Dokland T, Bernal R A, Burch A, Pletnev S, Fane B A, Rossmann M G
Department of Biological Sciences, Purdue University, West Lafayette, IN, 47907-1392, USA.
J Mol Biol. 1999 May 14;288(4):595-608. doi: 10.1006/jmbi.1999.2699.
An empty precursor particle called the procapsid is formed during assembly of the single-stranded DNA bacteriophage phiX174. Assembly of the phiX174 procapsid requires the presence of the two scaffolding proteins, D and B, which are structural components of the procapsid, but are not found in the mature virion. The X-ray crystallographic structure of a "closed" procapsid particle has been determined to 3.5 A resolution. This structure has an external scaffold made from 240 copies of protein D, 60 copies of the internally located B protein, and contains 60 copies of each of the viral structural proteins F and G, which comprise the shell and the 5-fold spikes, respectively. The F capsid protein has a similar conformation to that seen in the mature virion, and differs from the previously determined 25 A resolution electron microscopic reconstruction of the "open" procapsid, in which the F protein has a different conformation. The D scaffolding protein has a predominantly alpha-helical fold and displays remarkable conformational variability. We report here an improved and refined structure of the closed procapsid and describe in some detail the differences between the four independent D scaffolding proteins per icosahedral asymmetric unit, as well as their interaction with the F capsid protein. We re-analyze and correct the comparison of the closed procapsid with the previously determined cryo-electron microscopic image reconstruction of the open procapsid and discuss the major structural rearrangements that must occur during assembly. A model is proposed in which the D proteins direct the assembly process by sequential binding and conformational switching.
在单链DNA噬菌体φX174的组装过程中会形成一种名为原衣壳的空的前体颗粒。φX174原衣壳的组装需要两种支架蛋白D和B的存在，它们是原衣壳的结构成分，但在成熟病毒体中不存在。已确定“封闭”原衣壳颗粒的X射线晶体结构分辨率为3.5埃。该结构有一个由240个蛋白质D拷贝组成的外部支架、60个位于内部的B蛋白拷贝，并且包含病毒结构蛋白F和G各60个拷贝，它们分别构成壳体和五重尖峰。F衣壳蛋白具有与成熟病毒体中所见类似的构象，与先前确定的分辨率为25埃的“开放”原衣壳的电子显微镜重建不同，在后者中F蛋白具有不同的构象。D支架蛋白主要具有α螺旋折叠，并且表现出显著的构象变异性。我们在此报告封闭原衣壳的改进和细化结构，并详细描述二十面体不对称单元中四个独立的D支架蛋白之间的差异，以及它们与F衣壳蛋白的相互作用。我们重新分析并校正了封闭原衣壳与先前确定的开放原衣壳的冷冻电子显微镜图像重建之间的比较，并讨论了组装过程中必定发生的主要结构重排。提出了一个模型，其中D蛋白通过顺序结合和构象转换来指导组装过程。
