Smith K F, Harrison R A, Perkins S J
Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London.
Biochemistry. 1992 Jan 28;31(3):754-64. doi: 10.1021/bi00118a017.
C9 is the most abundant component of the membrane attack complex of the complement system of immune defense. This is a typical mosaic protein with thrombospondin (TSR) and low density lipoprotein receptor (LDLr) domains at its N-terminus and an epidermal growth factor-like (EGF) domain at its C-terminus. Between these lies a perforin-like sequence. In order to define the arrangement in solution of these four moieties in C9, high-flux neutron and synchrotron X-ray solution scattering studies were carried out. The neutron radius of gyration RG at infinite contrast is 3.33 nm, and its cross-sectional RG (RXS) is 1.66 nm. Similar values were obtained by synchrotron X-ray scattering after allowance for radiation effects. Stuhrmann analyses showed that the neutron radial inhomogeneity of scattering density alpha is 35 X 10(-5) from the RG data and 16 X 10(-5) from the RXS data. These values are typical for soluble glycoproteins and show no evidence for the existence of any large hydrophobic surface patches on free C9 that might form contacts with lipids. Indirect transformation of the neutron and X-ray scattering curves into real space showed that C9 had a maximum dimension estimated at 12 +/- 2 nm, and this suggests that the lengths of 7-8 nm deduced from previous electron microscopy studies in vacuo are underestimated. Molecular modeling of the C9 scattering curves utilized small spheres in the Debye equation, in which the analyses were constrained by the known volumes of the four moieties of C9 and the known sizes of the TSR and EGF-like domains. The most likely models for C9 suggest that these four regions of C9 are arranged in a V-shaped structure, with an angle of 10 degrees between the two arms, each of length 11.1 nm. This structure has a more hydrophobic character between the two arms. The scattering model is fully consistent with hydrodynamic sedimentation data on C9. Similar V-shaped hydrodynamic models could be developed for C6, C7, C8, and C9 of complement. Such a compact structure is atypical of other multidomain complement proteins so far studied by solution scattering and is fully compatible with mechanisms in which C9 is postulated, on activation, to undergo a drastic unfolding of its domain structure and to expose a more hydrophobic surface which can be embedded into lipid bilayers.
C9是免疫防御补体系统膜攻击复合物中含量最丰富的成分。这是一种典型的镶嵌蛋白，其N端有血小板反应蛋白（TSR）和低密度脂蛋白受体（LDLr）结构域，C端有一个表皮生长因子样（EGF）结构域。在这两者之间有一个穿孔素样序列。为了确定C9中这四个部分在溶液中的排列方式，进行了高通量中子和同步加速器X射线溶液散射研究。无限对比度下的中子回转半径RG为3.33纳米，其截面回转半径（RXS）为1.66纳米。考虑辐射效应后，同步加速器X射线散射得到了类似的值。施图尔曼分析表明，根据RG数据，散射密度α的中子径向不均匀性为35×10⁻⁵，根据RXS数据为16×10⁻⁵。这些值是可溶性糖蛋白的典型值，没有证据表明游离C9上存在任何可能与脂质形成接触的大的疏水表面斑块。将中子和X射线散射曲线间接转换到实空间表明，C9的最大尺寸估计为12±2纳米，这表明先前在真空中进行的电子显微镜研究推断的7 - 8纳米长度被低估了。利用德拜方程中的小球对C9散射曲线进行分子建模，其中分析受到C9四个部分的已知体积以及TSR和EGF样结构域的已知大小的限制。C9最可能的模型表明，C9的这四个区域呈V形结构排列，两条臂之间的夹角为10度，每条臂长11.1纳米。这种结构在两条臂之间具有更强的疏水性。散射模型与C9的流体动力学沉降数据完全一致。可以为补体的C6、C7、C8和C9开发类似的V形流体动力学模型。这种紧凑的结构对于迄今为止通过溶液散射研究的其他多结构域补体蛋白来说是非典型的，并且与假设C9在激活时经历其结构域结构的剧烈展开并暴露一个可以嵌入脂质双层的更疏水表面的机制完全兼容。
