The second and third extracellular domains of FcγRI (CD64) confer the unique high affinity binding of IgG2a

Fc gamma RI (CD64) is functionally unique as it is the only Fc gamma R able to bind monomeric IgG with high affinity. Fc gamma RI is also structurally distinct, containing an extracellular Ig-interactive region of three Ig-like domains in contrast to the two domains of the low affinity receptors Fc gamma RII and Fc gamma RIII. Previous studies have demonstrated that the third domain of Fc gamma RI plays a crucial role in high affinity IgG binding of the receptor, with the first and second domains together forming a low affinity IgG binding motif. In this study the individual functional contributions of the first and second domains of Fc gamma RI to IgG binding have been investigated. Chimeric Fc gamma R were generated by exchanging extracellular domains between mouse Fc gamma RI and the structurally related yet distinct low affinity receptor for IgG, mouse Fc gamma RII. The replacement of both domains 1 and 2 of Fc gamma RI with domains 1 and 2 of Fc gamma RII results in a dramatic change in IgG binding characteristics, as this receptor loses the capacity to bind monomeric IgG with high affinity and also demonstrates a broader specificity (binding not only IgG2a bur also IgG1 and 2b. IgG3 was not tested). However, the substitution of Fc gamma RII domain 2 of this chimeric receptor with domain 2 of Fc gamma RI (generating a chimeric receptor with domain 1 of Fc gamma RII linked to domains 2 and 3 of Fc gamma RI) was found to reconstitute the specific high affinity monomeric IgG2a binding of wild-type Fc gamma RI, albeit with a slightly reduced affinity (1.8-fold lower than wild-type Fc gamma RI). These findings suggest that it is the specific interaction between domains 2 and 3 of Fc gamma RI, with domain 1 playing a supporting role ill maintaining the conformational stability of the receptor. that is the major structural requirement to confer the unique Ig binding characteristics of Fc gamma RI. (C) 1998 Elsevier Science Ltd. All rights reserved.