Biochemical characterization of murine FcγRI

The biochemical analysis of murine FcγRI has been difficult because this receptor is co-expressed with other Fc receptors on the cell surface of myeloid cells and all precipitate together with immune complexes. To overcome this problem and to study cells which expressed FcγRI and only this Fc receptor, stable transfection of Chinese hamster ovary (CHO) cells (FcγRI^-) with FcγRI cDNA was performed thereby permitting biochemical characterization of FcγRI in isolation from other Fc receptors. Studies of FcγRI⁺CHO cells showed the mature FcγRI to be a 70,000 MW single-chain receptor on the cell surface. The 70,000 MW molecule was also identified on the FcγRI⁺ cell lines, J774 (a macrophage-like cell line), and 18.81 (a pre-B-cell line). FcγRI was shown to be N-linked glycosylated and after deglycosylation to have a protein core of 49,000 MW. FcγRI was found to be phosphorylated and after PMA stimulation, the level of phosphorylation was increased; serine residues in the cytoplasmic tail were identified as the phosphate acceptors. Thus FcγRI can now be definitely characterized as a 70,000 MW phosphoprotein.