Lyn and Syk Kinases Are Sequentially Engaged in Phagocytosis Mediated by FcγR

Abstract

Recent data indicate that phagocytosis mediated by FcγRs is controlled by the Src and Syk families of protein tyrosine kinases. In this study, we demonstrate a sequential involvement of Lyn and Syk in the phagocytosis of IgG-coated particles. The particles isolated at the stage of their binding to FcγRs (4°C) were accompanied by high amounts of Lyn, in addition to the signaling γ-chain of FcγRs. Simultaneously, the particle binding induced rapid tyrosine phosphorylation of numerous proteins. During synchronized internalization of the particles induced by shifting the cell to 37°C, Syk kinase and Src homology 2-containing tyrosine phosphatase-1 (SHP-1) were associated with the formed phagosomes. At this step, most of the proteins were dephosphorylated, although some underwent further tyrosine phosphorylation. Quantitative immunoelectron microscopy studies confirmed that Lyn accumulated under the plasma membrane beneath the bound particles. High amounts of the γ-chain and tyrosine-phosphorylated proteins were also observed under the bound particles. When the particles were internalized, the γ-chain was still detected in the region of the phagosomes, while amounts of Lyn were markedly reduced. In contrast, the vicinity of the phagosomes was heavily decorated with anti-Syk and anti-SHP-1 Abs. The local level of protein tyrosine phosphorylation was reduced. The data indicate that the accumulation of Lyn during the binding of IgG-coated particles to FcγRs correlated with strong tyrosine phosphorylation of numerous proteins, suggesting an initiating role for Lyn in protein phosphorylation at the onset of the phagocytosis. Syk kinase and SHP-1 phosphatase are mainly engaged at the stage of particle internalization.