Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada.
Beatrice Hunter Cancer Research Institute, Halifax, Nova Scotia, Canada.
J Virol. 2019 Oct 15;93(21). doi: 10.1128/JVI.00854-19. Print 2019 Nov 1.
Mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of cellular metabolism. In nutrient-rich environments, mTORC1 kinase activity stimulates protein synthesis to meet cellular anabolic demands. Under nutrient-poor conditions or under stress, mTORC1 is rapidly inhibited, global protein synthesis is arrested, and a cellular catabolic process known as autophagy is activated. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes multiple proteins that stimulate mTORC1 activity or subvert autophagy, but precise roles for mTORC1 in different stages of KSHV infection remain incompletely understood. Here, we report that during latent and lytic stages of KSHV infection, chemical inhibition of mTORC1 caused eukaryotic initiation factor 4F (eIF4F) disassembly and diminished global protein synthesis, which indicated that mTORC1-mediated control of translation initiation was largely intact. We observed that mTORC1 was required for synthesis of the replication and transcription activator (RTA) lytic switch protein and reactivation from latency, but once early lytic gene expression had begun, mTORC1 was not required for genome replication, late gene expression, or the release of infectious progeny. Moreover, mTORC1 control of autophagy was dysregulated during lytic replication, whereby chemical inhibition of mTORC1 prevented ULK1 phosphorylation but did not affect autophagosome formation or rates of autophagic flux. Together, these findings suggest that mTORC1 is dispensable for viral protein synthesis and viral control of autophagy during lytic infection and that KSHV undermines mTORC1-dependent cellular processes during the lytic cycle to ensure efficient viral replication. All viruses require host cell machinery to synthesize viral proteins. A host cell protein complex known as mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of protein synthesis. Under nutrient-rich conditions, mTORC1 is active and promotes protein synthesis to meet cellular anabolic demands. Under nutrient-poor conditions or under stress, mTORC1 is rapidly inhibited, global protein synthesis is arrested, and a cellular catabolic process known as autophagy is activated. Kaposi's sarcoma-associated herpesvirus (KSHV) stimulates mTORC1 activity and utilizes host machinery to synthesize viral proteins. However, we discovered that mTORC1 activity was largely dispensable for viral protein synthesis, genome replication, and the release of infectious progeny. Likewise, during lytic replication, mTORC1 was no longer able to control autophagy. These findings suggest that KSHV undermines mTORC1-dependent cellular processes during the lytic cycle to ensure efficient viral replication.
雷帕霉素靶蛋白复合体 1（mTORC1）是细胞代谢的主要调节因子。在营养丰富的环境中，mTORC1 激酶活性刺激蛋白质合成以满足细胞合成代谢的需求。在营养匮乏的情况下或应激条件下，mTORC1 会迅速被抑制，导致全局蛋白质合成停止，同时激活细胞分解代谢过程自噬。卡波济肉瘤相关疱疹病毒（KSHV）编码多种刺激 mTORC1 活性或颠覆自噬的蛋白，但 mTORC1 在 KSHV 感染的不同阶段的确切作用仍不完全清楚。在这里，我们报告在 KSHV 感染的潜伏和裂解阶段，mTORC1 的化学抑制导致真核起始因子 4F（eIF4F）解体和全局蛋白质合成减少，这表明 mTORC1 介导的翻译起始控制基本完整。我们观察到，mTORC1 是复制和转录激活剂（RTA）裂解开关蛋白的合成和从潜伏状态重新激活所必需的，但一旦早期裂解基因表达开始，mTORC1 就不再需要基因组复制、晚期基因表达或释放感染性后代。此外，mTORC1 对自噬的控制在裂解复制过程中失调，化学抑制 mTORC1 可阻止 ULK1 磷酸化，但不影响自噬体形成或自噬流的速率。总之，这些发现表明，在裂解感染过程中，mTORC1 对于病毒蛋白合成和病毒对自噬的控制是可有可无的，并且 KSHV 在裂解周期中破坏 mTORC1 依赖性细胞过程以确保有效的病毒复制。所有病毒都需要宿主细胞机制来合成病毒蛋白。一种称为雷帕霉素靶蛋白复合体 1（mTORC1）的宿主细胞蛋白复合物是蛋白质合成的主要调节因子。在营养丰富的条件下，mTORC1 是活跃的，并促进蛋白质合成以满足细胞合成代谢的需求。在营养匮乏的情况下或应激条件下，mTORC1 会迅速被抑制，全局蛋白质合成停止，一种称为自噬的细胞分解代谢过程被激活。卡波济肉瘤相关疱疹病毒（KSHV）刺激 mTORC1 活性并利用宿主机制合成病毒蛋白。然而，我们发现 mTORC1 活性在很大程度上对于病毒蛋白合成、基因组复制和感染性后代的释放是可有可无的。同样，在裂解复制过程中，mTORC1 不再能够控制自噬。这些发现表明，KSHV 在裂解周期中破坏 mTORC1 依赖性细胞过程以确保有效的病毒复制。
