Structural studies of T lymphocyte Fc receptors. Association of Gc protein with IgG binding to Fc gamma.

To obtain further information concerning the structure of Fc IgG-binding sites on human peripheral blood lymphocytes, enriched T-cells were surface-radioiodinated and treated with nonionic detergent, and the soluble supernatant was submitted to affinity chromatography selecting for components binding complexed IgG. Analysis of eluted material by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, isoelectric focusing, and two-dimensional electrophoresis demonstrated the major proteins to be of Mr 56,000, pI 4.8-5.1 and Mr 60,000, pI 5.0-5.6, and these were radiolabeled, indicating an origin in part from the T-cell membrane. While the Mr 56,000 band gave positive reactions upon transblotting with antisera to Gc protein, the identity of the Mr 60,000 protein remains unknown. Three other components were detected, although with less consistency; Mr 78,000, 42,000, and 20,000-25,000, respectively. Immunocytochemical experiments showed that less than 5% freshly isolated native T-cells were positive with antiserum to human Gc, but, after IgG antibody-coated erythrocyte rosetting, the number of positive cells increased to 15-25%, in close agreement with the percentage of IgG antibody-coated erythrocyte rosette-positive T-cells. These findings therefore indicate that, in addition to interaction with components of Mr 60,000, 42,000, and 20,000-25,000, complexed IgG binding to Fc gamma of human peripheral blood T-cells also becomes spatially associated with Gc protein.