中科院微生物所郭惠珊组揭示虫媒DNA病毒劫持宿主印迹基因逃逸表观抑制新进展

来源：病毒学界、中科院微生物所

近日，中国科学院微生物研究所郭惠珊研究组在 The Plant Cell 杂志在线发表了题为 DNA Geminivirus Infection Induces an Imprinted E3 Ligase Gene to Epigenetically Activate Viral Gene Transcription 的研究论文。研究发现虫媒DNA双生病毒诱导植物异常表达一个编码E3 ligase的印迹基因，通过泛素化降解DNA甲基转移酶，从而逃逸宿主甲基化抑制，促使病毒早期基因转录和病毒积累。 

双生病毒是一类单链环状的植物DNA病毒，能够侵染包括玉米，番茄，木薯等多种重要的农作物，对农业生产造成十分严重的危害。宿主植物主要通过转录水平 (Transcriptional Gene Silencing, TGS) 和转录后水平(Post-Transcriptional Gene Silencing, PTGS)的基因沉默抵御双生病毒的侵染。植物细胞核内的TGS主要通过对病毒基因组DNA进行甲基化从而抑制病毒的转录和扩增；而双生病毒则编码沉默抑制子 (Viral Suppressors of RNA silencing, VSRs) 抵御TGS。大量的报道主要阐述双生病毒VSR蛋白自身的功能，如C2蛋白通过干扰植物细胞甲基循环消除植物TGS对病毒基因组DNA的甲基化。但是这些VSR蛋白自身在病毒侵染早期是如何逃逸被抑制的分子机制却从未被探究。 

郭惠珊课题组以甜菜叶蝉传播的单分体双生病毒-甜菜严重曲顶病毒(Beet severe curly top virus, BSCTV)-为研究对象，在转化了包含C2/C3共转录启动子DNA片段的植物拟南芥中，意外发现其中一株因诱导了原来只在胚乳中特异表达的印迹基因VARIANT IN METHYLATION 5 (VIM5)，而激活C2/C3启动子；进而发现在BSCTV侵染拟南芥的早期，VIM5在叶片异常表达；生化实验揭示VIM5编码一种泛素化E3 ligase，介导拟南芥DNA甲基转移酶MET1和CMT3蛋白通过26S蛋白酶体降解，解除C2/C3启动子对称型CG和CHG胞嘧啶甲基化，促使VSR C2和复制增强子C3在BSCTV侵染早期得以转录。BSCTV侵染vim5突变体，C2和C3转录、病毒复制以及症状形成都延迟；而BSCTV侵染met1和cmt3突变体则恢复了病毒复制和症状延迟等现象。 

图.  BSCTV通过诱导VIM5的异位表达降解植物的甲基转移酶从而促进病毒的扩繁 

该研究首次揭露了双生病毒通过诱导宿主胚乳印迹基因在叶片的异常表达，通过泛素化降解DNA甲基转移酶，特异性降低病毒早期基因启动子甲基化水平促使早期基因转录。该研究揭示了病毒劫持宿主感病基因的新计谋，为双生病毒和植物互作提供了新的视角；也提示VIM5介导翻译后修饰在胚乳发育的调控作用。

AbstractFlowering plants and mammals encode imprinted genes that are primarily expressed in the endosperm and placenta in a parent-of-origin manner. In this study, we show that early activation of the geminiviral C2 and C3 genes in Arabidopsis plants, which encode a viral suppressor of RNA interference (RNAi) and a replication enhancer protein, respectively, correlates with transiently vegetative expression of Variant In Methylation 5 (VIM5), an endosperm imprinted gene conserved in diverse plant species. We demonstrate that VIM5 is a ubiquitin E3 ligase and directly targets DNA methyltransferases MET1 and CMT3 for degradation by the ubiquitin 26S proteasome proteolytic pathway. Infection with Beet severe curly top virus induces expression of VIM5 in rosette leaf tissues, possibly by the expression of the viral Rep protein, leading to the early activation of C2 and C3 coupled with reduced symmetric methylation in C2-3 promoter and the onset of disease symptoms. These findings show that this small DNA virus recruits a host imprinted gene for epigenetic activation of viral transcription. Our work reveals a distinct strategy of plant pathogens to exploit the host machinery to inhibit methylation-mediated defense response in establishing infection。