Isolation of biosynthetically-labeled Fc-binding proteins from detergent lysates and spent culture fluid of a macrophage-like cell line (P388D1).

Lactoperoxidase-catalyzed surface iodination of P388D1 mouse macrophage cells has led to the identification of a 57,000 m.w. protein as a candidate for the cell's receptor for the Fc region of IgG. THe present work extends these studies and shows, by biosynthetic incorporation of radiolabeled amino acids and sugars, that the protein in question is a glycoprotein, which appears to be shed or released from the cells in a slightly altered form during the labeling period. Use of labeled amino acids as probes permitted the detection of a large number of contaminating proteins in these preparations; some of these may represent degradation products of the putative FcR. One of the contaminants was identified as actin; the binding of the putative receptor to IgG appeared to be quite independent of any actin-IgG interactions.