Perforin-2 (PFN2, MPEG1) is a key pore-forming protein in mammalian innate immunity restricting intracellular bacteria proliferation. It forms a membrane-bound pre-pore complex that converts to a pore-forming structure upon acidification; but its mechanism of conformational transition has been debated. Here we used cryo-electron microscopy, tomography and subtomogram averaging to determine structures of PFN2 in pre-pore and pore conformations in isolation and bound to liposomes. In isolation and upon acidification, the pre-assembled complete pre-pore rings convert to pores in both flat ring and twisted conformations. On membranes, in situ assembled PFN2 pre-pores display various degrees of completeness; whereas PFN2 pores are mainly incomplete arc structures that follow the same subunit packing arrangements as found in isolation. Both assemblies on membranes use their P2 β-hairpin for binding to the lipid membrane surface. Overall, these structural snapshots suggest a molecular mechanism for PFN2 pre-pore to pore transition on a targeted membrane, potentially using the twisted pore as an intermediate or alternative state to the flat conformation, with the capacity to cause bilayer distortion during membrane insertion.

中文翻译：

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分离并组装在膜上的穿孔素 2 的冷冻电镜结构提示了孔形成的机制


Perforin-2（PFN2、MPEG1）是哺乳动物先天免疫中限制细胞内细菌增殖的关键成孔蛋白。它形成膜结合的预孔复合物，在酸化时转化为成孔结构；但其构象转变机制一直存在争议。在这里，我们使用冷冻电子显微镜、断层扫描和亚断层图平均来确定分离的和与脂质体结合的前孔和孔构象中的 PFN2 的结构。在分离和酸化后，预组装的完整预孔环转化为扁平环和扭曲构象的孔。在膜上，原位组装的 PFN2 预孔表现出不同程度的完整性；而PFN2孔主要是不完整的弧形结构，遵循与隔离中发现的相同的亚基堆积排列。膜上的两个组件都使用其 P2 β-发夹与脂质膜表面结合。总体而言，这些结构快照表明了 PFN2 在目标膜上从前孔到孔转变的分子机制，可能使用扭曲孔作为平坦构象的中间或替代状态，能够在膜插入过程中引起双层扭曲。