Chamberlain D, Ullman C G, Perkins S J
Department of Biochemistry and Molecular Biology, Royal Free and University College Medical School of University College London, UK.
Biochemistry. 1998 Oct 6;37(40):13918-29. doi: 10.1021/bi9805184.
Human factor I is a multidomain plasma serine protease with one factor I-membrane attack complex (FIMAC) domain, one CD5 domain, two low-density lipoprotein receptor (LDLr) domains, and one serine protease (SP) domain and is essential for the regulation of complement. The domain arrangement in factor I was determined by X-ray and neutron scattering on serum-derived human factor I (sFI) and recombinant insect cell factor I (rFI). While the radii of gyration of both were the same at 4.05 nm and both had overall lengths of 14 nm, the cross-sectional radii of gyration were different at 1.70 nm for sFI and 1.57 nm for rFI. This difference was attributed to their different means of glycosylation which is complex-type for sFI and high-mannose-type for rFI. Homology models were constructed for the FIMAC, LDLr, and SP domains of factor I using related crystal structures, and CD5 was represented as a globular protein by referencing its electron microscopy dimensions. In these models, 38 of the 40 Cys residues in factor I were predicted to form internal disulfide bridges. The two remaining Cys residues at the N terminus of the FIMAC domain and at the center of the first LDLr domain were potentially not bridged. It was postulated that, if these two Cys residues were bridged to each other, the FIMAC, CD5, and LDLr-1 domains would form a compact triangular arrangement. This hypothesis was tested by automated scattering curve fit searches based on 9600 bilobal models, setting the FIMAC, CD5, and LDLr-1 domains as one lobe and the large SP domain as the other lobe. The searches gave a single small family of similar structures with a separation of 5.9 nm between the centers of the lobes which gave similar good X-ray and neutron fits for both sFI and rFI, despite the different glycosylations of sFI and rFI. These best-fit structures for factor I showed that this domain model is plausible, and suggested that the SP and the CD5 and LDLr-1 domains may present exposed surfaces in factor I whose roles are to interact separately with its substrates C3b and C4b and with cofactor proteins.
人因子I是一种多结构域血浆丝氨酸蛋白酶，具有一个因子I-膜攻击复合物（FIMAC）结构域、一个CD5结构域、两个低密度脂蛋白受体（LDLr）结构域和一个丝氨酸蛋白酶（SP）结构域，对补体调节至关重要。通过对血清来源的人因子I（sFI）和重组昆虫细胞因子I（rFI）进行X射线和中子散射，确定了因子I中的结构域排列。虽然两者的回转半径均为4.05nm，总长度均为14nm，但sFI的截面回转半径为1.70nm，rFI的截面回转半径为1.57nm。这种差异归因于它们不同的糖基化方式，sFI为复合型糖基化，rFI为高甘露糖型糖基化。利用相关晶体结构构建了因子I的FIMAC、LDLr和SP结构域的同源模型，并通过参考其电子显微镜尺寸将CD5表示为球状蛋白。在这些模型中，因子I中40个半胱氨酸残基中的38个预计形成内部二硫键。FIMAC结构域N端和第一个LDLr结构域中心的两个剩余半胱氨酸残基可能未形成二硫键。据推测，如果这两个半胱氨酸残基相互形成二硫键，FIMAC、CD5和LDLr-1结构域将形成紧密的三角形排列。通过基于9600个双叶模型的自动散射曲线拟合搜索对这一假设进行了检验，将FIMAC、CD5和LDLr-1结构域设为一个叶，将大的SP结构域设为另一个叶。搜索得到了一个单一的小家族相似结构，叶中心之间的间距为5.9nm，尽管sFI和rFI的糖基化不同，但对sFI和rFI的X射线和中子拟合效果都很好。这些因子I的最佳拟合结构表明该结构域模型是合理的，并表明SP以及CD5和LDLr-1结构域可能在因子I中呈现出暴露的表面，其作用是分别与底物C3b和C4b以及辅因子蛋白相互作用。
