Membrane-proximal Ig-like domain of FcγRIII (CD16) contains residues critical for ligand binding

Margaret L. Hibbs, Martti Tolvanen, Olli Carpén

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Abstract

Ag-Ab complexes are cleared from the circulation through a complex system of receptors for the Fc portion of Ig (FcRs). FcγRIII (CD16) is a low affinity FcR for IgG that is composed of two highly homologous Ig-like extracellular domains. Using secondary structure predictions, we located a strongly hydrophilic region in the second Ig-like domain of FcγRIII that is predicted to lie between β-strands C and C'. Substitutions of seven out of eight amino acids in this region abolished binding to IgG. Substitution of a conformationally adjacent amino acid in a bend just before β-strand F and an amino acid in the B-C loop also affected ligand binding. However, amino acid substitutions in two different predicted loops in the second Ig-like domain as well as substitutions to three predicted loops in the first Ig-like domain had no effect on function. A chimeric FcγRIII molecule lacking the second Ig-like domain was unable to bind IgG further, suggesting the presence of the binding site in the second domain. Neutralizing mAbs that inhibit FcγRIII interaction with IgG were mapped to the E-F loop in the membrane proximal domain of FcγRIII, providing further evidence of the importance of this region of the molecule in ligand interaction.

Original languageEnglish
Pages (from-to)4466-4474
Number of pages9
JournalJournal of Immunology
Volume152
Issue number9
Publication statusPublished - 1 May 1994

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