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 ind uces receptor phosphorylation and phagocytosis of immunoglobulin G (Ig G)-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 Ig G-toated cells and whether Src is required for Fc gamma RIIA phosphory lation and for phagocytosis in vivo. Activation of Fc gamma RIIA in a fibroblast cell line deficient in Src kinase resulted in phosphorylati on of the receptor on tyrosine. In addition, Fc gamma RIIA-mediated ph agocytosis was observed in these fibroblasts in both the presence and absence of Src. in the presence of Src, however, phagocytosis of IgG-c oated cells was more efficient. The data indicate that the SRTK Src is not required for Fc gamma RIIA phosphorylation or for Fc gamma RIIA-m ediated phagocytosis in these cells. In vitro kinase assays demonstrat ed 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 s ome intracellular kinases in receptor phosphorylation.