Li Ka Shing Institute of Virology, Department of Medical Microbiology and Immunology, University of Alberta, Edmonton, Alberta, Canada.
Li Ka Shing Institute of Virology, Department of Medical Microbiology and Immunology, University of Alberta, Edmonton, Alberta, Canada
J Virol. 2018 Sep 12;92(19). doi: 10.1128/JVI.01167-18. Print 2018 Oct 1.
Poxviruses encode many proteins with the ability to regulate cellular signaling pathways. One such protein is the vaccinia virus innate immunity modulator E3. Multiple functions have been ascribed to E3, including modulating the cellular response to double-stranded RNA, inhibiting the NF-κB and IRF3 pathways, and dampening apoptosis. Apoptosis serves as a powerful defense against damaged and unwanted cells and is an effective defense against viral infection; many viruses therefore encode proteins that prevent or delay apoptosis. Here, we present data indicating that E3 does not directly inhibit the intrinsic apoptotic pathway; instead, it suppresses apoptosis indirectly by stimulating expression of the viral F1 apoptotic inhibitor. Our data demonstrate that E3 promotes F1 expression by blocking activation of the double-stranded RNA-activated protein kinase R (PKR). F1 mRNA is present in cells infected with E3-null virus, but the protein product does not detectably accumulate, suggesting a block at the translational level. We also show that two 3' coterminal transcripts span the F1 open reading frame (ORF), a situation previously described for the vaccinia virus mRNAs encoding the J3 and J4 proteins. One of these is a conventional monocistronic transcript of the F1L gene, while the other arises by read-through transcription from the upstream F2L gene and does not give rise to appreciable levels of F1 protein. Previous studies have shown that E3-deficient vaccinia virus triggers apoptosis of infected cells. Our study demonstrates that this proapoptotic phenotype stems, at least in part, from the failure of the mutant virus to produce adequate quantities of the viral F1 protein, which acts at the mitochondria to directly block apoptosis. Our data establish a regulatory link between the vaccinia virus proteins that suppress the innate response to double-stranded RNA and those that block the intrinsic apoptotic pathway.
痘病毒编码许多具有调节细胞信号通路能力的蛋白质。其中一种蛋白质是牛痘病毒先天免疫调节剂 E3。E3 具有多种功能，包括调节细胞对双链 RNA 的反应、抑制 NF-κB 和 IRF3 途径以及抑制细胞凋亡。细胞凋亡是一种针对受损和不需要的细胞的强大防御机制，也是一种有效抵御病毒感染的防御机制；因此，许多病毒编码防止或延迟细胞凋亡的蛋白质。在这里，我们提供的数据表明 E3 不会直接抑制内在凋亡途径；相反，它通过刺激病毒 F1 凋亡抑制剂的表达间接抑制凋亡。我们的数据表明，E3 通过阻断双链 RNA 激活的蛋白激酶 R (PKR) 的激活来促进 F1 的表达。感染 E3 缺失病毒的细胞中存在 F1 mRNA，但蛋白质产物无法检测到积累，表明在翻译水平上存在阻滞。我们还表明，跨越 F1 开放阅读框 (ORF) 的两个 3' 端共终止转录本，这种情况以前在编码 J3 和 J4 蛋白的牛痘病毒 mRNA 中已有描述。其中一个是 F1L 基因的常规单顺反子转录本，而另一个是由上游 F2L 基因的通读转录产生的，不会产生可观水平的 F1 蛋白。先前的研究表明，E3 缺失的牛痘病毒会触发感染细胞的凋亡。我们的研究表明，这种促凋亡表型至少部分源于突变病毒无法产生足够数量的病毒 F1 蛋白，该蛋白在线粒体处直接阻断细胞凋亡。我们的数据建立了一种调节联系，即在抑制对双链 RNA 的先天反应的牛痘病毒蛋白与阻断内在凋亡途径的蛋白之间建立了一种调节联系。
