The Cγ2 domain of immunoglobulin G (IgG) is reported to have a tryptophan residue and cationic residues at or near its Clq-binding site. The present study has used synthetic IgG peptides to explore the involvement of the 275-290 region of the Cγ2 domain of the heavy chain of human IgGl in binding to Cl. This region (Phe-Asn-Trp-Tyr-Val-Asp-Gly-Val-Gln-Val-His-Asn-Ala-Lys-Thr-Lys) contains Trp-277 and the cationic residues His-285, Lys-288, and Lys-290. The following peptides were synthesized by the solid-phase method and purified to homogeneity by using reverse-phase high-pressure liquid chromatography: the hexadecapeptide 275-290 and its N'-formyl derivative 275-290F containing both Trp-277 and the Cationic residues; the Nα-acetylpentapeptide 275-279A comprising the hydrophobic region around Trp-277; and the cationic decapeptide 281-290. When examined in the Augener assay for inhibition of Clmediated immune hemolysis, peptides 275-290F and 281-290 were about half as active as monomeric 7S human IgG on a molar basis and essentially as active on a site basis. Since both peptides containing residues 281-290 inhibited hemolysis in a manner similar to the Cγ2 domain, the cationic 281-290 region containing His-285, Lys-288, and Lys-290 may be a part of the Clq-binding site of Cγ2. These results are consistent with the tertiary structure of the Fc fragment of IgG, in which the 275-279 region is part of the hydrophobic core of the Cγ2 domain and the 281-290 region is exposed on the surface.
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