CD28 has a crucial role in regulating immune responses by enhancing T cell activation and differentiation. Recent studies have shown that the transmembrane helix (TMH) of CD28 mediates receptor assembly and activity, but a structural characterization of TMH is still lacking. Here, we determined the dimeric helix-helix packing of CD28-TMH using nuclear magnetic resonance (NMR) technology. Unexpectedly, wild-type CD28-TMH alone forms stable tetramers in lipid bicelles instead of dimers. The NMR structure of the CD28-TMH C165F mutant reveals that a GxxxA motif, which is highly conserved in many dimeric assemblies, is located at the dimerization interface. Mutating G160 and A164 can disrupt the transmembrane helix assembly and reduces CD28 enhancement in cells. In contrast, a previously proposed YxxxxT motif does not affect the dimerization of full-length CD28, but it does affect CD28 activity. These results imply that the transmembrane domain of CD28 regulates the signaling transduction in a complicated manner.

CD28 通过增强 T 细胞活化和分化在调节免疫反应中起关键作用。最近的研究表明，CD28 的跨膜螺旋 (TMH) 介导受体组装和活性，但仍然缺乏 TMH 的结构表征。在这里，我们使用核磁共振 (NMR) 技术确定了 CD28-TMH 的二聚螺旋-螺旋堆积。出乎意料的是，单独的野生型 CD28-TMH 在脂质双胞胎中形成稳定的四聚体，而不是二聚体。CD28-TMH C165F 突变体的 NMR 结构表明，在许多二聚体组装中高度保守的 GxxxA 基序位于二聚化界面。突变 G160 和 A164 可以破坏跨膜螺旋组装并减少细胞中 CD28 的增强。相比之下，先前提出的 YxxxxT 基序不影响全长 CD28 的二聚化，但它确实影响 CD28 活性。这些结果表明CD28的跨膜结构域以复杂的方式调节信号转导。