A NOVEL ROLE FOR THE FC RECEPTOR-GAMMA SUBUNIT - ENHANCEMENT OF FC-GAMMA-R LIGAND AFFINITY

The Fc receptors (FcR), which belong to the immunoglobulin (Ig) superf amily, bind to specific Ig isotypes with varying affinities triggering complex immune defense responses. Several of the FcR that lack signal ing motifs in their cytoplasmic domains rely on associated subunits to transmit signals. Two classes of FcR that bind the Fc portion of IgG, Fc gamma RI, and Fc gamma RIIIa associate with a subunit shared among several FcR, the gamma chain, which is involved in receptor expressio n and signal transduction. In this report, we propose that a novel rol e for gamma chain is to enhance the affinity of Fc gamma R for ligand. Our findings demonstrate that Fc gamma RI requires gamma-chain associ ation to attain high affinity binding for monomeric IgG, and suggest t hat the intermediate binding affinity of the Fc gamma RIIIa isoform re sults from its association with gamma chain. The affinity increase con ferred by gamma chain appears to be mediated through the transmembrane domain or the Fc gamma R, with no requirement for the cytoplasmic dom ain of the receptor.