Characterization of the family of dimers associated with Fc receptors (Fc epsilon RI and Fc gamma RIII)

The receptor for IgE (Fc epsilon RI) is a multimeric complex containing one alpha chain, one beta chain with four transmembrane domains and one homodimer of disulfide-linked gamma-chains. The Fc epsilon RI gamma-chains form additional disulfide-linked dimers with the homologous zeta- and eta-chains, as part of the TCR complex. The low affinity receptor for IgG (Fc gamma RIII)2 on NK cells is also associated with zeta-chains. Here we show that the gamma-chain is expressed in NK cells both as a group of heterogenous gamma gamma homodimers and also as a heterodimer bound to zeta. Fc gamma RIIIA is associated with three types of dimers zeta zeta, gamma zeta, and notably gamma gamma as well. In fact, gamma gamma appears to be the predominant species associating with Fc gamma RIIIA. The surface expressed Fc epsilon RI also associates with the same group of heterogenous gamma gamma homodimers. We also show that there is no C-terminal posttranslational cleavage of gamma occurring before its insertion into the plasma membrane as previously suggested. Thus, like the TCR, Fc gamma RIIIA may form a variety of receptor isoforms, though at present we do not understand the functional implications of these structures.