Baculovirus Inhibitor-of-Apoptosis Op-IAP3 Blocks Apoptosis by Interaction with and Stabilization of a Host Insect Cellular IAP
Baculovirus Inhibitor-of-Apoptosis Op-IAP3 Blocks Apoptosis by Interaction with and Stabilization of a Host Insect Cellular IAP
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DOI：
10.1128/jvi.02320-15
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发表时间：
2016-01-01
影响因子：
5.4
通讯作者：
Friesen, Paul D.
中科院分区：
文献类型：
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作者：
Byers, Nathaniel M.;Vandergaast, Rianna L.;Friesen, Paul D.
杆状病毒编码的凋亡抑制因子（IAP）蛋白可能是从其宿主细胞IAP同源物进化而来的，其作为细胞死亡的关键调节因子发挥作用。尽管它们与细胞IAP有着惊人的相关性，包括两个杆状病毒IAP重复（BIR）结构域和一个C-末端RING的保守性，但病毒IAP使用一种尚未解决的机制来抑制昆虫的细胞凋亡。为了定义这种机制，我们研究了Op-IAP 3，即来自杆状病毒OpMNPV的原型IAP。我们发现Op-IAP 3与SfIAP形成稳定的复合物，SfIAP是宿主昆虫草地贪夜蛾的天然的、短寿命的IAP。长寿命的Op-IAP 3阻止病毒诱导的SfIAP降解，其通常导致半胱天冬酶活化和细胞凋亡。在未感染的细胞中，Op-IAP 3还增加了SfIAP稳态水平并延长了SfIAP的半衰期。相反，在突变的Op-IAP 3存在下，SfIAP稳定性丧失或逆转，所述突变的Op-IAP 3被工程化以降低稳定性。因此，Op-IAP 3稳定SfIAP并保留其抗凋亡功能。与SfIAP相反，Op-IAP 3不能结合或抑制天然的Spodoptera半胱天冬酶。此外，BIR突变，废除了良好保守的IAP拮抗剂的结合不影响Op-IAP 3的能力，以防止病毒诱导的细胞凋亡。值得注意的是，当内源性SfIAP被RNA沉默消除时，Op-IAP 3也未能阻止细胞凋亡。因此，Op-IAP 3需要SfIAP作为辅因子。我们的研究结果提出了一种新的模型，其中Op-IAP 3直接与SfIAP相互作用以维持其细胞内水平，从而间接抑制病毒诱导的细胞凋亡。与此模型一致，Op-IAP 3已经进化出内在的稳定性，可能有助于抑制信号诱导的营业额和autoubiquitination时，绑定到其靶向的细胞IAP.IMPORTANCEThe IAP首次发现杆状病毒，因为它们的效力，防止细胞凋亡。然而，病毒IAP在宿主昆虫中的抗凋亡机制一直难以捉摸。我们在这里表明，原型病毒IAP，Op-IAP 3，通过与不稳定的，autoubiquitinating主机IAP在这样一种方式，细胞IAP水平和抗凋亡活性得到维持，间接阻断细胞凋亡。该机制解释了Op-IAP 3需要天然细胞IAP作为辅因子以及半胱天冬酶抑制的可免除性。病毒IAP介导的宿主IAP同源物的保存利用了正常的IAP-IAP相互作用，并且可能是病毒IAP进化的结果，其中degron介导的去稳定化和泛素化潜力已经降低。这种机制说明了另一种新的方法，通过这种方法，DNA病毒掺入宿主死亡调节因子，这些调节因子被修饰为对宿主调节控制具有抗性，目的是抑制宿主细胞凋亡并获得复制优势。
Baculovirus-encoded inhibitor of apoptosis (IAP) proteins likely evolved from their host cell IAP homologs, which function as critical regulators of cell death. Despite their striking relatedness to cellular IAPs, including the conservation of two baculovirus IAP repeat (BIR) domains and a C-terminal RING, viral IAPs use an unresolved mechanism to suppress apoptosis in insects. To define this mechanism, we investigated Op-IAP3, the prototypical IAP from baculovirus OpMNPV. We found that Op-IAP3 forms a stable complex with SfIAP, the native, short-lived IAP of host insect Spodoptera frugiperda. Long-lived Op-IAP3 prevented virus-induced SfIAP degradation, which normally causes caspase activation and apoptosis. In uninfected cells, Op-IAP3 also increased SfIAP steady-state levels and extended SfIAP's half-life. Conversely, SfIAP stabilization was lost or reversed in the presence of mutated Op-IAP3 that was engineered for reduced stability. Thus, Op-IAP3 stabilizes SfIAP and preserves its antiapoptotic function. In contrast to SfIAP, Op-IAP3 failed to bind or inhibit native Spodoptera caspases. Furthermore, BIR mutations that abrogate binding of well-conserved IAP antagonists did not affect Op-IAP3's capacity to prevent virus-induced apoptosis. Remarkably, Op-IAP3 also failed to prevent apoptosis when endogenous SfIAP was ablated by RNA silencing. Thus, Op-IAP3 requires SfIAP as a cofactor. Our findings suggest a new model wherein Op-IAP3 interacts directly with SfIAP to maintain its intracellular level, thereby suppressing virus-induced apoptosis indirectly. Consistent with this model, Op-IAP3 has evolved an intrinsic stability that may serve to repress signal-induced turnover and autoubiquitination when bound to its targeted cellular IAP.IMPORTANCEThe IAPs were first discovered in baculoviruses because of their potency for preventing apoptosis. However, the antiapoptotic mechanism of viral IAPs in host insects has been elusive. We show here that the prototypical viral IAP, Op-IAP3, blocks apoptosis indirectly by associating with unstable, autoubiquitinating host IAP in such a way that cellular IAP levels and antiapoptotic activities are maintained. This mechanism explains Op-IAP3's requirement for native cellular IAP as a cofactor and the dispensability of caspase inhibition. Viral IAP-mediated preservation of the host IAP homolog capitalizes on normal IAP-IAP interactions and is likely the result of viral IAP evolution in which degron-mediated destabilization and ubiquitination potential have been reduced. This mechanism illustrates another novel means by which DNA viruses incorporate host death regulators that are modified for resistance to host regulatory controls for the purpose of suppressing host cell apoptosis and acquiring replication advantages.