Li Keying, Okemefuna Azubuike I, Gor Jayesh, Hannan Jonathan P, Asokan Rengasamy, Holers V Michael, Perkins Stephen J
Institute of Structural and Molecular Biology, Darwin Building, University College London, Gower Street, London WC1E6BT, UK.
J Mol Biol. 2008 Dec 5;384(1):137-50. doi: 10.1016/j.jmb.2008.08.084. Epub 2008 Sep 9.
Complement receptor type 2 (CR2, CD21) is a cell surface protein that links the innate and adaptive immune response during the activation of B-cells through its binding to C3d, a cleavage fragment of the major complement component C3. The extracellular portion of CR2 comprises 15 or 16 short complement regulator (SCR) domains in a partially folded-back but flexible structure. Here, the effect of C3d binding to CR2 was determined by analytical ultracentrifugation and X-ray scattering. The sedimentation coefficient of unbound CR2 is 4.03 S in 50 mM NaCl. Because this agrees well with a value of 3.93 S in 137 mM NaCl, the overall CR2 structure is unaffected by change in ionic strength. Unbound C3d exists in monomer-dimer and monomer-trimer equilibria in 50 mM NaCl, but as a monomer only in 137 mM NaCl. In c(s) size-distribution analyses, an equimolar mixture of the CR2-C3d complex in 50 mM NaCl revealed a single peak shifted to 4.52 S when compared to unbound CR2 at 4.03 S to show that the complex had formed. The CR2-C3d complex in 137 mM NaCl showed two peaks at 2.52 S and 4.07 S to show that this had dissociated. Solution structural models for the CR2 SCR-1/2 complex with C3d and CR2 SCR-1/15 were superimposed. These gave an average sedimentation coefficient of 4.57 S for the complex, in good agreement with the observed value of 4.52 S. It is concluded that CR2 does not detectably change conformation when C3d is bound to it. Consistent with previous analyses, its C3d complex is not formed in physiological salt conditions. The implications of these solution results for its immune role are discussed. To our knowledge, this is the first solution structural study of a large multidomain SCR protein CR2 bound to its physiological ligand C3d.
补体受体2（CR2，CD21）是一种细胞表面蛋白，在B细胞激活过程中，通过与主要补体成分C3的裂解片段C3d结合，连接先天性免疫应答和适应性免疫应答。CR2的细胞外部分由15或16个短补体调节蛋白（SCR）结构域组成，呈部分回折但灵活的结构。在此，通过分析超速离心和X射线散射确定了C3d与CR2结合的效果。在50 mM NaCl中，未结合的CR2的沉降系数为4.03 S。由于这与在137 mM NaCl中的3.93 S值非常吻合，所以CR2的整体结构不受离子强度变化的影响。在50 mM NaCl中，未结合的C3d以单体 - 二聚体和单体 - 三聚体平衡存在，但在137 mM NaCl中仅以单体形式存在。在c(s)尺寸分布分析中，50 mM NaCl中CR2 - C3d复合物的等摩尔混合物显示，与4.03 S的未结合CR2相比，单个峰移至4.52 S，表明复合物已形成。137 mM NaCl中的CR2 - C3d复合物在2.52 S和4.07 S处显示两个峰，表明其已解离。将CR2 SCR - 1/2与C3d复合物和CR2 SCR - 1/15的溶液结构模型进行叠加。这些给出了复合物平均沉降系数为4.57 S，与观察到的4.52 S值非常吻合。结论是当C3d与CR2结合时，CR2的构象没有明显变化。与先前的分析一致，其C3d复合物在生理盐条件下不会形成。讨论了这些溶液结果对其免疫作用的影响。据我们所知，这是首次对与生理配体C3d结合的大型多结构域SCR蛋白CR2进行溶液结构研究。
