Beavil A J, Young R J, Sutton B J, Perkins S J
Randall Institute, King's College London, U.K.
Biochemistry. 1995 Nov 7;34(44):14449-61. doi: 10.1021/bi00044a023.
Human immunoglobulin E (IgE) consists of 14 domains, each with the characteristic immunoglobulin fold structure. Compared with the 12-domain structure of immunoglobulin G (IgG), IgE has an additional pair of domains (C epsilon 2) in the Fc region in place of the hinge of IgG. The crystal structure of the 4-domain Fc fragment of IgG is known, but not that of the 6-domain Fc fragment of IgE (IgE-Fc). In order to elucidate the position of the C epsilon 2 domains in the domain structure of IgE-Fc, IgE-Fc was studied by synchrotron X-ray and pulsed neutron scattering. The upper limit on the X-ray radius of gyration RG which determines macromolecular elongation was determined to be 3.52 +/- 0.14 nm. That for the neutron RG (measured in 100% 2H2O buffers) was 3.53 +/- 0.05 nm. The X-ray and neutron cross-sectional radii of gyration were 1.89 +/- 0.05 and 1.56 +/- 0.09 nm, respectively. The scattering curves were modeled on the basis of a previously-predicted model for IgE-Fc (Helm, B. A., Ling, Y., Teale, C., Padlan, E. A., & Brüggemann, M. (1991) Eur. J. Immunol. 21, 1543-1548). The extended arrangement of domains in that model resulted in poor agreement with experimental data. Interactive and automated procedures for the fitting of crystallographically-derived domain models to scattering data were developed. Each pair of C epsilon 2, C epsilon 3, and C epsilon 4 domains was translated and rotated relative to the remaining structure in a comprehensive five-parameter search of more than 37,000 models. Substantially improved agreement between the experimental and calculated scattering curves was obtained. Bent models for IgE-Fc in which the C epsilon 2 domain pair is rotated by at least 40-50 degrees from its position in the previously predicted linear IgE model consistently gave the best agreement with the X-ray and neutron scattering curves. Such a structure for the Fc fragment accounts in part for the bent structure previously proposed for intact human IgE, which is important for understanding the interaction between IgE and its receptors.
人免疫球蛋白E（IgE）由14个结构域组成，每个结构域都具有特征性的免疫球蛋白折叠结构。与免疫球蛋白G（IgG）的12结构域结构相比，IgE在Fc区域有额外的一对结构域（Cε2），取代了IgG的铰链区。IgG的4结构域Fc片段的晶体结构是已知的，但IgE的6结构域Fc片段（IgE-Fc）的晶体结构尚不清楚。为了阐明Cε2结构域在IgE-Fc结构域结构中的位置，利用同步辐射X射线和脉冲中子散射对IgE-Fc进行了研究。确定了决定大分子伸长率的X射线回转半径RG的上限为3.52±0.14nm。在100% 2H2O缓冲液中测得的中子RG为3.53±0.05nm。X射线和中子的截面回转半径分别为1.89±0.05和1.56±0.09nm。散射曲线是基于先前预测的IgE-Fc模型（Helm, B. A., Ling, Y., Teale, C., Padlan, E. A., & Brüggemann, M. (1991) Eur. J. Immunol. 21, 1543-1548）建立的。该模型中结构域的延伸排列与实验数据的吻合度较差。开发了用于将晶体学衍生的结构域模型拟合到散射数据的交互式和自动化程序。在对超过37000个模型进行的全面五参数搜索中，每对Cε2、Cε3和Cε4结构域相对于其余结构进行平移和旋转。实验散射曲线和计算散射曲线之间的吻合度得到了显著改善。在弯曲的IgE-Fc模型中，Cε2结构域对相对于先前预测的线性IgE模型中的位置旋转了至少40-50度，始终与X射线和中子散射曲线吻合得最好。Fc片段的这种结构部分解释了先前提出的完整人IgE的弯曲结构，这对于理解IgE与其受体之间的相互作用很重要。
