FC-GAMMA-RIIA-MEDIATED PHAGOCYTOSIS AND RECEPTOR PHOSPHORYLATION IN CELLS DEFICIENT IN THE PROTEIN-TYROSINE KINASE SRC

In the absence of other Fc receptors, stimulation of Fc gamma RIIA induces receptor phosphorylation and phagocytosis of immunoglobulin G (IgG)-coated cells. In vitro, Fc gamma RIIA is phosphorylated by the Src-related tyrosine kinase (SRTK) Src. Therefore, we investigated whether fibroblasts transfected with Fc gamma RIIA mediate phagocytosis of IgG-toated cells and whether Src is required for Fc gamma RIIA phosphorylation and for phagocytosis in vivo. Activation of Fc gamma RIIA in a fibroblast cell line deficient in Src kinase resulted in phosphorylation of the receptor on tyrosine. In addition, Fc gamma RIIA-mediated phagocytosis was observed in these fibroblasts in both the presence and absence of Src. in the presence of Src, however, phagocytosis of IgG-coated cells was more efficient. The data indicate that the SRTK Src is not required for Fc gamma RIIA phosphorylation or for Fc gamma RIIA-mediated phagocytosis in these cells. In vitro kinase assays demonstrated that the SRTK Fyn also is able to phosphorylate Fc gamma RLIA. Thus, Fc gamma RIIA can be phosphorylated by more than one tyrosine kinase in vitro. The data suggest that there may be shared functions among some intracellular kinases in receptor phosphorylation.