Fc receptors for the Fc part of IgG are the mediators for antibody effector functions. FcRIII and FcRII are low affinity receptors that, through the interaction with immune complexes, initiate a variety of immunological responses, such as phagocytosis, antibody-dependent cellular cytotoxicity, and release of inflammatory mediators. We set out to define the IgG binding site on human FcRIII. We assumed that potential -turns in Ig-like domains are the most probable determinants for ligand binding, and chimeric FcRIIIB/FcRI receptors as well as single residue mutants were constructed in these regions of FcRIIIB. Substitution of four amino acids in the membrane-proximal domain (Gln, Arg, Lys, Val) resulted in decreased binding of human IgG1. Lys and Val were found also to be crucial for the interaction with the IgG-binding inhibitory monoclonal antibody 3G8. In a putative three-dimensional model constructed in this study, these residues map on the CC` loop (Gln), on F -sheet (Arg), and on the FG loop (Lys, Val). Our data are consistent with the study about human FcRII (Hulett, M. D., Witort, E., Brinkworth, R. I., McKenzie, I. F. C., and Hogarth, P. M.(1994) J. Biol. Chem. 269, 15287-15293), suggesting that common structural determinants, i.e. FG loop or the GFC surface of the membrane-proximal domain, can be involved in interactions with IgG by both low affinity receptor classes FcRII and FcRIII.

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