Pt. Harrison et Jm. Allen, HIGH-AFFINITY IGG BINDING BY FC-GAMMA-RI (CD64) IS MODULATED BY 2 DISTINCT IGSF DOMAINS AND THE TRANSMEMBRANE DOMAIN OF THE RECEPTOR, Protein engineering, 11(3), 1998, pp. 225-232
The high affinity IgG receptor, Fc gamma RI, is comprised of three imm
unoglobulin superfamily (IgSF) domains (EC1, EC2 and EC3), a single tr
ansmembrane spanning region, and a short cytoplasmic tail. We have sho
wn a role for three separate domains of Fc gamma RI in the high affini
ty binding of IgG, Affinity measurements of chimeric Fc gamma Rs in wh
ich EC1 and EC2 of Fc gamma RI have been replaced with the homologous
EC1 and/or EC2 domains of the low affinity IgG receptor, Fc gamma RII
indicate that both EC2 and EC3 are essential for high affinity binding
of monomeric IgG, Identification of EC3 from Fc gamma RI as the bindi
ng site for the monoclonal antibody 10.1, which blocks IgG binding, pr
ovides further evidence for the role of this domain in binding. In add
ition, we have found that the affinity of Fc gamma RI is increased thr
eefold when co-expressed with its accessory molecule, gamma-chain. Aff
inity measurements of further chimeras indicates that the transmembran
e domain of Fc gamma RI has a negative influence upon the affinity of
the receptor. To account for these observations, we propose that recep
tor dimerization is required for maximal affinity of Fc gamma RI, Dime
rization may serve as the mechanism by which IgG binding triggers seve
ral Fc gamma RI-mediated events.