A subunit common to an IgG Fc receptor and the T-cell receptor mediates assembly through different interactions.

The zeta subunit is a component of the Fc gamma receptor of natural killer cells (Fc gamma RIII or CD16), as well as the multimeric T-cell receptor/CD3 complex, and is required for assembly of both native receptors. The role of the zeta subunit in human Fc gamma RIIIA assembly differs from its role in T-cell receptor/CD3 complex assembly. The transmembrane domain of the Fc gamma RIIIA alpha subunit forms noncovalent interactions with the comparable domain of the zeta subunit and is sufficient for surface expression of the Fc gamma RIIIA complex. In the absence of these interactions, sequences in the transmembrane domain of the Fc gamma RIIIA alpha subunit signal its degradation. Leu-46, present in the transmembrane domain of the human zeta subunit, is important for assembly with the Fc gamma RIIIA alpha subunit. Substitution of this leucine with an isoleucine, as found in the mouse zeta subunit, significantly reduces this interaction. In contrast, the mouse and human zeta subunits interact with the pentameric T-cell receptor/CD3 complex, resulting in surface expression of this receptor.