ABSTRACT
The receptor for IgA, FcalphaRI or CD89, is expressed on myeloid cells and can trigger phagocytosis, tumor cell lysis, and release of inflammatory mediators. These functions critically depend on the associated FcR gamma-chain; however, some biological functions, like receptor internalization, are solely mediated by FcalphaRI alpha-chain. Little is known as to how FcalphaRI regulates these processes and the FcalphaRI intracellular domain does not contain recognized signalling motifs. We searched for associating proteins and identified c-Jun activating binding protein 1 (JAB1) as a binding partner specifically for FcalphaRI. We found increased FcalphaRI surface expression after ectopic expression of JAB1 as well as diminished protein levels of total FcR gamma-chain levels after JAB1 knock-down. These data functionally link JAB1 with controlling protein expression levels of FcalphaRI-FcR gamma-chain protein complex.
Subject(s)
Antigens, CD/metabolism , Intracellular Signaling Peptides and Proteins/metabolism , Peptide Hydrolases/metabolism , Receptors, Fc/metabolism , Receptors, IgG/biosynthesis , Amino Acid Motifs/genetics , Animals , Antigens, CD/genetics , COP9 Signalosome Complex , Humans , Intracellular Signaling Peptides and Proteins/genetics , Intracellular Signaling Peptides and Proteins/immunology , Mice , NIH 3T3 Cells , Peptide Hydrolases/genetics , Peptide Hydrolases/immunology , Protein Binding/genetics , Protein Interaction Domains and Motifs/genetics , Protein Stability , RNA, Small Interfering/genetics , Receptors, Fc/genetics , Receptors, IgG/genetics , Transcriptional Activation/genetics , Transgenes/genetics , U937 CellsABSTRACT
The prototypic receptor for IgA (FcalphaRI, CD89) is expressed on myeloid cells and can trigger phagocytosis, tumor cell lysis, and release of inflammatory mediators. The functions of FcalphaRI and activating receptors for IgG (FcgammaRI and FcgammaRIII) are dependent on the FcR gamma-chain dimer. This study increases our understanding of the molecular basis of the FcalphaRI-FcR gamma-chain transmembrane interaction, which is distinct from that of other activatory FcRs. FcalphaRI is unique in its interaction with the common FcR gamma-chain, because it is based on a positively charged residue at position 209, 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 FcalphaRI for FcR gamma-chain association and FcalphaRI functioning with the use of site-directed mutagenesis. In an FcalphaRI 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 FcalphaRI. A lateral transfer of the positive charge, however, completely abrogated FcR gamma-chain-dependent functions in an FcalphaRI R209L/M210R mutant. By coimmunoprecipitation, we have demonstrated the loss of a physical interaction between FcR gamma-chain and FcalphaRI 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 FcalphaRI, but also the orientation of FcalphaRI toward the FcR gamma-chain dimer is essential for FcR gamma-chain association. This suggests the involvement of additional amino acids in the FcalphaRI-FcR gamma-chain interaction.
