Human C-reactive protein (CRP) is a pentameric complex involved in immune defense and regulation of autoimmunity. CRP is also a therapeutic target, with both administration and depletion of serum CRP being pursued as a possible treatment for autoimmune and cardiovascular diseases, among others. CRP binds to phosphocholine (PC) moieties on membranes to activate the complement system via the C1 complex, but it is unknown how CRP, or any pentraxin, binds to C1. Here, we present a cryoelectron tomography (cryoET)-derived structure of CRP bound to PC ligands and the C1 complex. To gain control of CRP binding, a synthetic mimotope of PC was synthesized and used to decorate cell-mimetic liposome surfaces. Structure-guided mutagenesis of CRP yielded a fully active complex able to bind PC-coated liposomes that was ideal for cryoET and subtomogram averaging. In contrast to antibodies, which form Fc-mediated hexameric platforms to bind and activate the C1 complex, CRP formed rectangular platforms assembled from four laterally associated CRP pentamers that bind only four of the six available globular C1 head groups. Potential residues mediating lateral association of CRP were identified from interactions between unit cells in existing crystal structures, which rationalized previously unexplained mutagenesis data regarding CRP-mediated complement activation. The structure also enabled interpretation of existing biochemical data regarding interactions mediating C1 binding and identified additional residues for further mutagenesis studies. These structural data therefore provide a possible mechanism for regulation of complement by CRP, which limits complement progression and has consequences for how the innate immune system influences autoimmunity.

中文翻译：

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短五聚体蛋白 C 反应蛋白表面激活经典补体级联反应的结构基础


人 C 反应蛋白 （CRP） 是一种五聚体复合物，参与免疫防御和自身免疫调节。CRP 也是一种治疗靶点，血清 CRP 的给药和耗竭都被作为自身免疫和心血管疾病等的可能治疗方法。CRP 与膜上的磷酸胆碱 （PC） 部分结合，通过 C1 复合物激活补体系统，但尚不清楚 CRP 或任何五聚体蛋白如何与 C1 结合。在这里，我们提出了一种与 PC 配体和 C1 复合物结合的 CRP 的冷冻电子断层扫描 （cryoET） 衍生结构。为了获得对 CRP 结合的控制，合成了 PC 的合成模拟物并用于装饰细胞模拟脂质体表面。CRP 的结构引导诱变产生一种完全活性的复合物，能够结合 PC 包被的脂质体，这是冷冻 ET 和亚断层照片平均的理想选择。与形成 Fc 介导的六聚体平台以结合和激活 C1 复合物的抗体相比，CRP 形成由四个横向连接的 CRP 五聚体组装而成的矩形平台，这些五聚体仅结合六个可用球状 C1 头基中的四个。从现有晶体结构中晶胞之间的相互作用中确定了介导 CRP 横向结合的潜在残基，这合理化了以前无法解释的关于 CRP 介导的补体激活的诱变数据。该结构还能够解释有关介导 C1 结合的相互作用的现有生化数据，并确定了用于进一步诱变研究的其他残基。因此，这些结构数据为 CRP 调节补体提供了一种可能的机制，CRP 限制了补体进展并对先天免疫系统如何影响自身免疫产生了影响。