Singleton Theresa E, Platzer Barbara, Dehlink Eleonora, Fiebiger Edda
Department of Medicine, Division of Gastroenterology and Nutrition, Children's Hospital Boston, 300 Longwood Avenue, EN720, Boston, MA 02115, USA.
Mol Immunol. 2009 Jul;46(11-12):2333-9. doi: 10.1016/j.molimm.2009.03.023. Epub 2009 Apr 29.
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, Fc epsilon RI, is a key regulator of immediate allergic responses. Tetrameric Fc epsilon RI consists of the IgE-binding alpha-chain, the multimembrane-spanning beta-chain and a dimer of the gamma-subunit (Fc epsilon R gamma). Comparative analysis of these seven transmembrane regions indicates that Fc epsilon RI 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, beta K97, is not involved in Fc epsilon RI stabilization or surface upregulation, a hallmark function of the beta-chain. Even a beta K97E 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 beta-chain contains the interface for receptor stabilization. In vitro translation experiments depict the first transmembrane region as the internal signal peptide of the beta-chain. We also show that this beta-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 Fc epsilon RI 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.
激活免疫受体家族通过三螺旋组装原理实现稳定。一个带正电荷的碱性跨膜残基与两个带负电荷的酸性跨膜残基相互作用，形成一个三螺旋界面，以稳定脂质双层中的受体复合物。该家族的一个成员，即IgE的高亲和力受体FcεRI，是速发型过敏反应的关键调节因子。四聚体FcεRI由IgE结合α链、多跨膜β链和γ亚基（FcεRγ）二聚体组成。对这七个跨膜区域的比较分析表明，FcεRI不符合三螺旋组装机制的电荷要求。我们进行了丙氨酸诱变，以表明跨膜区域中唯一的碱性氨基酸βK97不参与FcεRI的稳定或表面上调，而表面上调是β链的标志性功能。即使是βK97E突变体，尽管跨膜区域有四个带负电荷的酸性氨基酸，仍具有功能。使用截短突变体，我们证明β链的第一个不带电荷的跨膜结构域包含受体稳定的界面。体外翻译实验将第一个跨膜区域描绘为β链的内部信号肽。我们还表明，当用作I型蛋白的前导肽时，这个β链结构域可以作为可裂解的信号肽发挥作用。我们的结果提供了证据，证明四聚体FcεRI不是按照三螺旋组装原理组装的。我们得出结论，由多跨膜蛋白形成的受体使用不同的机制来屏蔽跨膜带电氨基酸。
