Isolation of an Fc_γ-Binding Protein from the Cell Membrane of a Macrophage-Like Cell Line (P388D₁) after Detergent Solubilization¹

Lactoperoxidase-catalyzed iodination, NP-40 lysis, and subsequent affinity chromatography on IgG-Sepharose were used in an attempt to define some of the molecular properties of the Fc receptor of P388D₁, a macrophage-like mouse tumor line. Radioiodinated material retained on columns of Sepharose coupled either to monomeric mouse IgG2a or monomeric human IgG1 appeared on SDS polyacrylamide gel electrophoresis to contain principally three labeled components, a major band of about 57,000 m.w., and two minor bands of 28,000 and 24,000 m.w. The mobilities of these components changed little on reduction, which suggested that they represented single polypeptide chains. An identical pattern was obtained with Sepharose-linked Fc fragments of human IgG1, but neither Fab fragments of IgG1 nor IgM appeared to bind these components. Since the specificity of binding to the immobilized proteins is the same as that observed in vivo, it is postulated that these proteins represent either all or some portion of the P388D₁ Fc receptor.