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Ameloblastin, the most abundant nonamelogenin enamel matrix protein, plays a role in ameloblast differentiation. Here, we found that ameloblastin was expressed in osteosarcoma cells; to explore the potential functions of ameloblastin in osteoblasts, we investigated whether this protein is involved in osteogenic differentiation and bone formation on the premise that CD63, a member of the transmembrane-4 glycoprotein superfamily, interacts with integrins in the presence of ameloblastin. Ameloblastin bound to CD63 and promoted CD63 binding to integrin β1. The interaction between CD63 and integrin β1 induced Src kinase inactivation via the binding of CD63 to Src. The reduction of Src activity and osteogenic differentiation mediated by ameloblastin were abrogated by treatment with anti-CD63 antibody and overexpression of constitutively active Src, respectively. Therefore, our results suggest that ameloblastin …