Suppression mechanism of Geminivirus-encoded TrAP protein

本研究的主要目的是在表观遗传水平上阐明宿主先天免疫反应和病毒抗防御反应的机制。真核生物已经进化出复杂的RNA沉默机制来防御入侵病毒。另一方面，包括感染人类的病毒在内的病毒编码蛋白质，称为病毒抑制因子，以阻断沉默途径以逃避宿主监视。目前，国内外对宿主-病毒相互作用的研究主要集中在转录后基因沉默（PTGS）的抗病毒作用和病毒对PTGS的抑制上。虽然我们对PTGS水平的病毒抑制的了解已经大大扩展，但我们对转录基因沉默（TGS）水平的病毒抑制的了解非常少。在真核生物中，细胞核DNA被包裹在组蛋白八聚体上形成染色质。染色质甲基化不仅调控基因复制和转录，还控制病毒在人类和植物中的潜伏期，发挥先天免疫系统的作用，限制病原体的侵袭。PI小组和其他实验室最近的研究表明，由模式生物拟南芥中的单链DNA病毒双病毒（Geminivirus）编码的TrAP抑制基因干扰TGS途径。通过细胞因子的蛋白质组学筛选，PI的研究小组已经确定组蛋白甲基转移酶（SUVH4）和组蛋白去甲基化酶（REF6）是TGS途径中的两个关键效应物，是TrAP的新靶点。这些结果和其他几个小组的工作导致了一个模型的概念化，即TGS作为防御侵入性DNA病原体的防御机制，而病毒抑制因子可以通过直接抑制TGS整合子来打破这一限制。针对这一模型，PI提出：1)确定TrAP对SUVH特异性抑制作用的生化基础。PI希望查明TrAP中参与SUVH4相互作用的关键残基，并研究这些残基如何影响SUVH4的功能和改变病毒的致病性；该项目还将研究TrAP是否靶向SUVH4的遗传类似物，如SUVH5和SUVH6，以调节双病毒感染；2)探讨TrAP-REF6相互作用在病毒感染中的作用和机制。PI实验室观察到，REF6与双病毒染色质结合，功能缺失的REF6突变体对双病毒感染的易感性降低。PI计划系统研究REF6的生化特性及其与TrAP在病毒转录和增殖中的功能相互作用。该研究将探讨组蛋白甲基转移酶和去甲基化酶如何协同赋予病毒潜伏期的基本机制，以及DNA病毒如何选择劫持关键的TGS成分作为反防御反应。TrAP的抑制机制可用于指导治疗或预防措施，以解决真核生物（包括人类）中由表观遗传功能障碍引起的生理障碍。

The primary goal of the proposed research is to elucidate the mechanism of host innate immune responses and viral counter-defense responses at an epigenetic level. Eukaryotes have evolved sophisticated mechanisms of RNA silencing to defense invasive viruses. On the other hand, viruses including those infecting humans encode proteins, referred to as viral suppressors, to block silencing pathways to evade host surveillance. The prevailing worldwide study on host-virus interaction focuses on the antivirus role of posttranscriptional gene silencing (PTGS) and viral suppression of PTGS. While our knowledge of viral suppression at the PTGS level has been drastically expanded, our understanding of viral suppression at the level of transcriptional gene silencing (TGS) is very poor. In eukaryotes, the nuclear DNA is wrapped onto histone octamers to form a chromatin. Chromatin methylation not only regulates gene replication and transcription, but also controls the latency of viruses in human and plants, functioning as an innate immune system to restrict invasive pathogens. Recent research from the PI's group and other laboratories has illuminated that TrAP suppressor encoded by Geminivirus, a family of single-stranded DNA viruses in the model organism Arabidopsis, genetically interferes with the TGS pathway. By proteomic screening of cellular factors, the PI's group has identified that a histone methyltransferase (SUVH4) and a histone demethylase (REF6), two key effectors in the TGS pathway, are new bona fide targets by TrAP. These results and work from several other groups led to conceptualization of a model that TGS serves as a defense mechanism to defend invasive DNA pathogens, whereas viral suppressors can break this restriction by directly inhibiting the TGS integrators. To address this model, the PI proposes: 1) to determine the biochemical basis for specific inhibitory effect of TrAP on SUVH. The PI wishes to pinpoint the critical residues of TrAP that participate in the interaction with SUVH4 and investigate how the residues affect SUVH4 function and alter the viral pathogenicity; The PI will also study whether TrAP targets the genetic paralogs of SUVH4 such as SUVH5 and SUVH6 to regulate Geminivirus infection; and 2) to investigate function and mechanism of TrAP-REF6 interaction in viral infection. The PI laboratory has observed that REF6 binds to Geminivirus chromatin and loss-of-function mutants of ref6 exhibit reduced susceptibility to Geminivirus infection. The PI plans to systemically study the biochemical features of REF6 and its functional interaction of TrAP in viral transcription and multiplication. The proposed study will address the fundamental but poorly understood mechanism how histone methyltransferases and demethylases coordinately confer viral latency and how DNA virus co-opts to hijack the critical TGS components as counter-defense responses. The suppression mechanism of TrAP may be exploited for directed therapies or preventative measures to address physiological disorders that arise from epigenetic dysfunction in eukaryotes including human.