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Platelet aggregation is mediated by conformational change of integrin αIIbβ3. Tyrosine-phosphorylation of cytoplasmic domain of β3 upon platelet activation has been demonstrated to play a critical role in this process. Recently, the adaptor protein ShcA has been shown to bind to the tyrosine-phosphorylated β3, while it remains open whether ShcA plays any role in platelet aggregation. Here, we show that ShcA bound to tyrosine-phosphorylated β3-tail peptide through its phosphotyrosine-binding domain in vitro. Then, we examined the involvement of ShcA in platelet aggregation by a previously established in vitro assay using platelets permeabilized with streptolysin-O, where exogenous addition of platelet cytosol is required for reconstitution of the Ca2+-induced aggregation. When ShcA was specifically depleted with anti-ShcA antibody from the cytosol, this ShcA-depleted cytosol lost the aggregation-supporting activity, which was rescued by addition of purified recombinant ShcA. Thus, ShcA is essential for the Ca2+-induced platelet aggregation. [Copyright &y& Elsevier]