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Biochemical characterization of an Fc receptor of rabbit lymphocytes

Isolation of Fc‐binding molecules was performed by complexing IgG antibodies and the corresponding antigens in lysates of radiolabeled rabbit lymphoid cells. A single‐ chain molecule of 110000 apparent mol. wt. (unreduced) or 120000 (reduced) was observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis when the antibodies were in the IgG form but not as F (ab′),. This molecule was very susceptible to proteolysis, but the fragments thus produced remained associated by disulfide bridges, and the binding properties of the molecule were conserved. The molecule and its proteolytic fragments (mol. wts. 75 000, 45 000 and 20 000) were very similar to those obtained for the mouse Fc receptor under similar conditions, and therefore the molecule was designated as rabbit Fc receptor. Among several precipitating systems used, some Ig‐anti‐Ig complexes appeared to be the most efficient in co precipitating the rabbit Fc receptor. Remarkably high titers of Fc receptor were found on lymphocyte membrane of infected rabbits, in agreement with a possible role of this molecule in immune regulation.