Multicomponent immune receptors are essential complexes in which distinct ligand-recognition and signaling subunits are held together by interactions between acidic and basic residues of their transmembrane helices. A 2:1 acidic-to-basic motif in the transmembrane domains of the subunits is necessary and sufficient to assemble these receptor complexes. Here, we study a prototype for these receptors, a DAP12-NKG2C 2:1 heterotrimeric complex, in which the two DAP12 subunits each contribute a single transmembrane Asp residue, and the NKG2C subunit contributes a Lys to form the complex. DAP12 can also associate with 20 other subunits using a similar motif. Here, we use molecular-dynamics simulations to understand the basis for the high affinity and diversity of interactions in this group of receptors. Simulations of the transmembrane helices with differing protonation states of the Asp-Asp-Lys triad identified a structurally stable interaction in which a singly-protonated Asp-Asp pair forms a hydrogen-bonded carboxyl-carboxylate clamp that clasps onto a charged Lys side chain. This polar motif was also supported by density functional theory and a Protein Data Bank–wide search. In contrast, the helices are dynamic at sites distal to the stable carboxyl-carboxylate clamp motif. Such a locally stable but globally dynamic structure is well suited to accommodate the sequence and structural variations in the transmembrane helices of multicomponent receptors, which mix and match subunits to create combinatorial functional diversity from a limited number of subunits. It also supports a signaling mechanism based on multisubunit clustering rather than propagation of rigid conformational changes through the membrane.

多组分免疫受体是必不可少的复合物，其中不同的配体识别和信号亚基通过其跨膜螺旋的酸性和碱性残基之间的相互作用保持在一起。亚基跨膜域中酸性与碱性比例为 2:1 的基序对于组装这些受体复合物是必要且足够的。在这里，我们研究了这些受体的原型，即 DAP12-NKG2C 2:1 异源三聚体复合物，其中两个 DAP12 亚基各贡献一个跨膜天冬氨酸残基，而 NKG2C 亚基贡献一个赖氨酸以形成复合物。DAP12 还可以使用类似的主题与其他 20 个亚基相关联。在这里，我们使用分子动力学模拟来了解这组受体中相互作用的高亲和力和多样性的基础。对具有不同质子化状态的 Asp-Asp-Lys 三联体的跨膜螺旋的模拟确定了一种结构稳定的相互作用，其中单质子化的 Asp-Asp 对形成一个氢键的羧基-羧酸盐夹钳，夹在带电的 Lys 侧链上。这种极性基序也得到了密度泛函理论和蛋白质数据库范围搜索的支持。相比之下，螺旋在稳定的羧基-羧酸钳位基序远端的位点是动态的。这种局部稳定但全局动态的结构非常适合适应多组分受体跨膜螺旋中的序列和结构变异，这些受体混合和匹配亚基，以从有限数量的亚基中产生组合功能多样性。