The IgG Fc contains distinct Fc receptor (FcR) binding sites: The leukocyte receptors Fc gamma RI and Fc gamma RIIa bind to a region in the Fc distinct from that recognized by neonatal FcR and protein A

The CH2-CH3 interface of the IgG Fc domain contains the binding sites for a number of Fc receptors including Staphylococcal protein A and the neonatal Fe receptor (FcRn), It has recently been proposed that the CH2-CH3 interfa ce also contains the principal binding site for an isoform of the low affin ity IgG Fc receptor II (Fc gamma RIIb), The Fc gamma RI and Fc gamma RII bi nding sites have previously been mapped to the lower hinge and the adjacent surface of the CH2 domain although contributions of the CH2-CH3 interface to binding have been suggested. This study addresses the question whether t he CH2-CH3 interface plays a role in the interaction of IgG with Fc gamma R I and Fc gamma RIIa, We demonstrate that recombinant soluble murine Fc gamm a RI and human Fc gamma RIIa did not compete with protein A and FcRn for bi nding to IgG, and that the CH2-CH3 interface therefore appears not to be in volved in Fc gamma RI and Fc gamma RIIa binding. The importance of the lowe r hinge was confirmed by introducing mutations in the proposed binding site (LL234,235AA) which abrogated binding of recombinant soluble Fc gamma RIIa to human IgG1, We conclude that the lower hinge and the adjacent region of the CH2 domain of IgG Fc is critical for the interaction between Fc gamma RIIa and human IgG, whereas contributions of the CH2-CH3 interface appear t o be insignificant.