Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Strasse 9, 60438 Frankfurt/Main, Germany.
Department of Structural Biology, Max Planck Institute of Biophysics, Max-von-Laue Strasse 3, 60438 Frankfurt/Main, Germany.
Nature. 2017 Nov 23;551(7681):525-528. doi: 10.1038/nature24627. Epub 2017 Nov 6.
The peptide-loading complex (PLC) is a transient, multisubunit membrane complex in the endoplasmic reticulum that is essential for establishing a hierarchical immune response. The PLC coordinates peptide translocation into the endoplasmic reticulum with loading and editing of major histocompatibility complex class I (MHC-I) molecules. After final proofreading in the PLC, stable peptide-MHC-I complexes are released to the cell surface to evoke a T-cell response against infected or malignant cells. Sampling of different MHC-I allomorphs requires the precise coordination of seven different subunits in a single macromolecular assembly, including the transporter associated with antigen processing (TAP1 and TAP2, jointly referred to as TAP), the oxidoreductase ERp57, the MHC-I heterodimer, and the chaperones tapasin and calreticulin. The molecular organization of and mechanistic events that take place in the PLC are unknown owing to the heterogeneous composition and intrinsically dynamic nature of the complex. Here, we isolate human PLC from Burkitt's lymphoma cells using an engineered viral inhibitor as bait and determine the structure of native PLC by electron cryo-microscopy. Two endoplasmic reticulum-resident editing modules composed of tapasin, calreticulin, ERp57, and MHC-I are centred around TAP in a pseudo-symmetric orientation. A multivalent chaperone network within and across the editing modules establishes the proofreading function at two lateral binding platforms for MHC-I molecules. The lectin-like domain of calreticulin senses the MHC-I glycan, whereas the P domain reaches over the MHC-I peptide-binding pocket towards ERp57. This arrangement allows tapasin to facilitate peptide editing by clamping MHC-I. The translocation pathway of TAP opens out into a large endoplasmic reticulum lumenal cavity, confined by the membrane entry points of tapasin and MHC-I. Two lateral windows channel the antigenic peptides to MHC-I. Structures of PLC captured at distinct assembly states provide mechanistic insight into the recruitment and release of MHC-I. Our work defines the molecular symbiosis of an ABC transporter and an endoplasmic reticulum chaperone network in MHC-I assembly and provides insight into the onset of the adaptive immune response.
肽加载复合物（PLC）是内质网中一种短暂的多亚基膜复合物，对于建立分层免疫反应至关重要。PLC 协调将肽转运到内质网中，同时加载和编辑主要组织相容性复合体 I 类（MHC-I）分子。在 PLC 中进行最终校对后，稳定的肽-MHC-I 复合物被释放到细胞表面，以引发针对感染或恶性细胞的 T 细胞反应。不同 MHC-I 同种型的采样需要在单个大分子组装中精确协调七个不同的亚基，包括抗原加工相关转运蛋白（TAP1 和 TAP2，统称 TAP）、氧化还原酶 ERp57、MHC-I 异二聚体以及伴侣蛋白 tapasin 和钙网蛋白。由于复合物的异质组成和固有动态性质，PLC 的分子组织和发生的机制事件尚不清楚。在这里，我们使用工程病毒抑制剂作为诱饵从 Burkitt 淋巴瘤细胞中分离出人 PLC，并通过电子冷冻显微镜确定天然 PLC 的结构。两个内质网驻留的编辑模块由 tapasin、钙网蛋白、ERp57 和 MHC-I 组成，围绕 TAP 呈伪对称取向。编辑模块内和跨编辑模块的多价伴侣蛋白网络在 MHC-I 分子的两个侧向结合平台上建立了校对功能。钙网蛋白的凝集素样结构域感知 MHC-I 聚糖，而 P 结构域伸向 MHC-I 肽结合口袋朝向 ERp57。这种排列允许 tapasin 通过夹住 MHC-I 来促进肽编辑。TAP 的转运途径打开到大的内质网腔室中，由 tapasin 和 MHC-I 的膜进入点限制。两个侧向窗口将抗原肽引导至 MHC-I。在不同组装状态下捕获的 PLC 结构提供了 MHC-I 组装中 ABC 转运蛋白和内质网伴侣蛋白网络募集和释放的机制见解。我们的工作定义了 ABC 转运蛋白和内质网伴侣蛋白网络在 MHC-I 组装中的分子共生，并深入了解适应性免疫反应的开始。
