Fc gamma Receptor contained in a mixture of plasma-membrane components released from pig peripheral blood lymphocytes following a 4----37 degrees C temperature shift was isolated by affinity chromatography on immobilized pig IgG. The main component of the receptor preparation exhibited an apparent molecular weight of 40-43 kDa in SDS polyacrylamide gel electrophoresis. Specificity of interaction of isolated Fc gamma receptor with immobilized pig IgG (Ka = 5.4 x 10(6) M-1) was investigated by competitive radioimmunoassay employing labelled Fc gamma receptor. The interaction was specifically inhibited by pig IgG, its Fc fragment, and less so by its pFc' fragment. Inhibition by peptides prepared from the pig IgG CH3 domain pointed to the heavy chain segment between residues 340 and 380 as the probable location of the binding site. In the competitive assay, bovine, human, mouse and guinea pig IgGs were as effective inhibitors as the homologous IgG, while rabbit IgG produced weaker inhibition. The amino acid composition of the pig lymphocyte Fc gamma receptor was determined. Comparison with the amino acid compositions of some other Fc receptors and other proteins revealed its structural relatedness to several Fc gamma receptors of lymphoid cells and to the poly-Ig receptor. In addition, the comparison of amino acid compositions suggested a structural relationship between the pig Fc gamma receptor and some seemingly unrelated proteins.

Institute of Molecular Genetics Czechoslovak Academy of Sciences Praha