CIB1 is an endogenous inhibitor of agonist-induced integrin αIIbβ3 activation

In response to agonist stimulation, the alpha IIb beta 3 integrin on platelets is converted to an active conformation that binds fibrinogen and mediates platelet aggregation. This process contributes to both normal hemostasis and thrombosis. Activation of alpha IIb beta 3 is believed to occur in part via engagement of the beta 3 cytoplasmic tail with talin; however, the role of the alpha IIb tail and its potential binding partners in regulating alpha IIb beta 3 activation is less clear. We report that calcium and integrin binding protein 1 (CIB1), which interacts directly with the alpha IIb tail, is an endogenous inhibitor of alpha IIb beta 3 activation; overexpression of CIB1 in megakaryocytes blocks agonist-induced alpha IIb beta 3 activation, whereas reduction of endogenous CIB1 via RNA interference enhances activation. CIB1 appears to inhibit integrin activation by competing with talin for binding to alpha IIb beta 3, thus providing a model for tightly controlled regulation of alpha IIb beta 3 activation.