Signaling through mutants of the IgA receptor CD89 and consequences for Fc receptor γ-chain interaction

The prototypic receptor for IgA (Fc alpha RI, CD89) is expressed on myeloid cells and can trigger phagocytosis, tumor cell lysis, and release of inflammatory mediators. The functions of FcaRI and activating receptors for IgG (Fc gamma RI and Fc gamma RIII) are dependent on the FcR gamma-chain dimer. This study increases our understanding of the molecular basis of the Fc alpha RI-FcR gamma-chain transmembrane interaction, which is distinct from that of other activatory FcRs. FcaRI is unique in its interaction with the common FcR gamma-chain, because it is based on a positively charged residue at position 20.9, which associates with a negatively charged amino acid of FcR gamma-chain. We explored the importance of the position of this positive charge within human FcaRI for FcR gamma-chain association and Fc alpha RI functioning with the use of site-directed mutagenesis. In an Fc alpha RI R209L/A213H mutant, which represents a vertical relocation of the positive charge, proximal and distal FcR gamma-chain-dependent functions, such as calcium flux, MAPK phosphorylation, and IL-2 release, were similar to those of wild-type Fc alpha RI. A lateral transfer of the positive charge, however, completely abrogated FcR gamma-chain-dependent functions in an Fc alpha RI R209L/M210R mutant. By coimmunoprecipitation, we have demonstrated the loss of a physical interaction between FcR gamma-chain and FcaRI M210R mutant, thus explaining the loss of FcR gamma-chain-dependent functions. In conclusion, not only the presence of a basic residue in the transmembrane region of FcaRI, but also the orientation of FcaRI toward the FcR gamma-chain dimer is essential for FcR gamma-chain association. This suggests the involvement of additional amino acids in the FcaRI-FcR gamma-chain interaction.