The human high affinity IgE receptor (Fc⑀RI) is a central component of the allergic response and is expressed as either a trimeric ␣␥2 or tetrameric ␣␥2 complex. It has been previously described that the cytoplasmic domain (CD) of the ␣-chain carries a dilysine motif at positions ؊3/؊7 from the C terminus that functions in intracellular retention prior to assembly with other Fc⑀RI subunits. In this report we have further explored the role of the ؊3/؊7 dilysine signal in controlling steady-state ␣-chain transport by mutational analysis and found little surface expression of a ؊3/؊7 dialanine ␣-chain mutant but significant Golgi localization. We compared the transport properties of a series of ␣-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 ␣-chain retention. By performing alanine-scanning mutagenesis we identified a dilysine sequence (Lys The high affinity IgE receptor (Fc⑀RI) 2 is a multisubunit complex comprised of either a tetrameric ␣␥2 complex or an alternative trimeric ␣␥2 isoform (1, 2). Aggregation of the tetrameric receptor on mast cells and basophils occurs upon cross-linking of receptor-bound IgE with a multivalent antigen that initiates Fc⑀RI-dependent signaling, culminating in cellular degranulation and the ensuing clinical symptoms of hypersensitivity. Expression of the trimeric Fc⑀RI isoform occurs on a number of cells, including monocytes, epidermal Langerhans cells, and dendritic cells, and a role for the ␣␥2 receptor in antigen presentation has now been established (3). The Fc⑀RI ␣-chain is exclusively involved in IgE binding, whereas the associated ␥-and -chains function in signaling by the presence of intracellular tyrosine activation motifs. Assembly with the -chain also functions to amplify Fc⑀RI cell-surface expression relative to the ␣␥2 isoform (4, 5) and represents a fundamental mechanism for the regulation of Fc⑀RI cell-surface expression. It has long been known that binding of IgE to cell-surface Fc⑀RI leads to enhanced receptor expression (6), an effect that has now been amply demonstrated for both receptor isoforms (6 -9) and is thought to involve receptor-ligand surface stabilization and possibly increased export of ␣-chain from an intracellular pool (10, 11). Additional mechanisms that regulate Fc⑀RI expression have also been identified, including the effects of interleukin-4 in mast cells (7, 12) and transforming growth factor-1 in both dendritic cells (13) and mast cells (14). Because the sensitivity of the allergic response is critically linked to Fc⑀RI surface expression (15) that in turn is dependent on Fc⑀RI subunit transport characteristics (16) and assembly (17), it remains an important goal to fully elucidate the Fc⑀RI structural features that regulate these processes to thereby provide a better understanding of the molecular origins of the allergic response.The underlying factors th...
