Characterization of the family of dimers associated with Fc receptors (Fc∈RI and FcγRIII)

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