Quantitative measurements of the in vitro binding of 125I-labeled rabbit IgG (IgGR), and its Fab-1, Fab-2, and Fc fragments to receptors on the fetal rabbit yolk sac membrane (YSM) were carried out by incubating equimolar solutions (1 × 10-5 M) of IgGR and of each of its fragments with formalin-fixed discs (3.5 cm2) of the YSM. It was found that 97, 4, 2, and 96 pmoles of IgGR, Fab-1, Fab-2, and Fc, respectively, were bound per YSM disc. Since the binding characteristics of intact IgGR were fully conserved in the Fc piece, the results establish that the YSM receptor for IgG is an Fc receptor. It was also shown that whereas IgGR retained its binding ability after exposure to stress (55°C, 10 min), free Fc did not. These findings, together with knowledge of the IgG structure, imply that the YSM receptor recognition unit of IgGR resides in the CH2 domain of its Fc piece.
This work was supported in part by Research Grant PCM-76-09979 from the National Science Foundation and Grant HD-7752 from the National Institutes of Health, United States Public Health Service.