating single αIIbß3 molecules in platelets using the designed peptide anti-αIIb TM also initiates αIIbß3-mediated outside-in signaling by causing activation of ß3-associated c-Src. Anti-αIIb TM caused activation of ß3-associated c-Src and the kinase Syk, but not the kinase FAK, under conditions that precluded extracellular ligand binding to αIIbß3. c-Src and Syk are activated by trans-autophosphorylation, suggesting that activation of individual αIIbß3 molecules can initiate αIIbß3 clustering in the absence of ligand binding. Consistent with this possibility, incubating platelets with anti-αIIb TM resulted in the redistribution of αIIbß3 from a homogenous ring located at the periphery of discoid platelets into nodular densities consistent with clustered αIIbß3. Thus, these studies indicate that not only is resting αIIbß3 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.