Properdin stabilizes convertases formed upon activation of the complement cascade within the immune system. The biological activity of properdin depends on the oligomerization state, but whether properdin oligomers are rigid and how their structure links to function remains unknown. We show by combining electron microscopy and solution scattering, that properdin oligomers adopt extended rigid and well-defined conformations that are well approximated by single models of apparent n-fold rotational symmetry with dimensions of 230-360 Å. Properdin monomers are pretzel shaped molecules with limited flexibility. In solution, properdin dimers are curved molecules whereas trimers and tetramers are close to being planar molecules. Structural analysis indicates that simultaneous binding through all binding sites to surface linked convertases is unlikely for properdin trimer and tetramers. We show that multivalency alone is insufficient for full activity in a cell lysis assay. Hence, the observed rigid extended oligomer structure is an integral component of properdin function.

中文翻译：

---

Properdin低聚物采用刚性扩展构象支持功能

Properdin 稳定免疫系统内补体级联激活后形成的转化酶。备解素的生物活性取决于寡聚化状态，但备解素寡聚体是否刚性以及它们的结构如何与功能联系仍然未知。我们通过结合电子显微镜和溶液散射表明，备解素低聚物采用扩展的刚性和明确定义的构象，这些构象很好地近似于具有 230-360 Å 尺寸的表观 n 重旋转对称的单个模型。Properdin 单体是具有有限灵活性的椒盐卷饼状分子。在溶液中，备解素二聚体是弯曲分子，而三聚体和四聚体接近于平面分子。结构分析表明，备解素三聚体和四聚体不可能通过所有结合位点同时结合到表面连接的转化酶。我们表明，单独的多价不足以在细胞裂解测定中充分发挥作用。因此，观察到的刚性延伸低聚物结构是备解素功能的一个组成部分。