The first transmembrane region of the β-chain stabilizes the tetrameric FcɛRI complex

The family of activating immune receptors stabilizes via the 3-helix assembly principle. A charged basic transmembrane residue interacts with two charged acidic transmembrane residues and forms a 3-helix interface to stabilize receptor complexes in the lipid bilayer. One family member, the high affinity receptor for IgE, FcɛRI, is a key regulator of immediate allergic responses. Tetrameric FcɛRI consists of the IgE-binding α-chain, the multimembrane-spanning β-chain and a dimer of the γ-subunit (FcɛRγ). Comparative analysis of these seven transmembrane regions indicates that FcɛRI does not meet the charge requirements for the 3-helix assembly mechanism. We performed alanine mutagenesis to show that the only basic amino acid in the transmembrane regions, βK97, is not involved in FcɛRI stabilization or surface upregulation, a hallmark function of the β-chain. Even a βK97E mutant is functional despite four negatively charged acidic amino acids in the transmembrane regions. Using truncation mutants, we demonstrate that the first uncharged transmembrane domain of the β-chain contains the interface for receptor stabilization. In vitro translation experiments depict the first transmembrane region as the internal signal peptide of the β-chain. We also show that this β-chain domain can function as a cleavable signal peptide when used as a leader peptide for a Type I protein. Our results provide evidence that tetrameric FcɛRI does not assemble according to the 3-helix assembly principle. We conclude that receptors formed with multispanning proteins use different mechanisms of shielding transmembrane charged amino acids.