MULTIPLE REGIONS OF HUMAN FC-GAMMA-RII (CD32) CONTRIBUTE TO THE BINDING OF IGG

The low affinity receptor for IgG, Fc gamma RII (CD32), has a wide dis tribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resista nce to infection. Fc gamma RII is a member of the immunoglobulin super family, containing an extracellular region of two Ig-like domains. The IgG binding site of human Fc gamma RII has been localized to an 8-ami no acid segment of the second extracellular domain, Asn(154)-Ser(161). In this study, evidence is presented to suggest that domain 1 and two additional regions of domain 2 also contribute to the binding of IgG by Fc gamma RII. Chimeric receptors generated by exchanging the extrac ellular domains and segments of domain 2 between Fc gamma RII and the structurally related Fc epsilon RI alpha chain were used to demonstrat e that substitution of domain 1 in its entirety or the domain 2 region s encompassing residues Ser(109)-Val(116) and Ser(130)-Thr(135) result ed in a loss of the ability of these receptors to bind hIgG1 in dimeri c form. Site-directed mutagenesis performed on individual residues wit hin and flanking the Ser(109)-Val(116) and Ser(130)-Thr(135) domain 2 segments indicated that substitution of Lys(113), Pro(114), Leu(115), Val(116), Phe(129) and His(131) profoundly decreased the binding of hI gG1, whereas substitution of Asp(133) and Pro(134) increased binding. These findings suggest that not only is domain 1 contributing to the a ffinity of IgG binding by Fc gamma RII but, importantly, that the doma in 2 regions Ser(109)-Val(116) and Phe(129)-Thr(135) also play key rol es in the binding of hIgG1. The location of these binding regions on a molecular model of the entire extracellular region of Fc gamma RII in dicates that they comprise loops that are juxtaposed in domain 2 at th e interface with domain 1, with the putative crucial binding residues forming a hydrophobic pocket surrounded by a wall of predominantly aro matic and basic residues.