An intracellular 50-kDa Fc gamma-binding protein is induced in human cells by alpha-IFN.

This report describes a novel intracellular Ig Fc-binding protein that is induced by alpha-IFN in a wide range of human cell types. The protein has a m.w. of 45,000 to 50,000 and it will bind human IgG but not IgM, IgA, or IgE. The IgG-binding specificity of this protein (designated Fc gamma BP) is similar to that of the Fc gamma RII receptor in that it binds only to mouse IgG1 and IgG2b but not IgG2a. It is an intracellular protein and is detectable by immunoprecipitation from [35S]methionine-labeled cells but it is undetectable on the cell surface. Fc gamma BP is a phosphoprotein, and the amount of phosphorylated Fc gamma BP increases after treatment with alpha-IFN. Although the function of this protein has yet to be determined, the induction of an Fc-binding protein by alpha-IFN suggests a role in the immune response, particularly during infection by viruses.