Transport of the IgE receptor alpha-chain is controlled by a multicomponent intracellular retention signal

The human high affinity IgE receptor (FcepsilonRI) is a central component of the allergic response and is expressed as either a trimeric alphagamma2 or tetrameric alphabetagamma2 complex. It has been previously described that the cytoplasmic domain (CD) of the alpha-chain carries a dilysine motif at positions -3/-7 from the C terminus that functions in intracellular retention prior to assembly with other FcepsilonRI subunits. In this report we have further explored the role of the -3/-7 dilysine signal in controlling steady-state alpha-chain transport by mutational analysis and found little surface expression of a -3/-7 dialanine alpha-chain mutant but significant Golgi localization. We compared the transport properties of a series of alpha-chain cytoplasmic domain truncation mutants and observed that truncation mutants lacking 23 or more C-terminal residues showed a dramatic increase in steady-state transport suggesting a role for the membrane-proximal CD sequence in alpha-chain retention. By performing alanine-scanning mutagenesis we identified a dilysine sequence (Lys(212)-Lys(216)) proximal to the transmembrane domain (TMD) that is important for both alpha-chain cell-surface expression and intracellular stability. Furthermore, co-mutation of the Lys(212)-Lys(216) residues with the -3/-7 dilysine signal produced a dramatic increase in alpha-chain surface expression that was further increased by co-mutation of the lone charged residue (Asp(192)) in the TMD thereby defining three regions that function to regulate alpha-chain transport and in a highly synergistic manner.