Sidney Kimmel Comprehensive Cancer Center, Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America.
PLoS Pathog. 2012;8(6):e1002748. doi: 10.1371/journal.ppat.1002748. Epub 2012 Jun 7.
Viral replication efficiency is in large part governed by the ability of viruses to counteract pro-apoptotic signals induced by infection of host cells. For HHV-8, viral interferon regulatory factor-1 (vIRF-1) contributes to this process in part via inhibitory interactions with BH3-only protein (BOP) Bim, recently identified as an interaction partner of vIRF-1. Here we recognize that the Bim-binding domain (BBD) of vIRF-1 resembles a region (BH3-B) of Bid, another BOP, which interacts intramolecularly with the functional BH3 domain of Bid to inhibit it pro-apoptotic activity. Indeed, vIRF-1 was found to target Bid in addition to Bim and to interact, via its BBD region, with the BH3 domain of each. In functional assays, BBD could substitute for BH3-B in the context of Bid, to suppress Bid-induced apoptosis in a BH3-binding-dependent manner, and vIRF-1 was able to protect transfected cells from apoptosis induced by Bid. While vIRF-1 can mediate nuclear sequestration of Bim, this was not the case for Bid, and inhibition of Bid and Bim by vIRF-1 could occur independently of nuclear localization of the viral protein. Consistent with this finding, direct BBD-dependent inactivation by vIRF-1 of Bid-induced mitochondrial permeabilization was demonstrable in vitro and isolated BBD sequences were also active in this assay. In addition to Bim and Bid BH3 domains, BH3s of BOPs Bik, Bmf, Hrk, and Noxa also were found to bind BBD, while those of both pro- and anti-apoptotic multi-BH domain Bcl-2 proteins were not. Finally, the significance of Bid to virus replication was demonstrated via Bid-depletion in HHV-8 infected cells, which enhanced virus production. Together, our data demonstrate and characterize BH3 targeting and associated inhibition of BOP pro-apoptotic activity by vIRF-1 via Bid BH3-B mimicry, identifying a novel mechanism of viral evasion from host cell defenses.
病毒复制效率在很大程度上受病毒对抗由宿主细胞感染引起的促凋亡信号的能力所支配。对于 HHV-8，病毒干扰素调节因子-1（vIRF-1）通过与 BH3 仅蛋白（BOP）Bim 的抑制性相互作用来促进这一过程，Bim 最近被鉴定为 vIRF-1 的相互作用伙伴。在这里，我们认识到 vIRF-1 的 Bim 结合结构域（BBD）类似于另一种 BOP Bid 的 BH3-B 区域，Bid 通过其分子内功能 BH3 结构域与 Bid 相互作用，从而抑制其促凋亡活性。事实上，除了 Bim 之外，vIRF-1 还被发现靶向 Bid，并且通过其 BBD 区域与每个 Bid 的 BH3 结构域相互作用。在功能测定中，BBD 可以替代 Bid 中的 BH3-B，以依赖 BH3 结合的方式抑制 Bid 诱导的细胞凋亡，并且 vIRF-1 能够保护转染细胞免受 Bid 诱导的凋亡。虽然 vIRF-1 可以介导 Bim 的核隔离，但 Bid 则不然，并且 vIRF-1 对 Bid 和 Bim 的抑制作用可以独立于病毒蛋白的核定位而发生。与这一发现一致，vIRF-1 通过直接的 BBD 依赖性失活来抑制 Bid 诱导的线粒体通透性，这在体外得到了证明，并且分离的 BBD 序列在该测定中也具有活性。除了 Bim 和 Bid BH3 结构域外，BOP Bik、Bmf、Hrk 和 Noxa 的 BH3 结构域也被发现与 BBD 结合，而前凋亡和抗凋亡多 BH 结构域 Bcl-2 蛋白的 BH3 结构域则没有。最后，通过在 HHV-8 感染的细胞中耗尽 Bid 来证明 Bid 对病毒复制的重要性，这增强了病毒的产生。总之，我们的数据通过 vIRF-1 通过 Bid BH3-B 模拟来证明和描述 BH3 靶向及其对 BOP 促凋亡活性的关联抑制作用，确定了病毒逃避宿主细胞防御的一种新机制。
