Crystal Structure Analysis of Spherical Viruses

TNV亚基由275个氨基酸组成。TNV晶体属于立方空间群P4_232，胞元尺寸为a=b=c=338*。对于含有两个分子量约为6.7x10^6的病毒颗粒的3.86x10^7*^3的单位细胞，Vm值为2.9*^3/道尔顿。在100* ~ 5*分辨率范围内，观测到的f> 3sigma (F)反射达到理论反射数的83%。以5*分辨率构建最终图的电子密度笼。c -亚基n端臂（β - aa臂）的23个残基呈现有序结构。它比SBMV短3个残基。亚基A和B对应的主链段与SBMV一样紊乱。在A-和b -亚基对应的电子密度笼上进行伪3倍旋转后，c -亚基模型得到了很好的叠加，除了残基66 ~ 88的β - aa臂。主结构间隙周围的主结构在电子密度笼上叠加良好。二级结构按照SBMV的命名法依次命名为β aa、β ab、β ac、α a、β ad、β ae、α b、β af、α c、β ag、α had、α e、β ah和β ai。A亚基和B亚基在二十面体表面各有186个有序残基，C亚基在β aa处各有24个有序残基。TNV亚基与SBMV亚基的三级结构比较表明，TNV亚基在α c和β ag之间的环区有2个间隙，在α had和α e的连续片段中有5个间隙，在β ah和β ai之间的环区有2个间隙。α - c和β - ag之间的间隙在病毒衣壳内部，其他间隙在病毒颗粒的表面。TNV的C亚基有序β - a臂比SBMV短3个残基。

The TNV subunit is composed of 275 amino acids. TNV crystals belong to the cubic space group P4_232 with the cell dimensions, a=b=c=338*. For the unit cell of 3.86x10^7*^3, containing two virus particles with a molecular weight of about 6.7x10^6, the Vm value is 2.9*^3/dalton. The observed reflections with F>3sigma (F) amounted to 83% of the theoretical number in the 100* to 5* resolution range.Electron density cages of the final map were constructed at 5* resolution. Twenty-three residues of an N-terminal arm (betaA arm) of the C-subunit exhibits ordered structure. It is shorter than that of SBMV by three residues. The corresponding main chain segments of the subunits A and B were disordered as those in SBMV.The C-subunit model was superposed well after pseudo 3-fold rotations on electron density cages corresponding to the A- and B-subunits except for the betaA arm from residues 66 to 88. Backbone structures around gaps in primary structure are superposed well on the electron density cages.Secondary structures were named betaA,betaB,betaC,alphaA,betaD,betaE,alphaB,betaF,alphaC,betaG,alphaD,alphaE,betaH and betaI along the polypeptide chain, following the nomenclature for SBMV.Each of A and B subunits has 186 ordered residues in the icosahedral surface, and C subunit has 24 additional ordered residues at betaA.Tertiary structural comparison between TNV and SBMV subunits shows that TNV subunits have two gaps in a loop region between alphaC and betaG,five gaps in continuous segments of alphaD and alphaE,and two gaps in a loop between betaH and betaI.The gaps between alphaC and betaG are inside the virus capsid, and the other gaps are on the surface of the virus particle. The ordered betaA arm of subunit C of TNV is shorter than that of SBMV by three residues.