By using radiolabeled myeloma proteins specific for each of two distinct Fc receptor activities on macrophage-like murine cell lines (“aggregated IgG FcR” activity and “monomer IgG2a FcR” activity), we have been able to detect solubilized FcR of both types in detergent lysates of these cells. The two solubilized Fc receptors can be distinguished and physically separated from one another by means of affinity chromatography and by sucrose gradient centrifugation. They also differ in their sensitivity to phospholipase C. The detergent solubilized monomer IgG2a FcR can be removed from solution with a Sepharose IgG2a column, has an S value of 4 to 5 and is resistant to phospholipase C. The solubilized aggregated IgG FcR does not bind to insolubilized IgG2a, has an S value of greater than 19, and is sensitive to phospholipase C treatment. We conclude from these studies that two different cell membrane macromolecules are responsible for the Fc receptor activities observed in macrophage-like cell lines.

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