Department of Bioengineering, University of California, Riverside, United States.
Department of Bioengineering, University of California, Riverside, United States.
Mol Immunol. 2015 Mar;64(1):112-22. doi: 10.1016/j.molimm.2014.11.006. Epub 2014 Nov 27.
The C3d:CR2(SCR1-2) interaction plays an important role in bridging innate and adaptive immunity, leading to enhanced antibody production at sites of complement activation. Over the past decade, there has been much debate over the binding mode of this interaction. An initial cocrystal structure (PDB: 1GHQ) was published in 2001, in which the only interactions observed were between the SCR2 domain of CR2 and a side-face of C3d whereas a cocrystal structure (PDB: 3OED) published in 2011 showed both the SCR1 and SCR2 domains of CR2 interacting with an acidic patch on the concave surface of C3d. The initial 1GHQ structure is at odds with the majority of existing biochemical data and the publication of the 3OED structure renewed uncertainty regarding the physiological relevance of 1GHQ, suggesting that crystallization may have been influenced by the presence of zinc acetate in the crystallization process. In our study, we used a variety of computational approaches to gain insight into the binding mode between C3d and CR2 and demonstrate that the binding site at the acidic patch (3OED) is electrostatically more favorable, exhibits better structural and dissociative stability, specifically at the SCR1 domain, and has higher binding affinity than the 1GHQ binding mode. We also observe that nonphysiological zinc ions enhance the formation of the C3d:CR2 complex at the side face of C3d (1GHQ) through increases in electrostatic favorability, intermolecular interactions, dissociative character and overall energetic favorability. These results provide a theoretical basis for the association of C3d:CR2 at the acidic cavity of C3d and provide an explanation for binding of CR2 at the side face of C3d in the presence of nonphysiological zinc ions.
C3d:CR2(SCR1-2) 相互作用在桥接先天免疫和适应性免疫方面发挥着重要作用，导致补体激活部位的抗体产生增强。在过去的十年中，关于这种相互作用的结合模式存在很多争议。2001 年发表了最初的共晶结构（PDB:1GHQ），其中仅观察到 CR2 的 SCR2 结构域与 C3d 的侧面对相互作用，而 2011 年发表的共晶结构（PDB:3OED）显示 CR2 的 SCR1 和 SCR2 结构域均与 C3d 凹面的酸性斑相互作用。最初的 1GHQ 结构与大多数现有的生化数据不一致，3OED 结构的发表使人们对 1GHQ 的生理相关性产生了新的不确定性，表明结晶过程可能受到结晶过程中醋酸锌存在的影响。在我们的研究中，我们使用了多种计算方法来深入了解 C3d 和 CR2 之间的结合模式，并证明酸性斑（3OED）处的结合位点在静电上更有利，表现出更好的结构和离解稳定性，特别是在 SCR1 结构域，并且具有比 1GHQ 结合模式更高的结合亲和力。我们还观察到非生理锌离子通过增加静电有利度、分子间相互作用、离解特性和整体能量有利度来增强 C3d 侧面上的 C3d:CR2 复合物的形成（1GHQ）。这些结果为 C3d:CR2 在 C3d 酸性腔内的关联提供了理论基础，并解释了在存在非生理锌离子的情况下 CR2 在 C3d 侧面上的结合。
