Perkins S J, Smith K F, Sim R B
Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London, U.K.
Biochem J. 1993 Oct 1;295 ( Pt 1)(Pt 1):101-8. doi: 10.1042/bj2950101.
Factor I is a typical multidomain protein of the complement system. It regulates complement activation by proteolytic degradation of C3b or C4b in the presence of factor H, complement receptor type 1, membrane cofactor protein or C4b-binding protein as cofactor. It is constructed from five presumed independently folded domains, namely a factor I module, a CD5-like domain, two low-density-lipoprotein receptor type A domains and a serine-proteinase domain. X-ray and neutron solution scattering was used to study the arrangement of these domains in factor I. Factor I was determined to be monomeric in solution, with an A280(1%,1cm) of 12.3-14.1. Its radius of gyration (RG) was 3.96 nm by X-rays in a high positive solute-solvent contrast, and 3.84 nm by neutrons at infinite solute-solvent contrast. The cross-sectional radius of gyration (RXS) was likewise found to be 1.64 nm by X-rays and 1.55 nm by neutrons. The RG data were not noticeably dependent on the solute-solvent contrast, whereas the RXS data showed a small dependence. The maximum dimension of factor I was determined to be 12.8 nm from the RG and RXS data, and 14-15 nm from the X-ray and neutron distance distribution functions. This length is too short to account for a linear arrangement of the domains in factor I. Small sphere models were developed for factor I in which the largest domain was modelled from the crystal structure for beta-trypsin. The attachment of either an elliptical cylinder or a two-armed V-shaped structure to this domain to represent the remaining four small domains gave good scattering curve-fits for factor I, and were compatible with experimental sedimentation coefficients. The non-extended domain models for factor I imply that the steric accessibility of each domain will be reduced, and this may be important for its functional activity.
因子I是补体系统中一种典型的多结构域蛋白。在因子H、1型补体受体、膜辅因子蛋白或C4b结合蛋白作为辅因子存在的情况下，它通过对C3b或C4b进行蛋白水解降解来调节补体激活。它由五个假定独立折叠的结构域构成，即一个因子I模块、一个CD5样结构域、两个A型低密度脂蛋白受体结构域和一个丝氨酸蛋白酶结构域。利用X射线和中子溶液散射来研究因子I中这些结构域的排列。因子I在溶液中被确定为单体，A280（1%，1cm）为12.3 - 14.1。在高正溶质 - 溶剂对比度下，通过X射线测得其回转半径（RG）为3.96 nm，在无限溶质 - 溶剂对比度下，通过中子测得为3.84 nm。同样通过X射线测得截面回转半径（RXS）为1.64 nm，通过中子测得为1.55 nm。RG数据对溶质 - 溶剂对比度没有明显依赖性，而RXS数据显示出较小的依赖性。根据RG和RXS数据确定因子I的最大尺寸为12.8 nm，根据X射线和中子距离分布函数确定为14 - 15 nm。这个长度太短，无法解释因子I中结构域的线性排列。针对因子I构建了小球体模型，其中最大的结构域根据β - 胰蛋白酶的晶体结构建模。将一个椭圆圆柱体或一个双臂V形结构连接到该结构域以代表其余四个小结构域，能很好地拟合因子I的散射曲线，并且与实验沉降系数相符。因子I的非伸展结构域模型表明每个结构域的空间可及性会降低，这可能对其功能活性很重要。
