Chipman P R, Agbandje-McKenna M, Renaudin J, Baker T S, McKenna R
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
Structure. 1998 Feb 15;6(2):135-45. doi: 10.1016/s0969-2126(98)00016-1.
Spiroplasma virus, SpV4, is a small, non-enveloped virus that infects the helical mollicute Spiroplasma melliferum. SpV4 exhibits several similarities to the Chlamydia phage, Chp1, and the Coliphages alpha 3, phi K, G4 and phi X174. All of these viruses are members of the Microviridae. These viruses have isometric capsids with T = 1 icosahedral symmetry, cause lytic infections and are the only icosahedral phages that contain single-stranded circular DNA genomes. The aim of this comparative study on these phages was to understand the role of their capsid proteins during host receptor recognition.
The three-dimensional structure of SpV4 was determined to 27 A resolution from images of frozen-hydrated particles. Cryo-electron microscopy (cryo-EM) revealed 20, approximately 54 A long, 'mushroom-like' protrusions on the surface of the capsid. Each protrusion comprises a trimeric structure that extends radially along the threefold icosahedral axes of the capsid. A 71 amino acid portion of VP1 (the SpV4 capsid protein) was shown, by structural alignment with the atomic structure of the F capsid protein of phi X174, to represent an insertion sequence between the E and F strands of the eight-stranded antiparallel beta-barrel. Secondary structure prediction of this insertion sequence provided the basis for a probable structural motif, consisting of a six-stranded antiparallel beta sheet connected by small turns. Three such motifs form the rigid stable trimeric structures (mushroom-like protrusions) at the threefold axes, with hydrophobic depressions at their distal surface.
Sequence alignment and structural analysis indicate that distinct genera of the Microviridae might have evolved from a common primordial ancestor, with capsid surface variations, such as the SpV4 protrusions, resulting from gene fusion events that have enabled diverse host ranges. The hydrophobic nature of the cavity at the distal surface of the SpV4 protrusions suggests that this region may function as the receptor-recognition site during host infection.
螺旋体病毒SpV4是一种小型无包膜病毒，可感染螺旋形支原体蜜螺旋体。SpV4与衣原体噬菌体Chp1以及大肠杆菌噬菌体α3、phi K、G4和phi X174有若干相似之处。所有这些病毒均为微小病毒科成员。这些病毒具有T = 1二十面体对称的等轴衣壳，引发裂解性感染，并且是唯一含有单链环状DNA基因组的二十面体噬菌体。这项对这些噬菌体的比较研究旨在了解其衣壳蛋白在宿主受体识别过程中的作用。
通过冷冻水合颗粒图像，将SpV4的三维结构解析到27 Å分辨率。冷冻电子显微镜（cryo-EM）显示衣壳表面有20个长约54 Å的“蘑菇状”突起。每个突起包含一个三聚体结构，该结构沿衣壳的三重二十面体轴径向延伸。通过与phi X174的F衣壳蛋白的原子结构进行结构比对，发现VP1（SpV4衣壳蛋白）的71个氨基酸部分代表八链反平行β桶的E链和F链之间的插入序列。该插入序列的二级结构预测为一个可能的结构基序提供了基础，该基序由通过小转角连接的六链反平行β折叠组成。三个这样的基序在三重轴处形成刚性稳定的三聚体结构（蘑菇状突起），其远端表面有疏水凹陷。
序列比对和结构分析表明，微小病毒科的不同属可能起源于一个共同的原始祖先，衣壳表面的变异，如SpV4突起，是由基因融合事件导致的，这些事件使得宿主范围多样化。SpV4突起远端表面腔的疏水性表明该区域可能在宿主感染期间作为受体识别位点发挥作用。
