Several interleukin-1 (IL-1) family members, including IL-1β and IL-18, require processing by inflammasome-associated caspases to unleash their activities. Here, we unveil, by cryoelectron microscopy (cryo-EM), two major conformations of the complex between caspase-1 and pro-IL-18. One conformation is similar to the complex of caspase-4 and pro-IL-18, with interactions at both the active site and an exosite (closed conformation), and the other only contains interactions at the active site (open conformation). Thus, pro-IL-18 recruitment and processing by caspase-1 is less dependent on the exosite than the active site, unlike caspase-4. Structure determination by nuclear magnetic resonance uncovers a compact fold of apo pro-IL-18, which is similar to caspase-1-bound pro-IL-18 but distinct from cleaved IL-18. Binding sites for IL-18 receptor and IL-18 binding protein are only formed upon conformational changes after pro-IL-18 cleavage. These studies show how pro-IL-18 is selected as a caspase-1 substrate, and why cleavage is necessary for its inflammatory activity.

一些白细胞介素-1 (IL-1) 家族成员，包括 IL-1β 和 IL-18，需要通过炎性体相关的半胱天冬酶进行处理才能释放其活性。在这里，我们通过冷冻电子显微镜 (cryo-EM) 揭示了 caspase-1 和 pro-IL-18 之间复合物的两种主要构象。一种构象类似于 caspase-4 和 pro-IL-18 的复合物，在活性位点和外部位点（闭合构象）都有相互作用，另一种构象仅包含在活性位点（开放构象）的相互作用。因此，与 caspase-4 不同，caspase-1 募集和加工的 pro-IL-18 对外部位点的依赖程度低于对活性位点的依赖程度。核磁共振结构测定揭示了 apo pro-IL-18 的紧凑折叠，其与 caspase-1 结合的 pro-IL-18 相似，但与裂解的 IL-18 不同。 IL-18 受体和 IL-18 结合蛋白的结合位点仅在 IL-18 前体裂解后构象发生变化时形成。这些研究展示了如何选择 pro-IL-18 作为 caspase-1 底物，以及为什么裂解对其炎症活性是必要的。