Differential Involvement of Src Family Kinases in Fcγ Receptor-Mediated Phagocytosis1

The tyrosine phosphorylation cascade originated from Fcγ receptors (FcγRs) is essential for macrophage functions including phagocytosis. Although the initial step is ascribed to Src family tyrosine kinases, the role of individual kinases in phagocytosis signaling is still to be determined. In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the FcγR-mediated signaling that leads to phagocytosis in a kinase-dependent manner. We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope. Those constructs derived from Lyn and Hck (a-Lyn and a-Hck) that associated with detergent-resistant membranes successfully reconstructed FcγR-mediated Syk activation, filamentous actin rearrangement, and phagocytosis. In contrast, c-Src-derived construct (a-Src), that was excluded from detergent-resistant membranes, could not restore the series of phagocytosis signaling. Tyrosine phosphorylation of Vav and c-Cbl was restored in common by a-Lyn, a-Hck, and a-Src, but FcγRIIB tyrosine phosphorylation, which is implicated in negative signaling, was reconstituted solely by a-Lyn and a-Hck. These findings suggest that Src family kinases are differentially involved in FcγR-signaling and that selective kinases including Lyn and Hck are able to fully transduce phagocytotic signaling.