High affinity IgG binding by FcgammaRI (CD64) is modulated by two distinct IgSF domains and the transmembrane domain of the receptor

The high affinity IgG receptor, FcgammaRI, is comprised of threeimmunoglobulin superfamily (IgSF) domains (EC1, EC2 and EC3), a singletransmembrane spanning region, and a short cytoplasmic tail. We have showna role for three separate domains of FcgammaRI in the high affinity bindingof IgG. Affinity measurements of chimeric FcgammaRs in which EC1 and EC2 ofFcgammaRI have been replaced with the homologous EC1 and/or EC2 domains ofthe low affinity IgG receptor, FcgammaRII indicate that both EC2 and EC3are essential for high affinity binding of monomeric IgG. Identification ofEC3 from FcgammaRI as the binding site for the monoclonal antibody 10.1,which blocks IgG binding, provides further evidence for the role of thisdomain in binding. In addition, we have found that the affinity ofFcgammaRI is increased threefold when co-expressed with its accessorymolecule, gamma-chain. Affinity measurements of further chimeras indicatesthat the transmembrane domain of FcgammaRI has a negative influence uponthe affinity of the receptor. To account for these observations, we proposethat receptor dimerization is required for maximal affinity of FcgammaRI.Dimerization may serve as the mechanism by which IgG binding triggersseveral FcgammaRI-mediated events.