Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
Department of Biology, The Catholic University of America, Washington, DC 20064.
Proc Natl Acad Sci U S A. 2017 Sep 26;114(39):E8184-E8193. doi: 10.1073/pnas.1708483114. Epub 2017 Sep 11.
The 3.3-Å cryo-EM structure of the 860-Å-diameter isometric mutant bacteriophage T4 capsid has been determined. WT T4 has a prolate capsid characterized by triangulation numbers (T numbers) T = 13 for end caps and T = 20 for midsection. A mutation in the major capsid protein, gp23, produced T=13 icosahedral capsids. The capsid is stabilized by 660 copies of the outer capsid protein, Soc, which clamp adjacent gp23 hexamers. The occupancies of Soc molecules are proportional to the size of the angle between the planes of adjacent hexameric capsomers. The angle between adjacent hexameric capsomers is greatest around the fivefold vertices, where there is the largest deviation from a planar hexagonal array. Thus, the Soc molecules reinforce the structure where there is the greatest strain in the gp23 hexagonal lattice. Mutations that change the angles between adjacent capsomers affect the positions of the pentameric vertices, resulting in different triangulation numbers in bacteriophage T4. The analysis of the T4 mutant head assembly gives guidance to how other icosahedral viruses reproducibly assemble into capsids with a predetermined T number, although the influence of scaffolding proteins is also important.
已确定直径为 860Å 的等轴突变体噬菌体 T4 衣壳的 3.3Å 冷冻电镜结构。WT T4 具有拉长的衣壳，其特征在于端盖的三角数（T 数）T=13，中段的 T=20。主要衣壳蛋白 gp23 中的突变产生了 T=13 的二十面体衣壳。660 个外壳蛋白 Soc 稳定了衣壳，Soc 分子夹在相邻的 gp23 六聚体上。Soc 分子的占有率与相邻六聚体衣壳之间平面之间的角度大小成正比。相邻六聚体衣壳之间的角度在五重顶点处最大，这里与平面六边形排列的偏差最大。因此，Soc 分子加强了 gp23 六边形晶格中应变最大的结构。改变相邻衣壳之间角度的突变会影响五重顶点的位置，从而导致噬菌体 T4 中的三角数不同。对 T4 突变体头部组装的分析为其他二十面体病毒如何可重复地组装成具有预定 T 数的衣壳提供了指导，尽管支架蛋白的影响也很重要。
