Meghna U. Naik, Jeffrey L. Caplan, and Ulhas P. Naik
Fibrinogen binding to activated integrin induces outside–in signaling that results in stable platelet aggregates and clot retraction. How integrin αIIbβ3 is discouraged from spontaneous activation is not known. We have recently shown that junctional adhesion molecule–A (JAM–A) renders protection from thrombosis by suppressing integrin outside–in signaling. In this study, we show that JAM–A associates with integrin αIIbβ3 in resting platelets and dissociates upon platelet activation by agonists. We also show that integrin–associated JAM–A is tyrosine phosphorylated and is rapidly dephosphorylated upon platelet activation. C–terminal Src kinase (Csk) binds to tyrosine phosphorylated JAM–A through its Src homology 2 domain. Thus, JAM–A recruits Csk to the integrin–c–Src complex in resting platelets. Csk, in turn, keeps integrin–associated c–Src in an inactive state by phosphorylating Y529 in its regulatory domain. Absence of JAM–A results in impaired c–SrcY529 phosphorylation and augmentation of outside–in signaling–dependent c–Src activation. Our results strongly suggest that tyrosine–phosphorylated JAM–A is a Csk–binding protein and functions as an endogenous inhibitor of integrin signaling. JAM–A recruits Csk to the integrin–c–Src complex, where Csk negatively regulates c–Src activation, thereby suppressing the initiation of outside–in signaling. Upon agonist stimulation, JAM–A is dephosphorylated on the tyrosine, allowing the dissociation of Csk from the integrin complex, and thus facilitating outside–in signaling.