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，晶胞尺寸a=b=c=338*。对于3.86×10^7*^3的单位细胞，含有两个分子量约为6.7×10^6的病毒颗粒，其Vm值为2.9*^3/Dalton。在100*到5*分辨率范围内，用F&GT；3sigma(F)观测到的反射达到理论值的83%，并在5*分辨率下构建了最终地图的电子密度笼。C-亚基N-端臂(βA臂)的23个残基呈现有序结构。比SBMV短3个残基。A和B亚基对应的主链段与SBMV中的主链段相同，C-亚基模型在与A-和B-亚基对应的电子密度笼上经过伪3倍旋转后很好地重叠在与A-和B-亚基对应的电子密度笼上，除了66到88个残基中的βA臂。二级结构被命名为βA、BETAB、Betac、αA、BetaD、BetaE、AlphaB、BetaF、AlphaC、BetaG、AlphaD、AlphaE、BETAH和BetaI，沿着SBMV的命名，A和B亚基的每个亚基在二十面体表面有186个有序残基，C亚基在BetaA有24个额外的有序残基。TnV和SBMV亚基的三级结构比较表明，TNV亚基在αC和βG之间的环区有2个间隙，在αD和αE的连续片段中有5个间隙在BETAH和BetaI之间的环路中有两个间隙。AlphaC和BetaG之间的间隙在病毒衣壳内，其他间隙在病毒颗粒的表面。TnV C亚基的有序βA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.