Chandrasekar V, Johnson J E
Department of Molecular Biology, Scripps Research Institute, La Jolla, CA 92037, USA.
Structure. 1998 Feb 15;6(2):157-71. doi: 10.1016/s0969-2126(98)00018-5.
Tobacco ringspot virus (TRSV) is a member of the nepovirus genus of icosahedral RNA plant viruses that cause disease in fruit crops. Nepoviruses, comoviruses and picornaviruses are classified in the picornavirus superfamily. Crystal structures of comoviruses and picornaviruses and the molecular mass of the TRSV subunit (sufficient to accommodate three beta-barrel domains) suggested that nepoviruses may represent a link in the evolution of the picornavirus capsids from a T = 3 icosahedral virus. This evolutionary process is thought to involve triplication of the capsid protein gene, to encode a three-domain polyprotein, followed by development of cleavage sites in the interdomain linking regions. Structural studies on TRSV were initiated to determine if the TRSV subunit corresponds to the proposed uncleaved three-domain polyprotein.
The 3.5 A resolution structure of TRSV shows that the capsid protein consists of three beta-barrel domains covalently linked by extended polypeptides. The order of connectivity of the domains in TRSV confirms the proposed connectivity for the precleaved comovirus and picornavirus capsid polyprotein. Structural differences between equivalent domains in TRSV and comoviruses are confined to the external surface loops, interdomain connecting polypeptides and N termini. The three different domains within TRSV and comoviruses are more closely related at the structural level than the three individual domains within picornaviruses.
The structural results confirm the notion of divergent evolution of the capsid polyproteins of nepoviruses, comoviruses and picornaviruses from a common ancestor. A number of residues were found to be conserved among various nepoviruses, some of which stabilize the quaternary structure of the three domains in the TRSV capsid protein subunit. Two conserved regions were identified on the external surface of TRSV, however, mutational studies will be needed to understand their functional significance. Nepoviruses transmitted by the same nematode species do not share regions with similar amino acid composition on the viral surface.
烟草环斑病毒（TRSV）是二十面体RNA植物病毒中的线虫传多面体病毒属成员，可导致果树作物发病。线虫传多面体病毒、豇豆花叶病毒和小RNA病毒被归类于小RNA病毒超家族。豇豆花叶病毒和小RNA病毒的晶体结构以及TRSV亚基的分子量（足以容纳三个β桶结构域）表明，线虫传多面体病毒可能代表了从T = 3二十面体病毒进化而来的小RNA病毒衣壳中的一个环节。这个进化过程被认为涉及衣壳蛋白基因的三倍化，以编码一个三结构域多聚蛋白，随后在结构域间连接区域形成切割位点。对TRSV的结构研究旨在确定TRSV亚基是否对应于所提出的未切割的三结构域多聚蛋白。
TRSV的3.5埃分辨率结构表明，衣壳蛋白由三个通过延伸多肽共价连接的β桶结构域组成。TRSV中结构域的连接顺序证实了所提出的未切割的豇豆花叶病毒和小RNA病毒衣壳多聚蛋白的连接方式。TRSV和豇豆花叶病毒中对应结构域之间的结构差异仅限于外部表面环、结构域间连接多肽和N末端。TRSV和豇豆花叶病毒中的三个不同结构域在结构水平上比小RNA病毒中的三个单个结构域更为密切相关。
结构结果证实了线虫传多面体病毒、豇豆花叶病毒和小RNA病毒的衣壳多聚蛋白从共同祖先发生趋异进化的观点。发现许多残基在各种线虫传多面体病毒中保守，其中一些残基稳定了TRSV衣壳蛋白亚基中三个结构域的四级结构。在TRSV的外部表面鉴定出两个保守区域，然而，需要进行突变研究以了解它们的功能意义。由相同线虫物种传播的线虫传多面体病毒在病毒表面不共享具有相似氨基酸组成的区域。
