Widana Gamage Shirani M K, Dietzgen Ralf G
Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, St LuciaQLD, Australia.
Front Microbiol. 2017 Apr 11;8:612. doi: 10.3389/fmicb.2017.00612. eCollection 2017.
Tospoviruses are among the most devastating viruses of horticultural and field crops. Capsicum chlorosis virus (CaCV) has emerged as an important pathogen of capsicum and tomato in Australia and South-east Asia. Present knowledge about CaCV protein functions in host cells is lacking. We determined intracellular localization and interactions of CaCV proteins by live plant cell imaging to gain insight into the associations of viral proteins during infection. Proteins were transiently expressed as fusions to autofluorescent proteins in leaf epidermal cells of and capsicum. All viral proteins localized at least partially in the cell periphery suggestive of cytoplasmic replication and assembly of CaCV. Nucleocapsid (N) and non-structural movement (NSm) proteins localized exclusively in the cell periphery, while non-structural suppressor of silencing (NSs) protein and Gc and Gn glycoproteins accumulated in both the cell periphery and the nucleus. Nuclear localization of CaCV Gn and NSs is unique among tospoviruses. We validated nuclear localization of NSs by immunofluorescence in protoplasts. Bimolecular fluorescence complementation showed self-interactions of CaCV N, NSs and NSm, and heterotypic interactions of N with NSs and Gn. All interactions occurred in the cytoplasm, except NSs self-interaction was exclusively nuclear. Interactions of a tospoviral NSs protein with itself and with N had not been reported previously. Functionally, CaCV NSs showed strong local and systemic RNA silencing suppressor activity and appears to delay short-distance spread of silencing signal. Cell-to-cell movement activity of NSm was demonstrated by -complementation of a movement-defective tobamovirus replicon. CaCV NSm localized at plasmodesmata and its transient expression led to the formation of tubular structures that protruded from protoplasts. The D residue in the 30K-like movement protein-specific LxD/NG motif of NSm was critical for plasmodesmata localization and movement activity. Compared to other tospoviruses, CaCV proteins have both conserved and unique properties in terms of localization, interactions and protein functions which will effect viral multiplication and movement in host plants.
番茄斑萎病毒属病毒是园艺作物和大田作物中最具毁灭性的病毒之一。辣椒褪绿病毒(CaCV)已成为澳大利亚和东南亚地区辣椒和番茄的一种重要病原体。目前尚缺乏关于CaCV蛋白在宿主细胞中功能的相关知识。我们通过活植物细胞成像确定了CaCV蛋白的细胞内定位和相互作用,以深入了解病毒蛋白在感染过程中的关联。在本氏烟草和辣椒的叶片表皮细胞中,将蛋白作为与自发荧光蛋白的融合蛋白进行瞬时表达。所有病毒蛋白至少部分定位于细胞周边,提示CaCV在细胞质中进行复制和组装。核衣壳(N)蛋白和非结构运动(NSm)蛋白仅定位于细胞周边,而沉默抑制子(NSs)蛋白以及Gc和Gn糖蛋白则在细胞周边和细胞核中均有积累。CaCV的Gn和NSs蛋白的核定位在番茄斑萎病毒属病毒中是独特的。我们通过原生质体中的免疫荧光验证了NSs蛋白的核定位。双分子荧光互补显示CaCV的N、NSs和NSm蛋白存在自身相互作用,以及N与NSs和Gn之间的异型相互作用。所有相互作用均发生在细胞质中,只有NSs蛋白的自身相互作用仅发生在细胞核中。此前尚未报道过番茄斑萎病毒属病毒的NSs蛋白与自身以及与N蛋白之间的相互作用。在功能上,CaCV的NSs蛋白表现出较强的局部和系统RNA沉默抑制活性,并且似乎会延迟沉默信号的短距离传播。通过对运动缺陷型烟草花叶病毒复制子进行互补试验,证明了NSm蛋白具有细胞间运动活性。CaCV的NSm蛋白定位于胞间连丝,其瞬时表达导致从原生质体突出的管状结构形成。NSm蛋白中类似30K运动蛋白特异性的LxD/NG基序中的D残基对于胞间连丝定位和运动活性至关重要。与其他番茄斑萎病毒属病毒相比,CaCV蛋白在定位、相互作用和蛋白功能方面具有保守和独特的特性,这将影响病毒在宿主植物中的增殖和运动。