美国加州大学Shaeri Mukherjee和Peter Walter研究组合作发现，军团菌毒素表现出tRNA模拟和糖基转移酶活性，以翻译机制为目标，引发核毒性应激反应。这一研究成果发表在2023年10月19日出版的国际学术期刊《自然—细胞生物学》上。

他们报道军团菌效应物SidI以前所未有的方式发挥作用，作为一种转移RNA模拟物，直接结合核糖体并使其糖基化。SidI的3.1 Å冷冻电镜结构揭示了一个具有“倒L”形状的N端结构域和tRNA模拟的表面电荷分布特征，以及一个采用糖基转移酶折叠的C端结构域，这使得SidI能够利用GDP -甘露糖作为糖前体。这种tRNA模拟和酶作用的耦合使SidI具有阻断蛋白质合成的能力，其效力可与蓖麻毒素相媲美，蓖麻毒素是已知最强大的毒素之一。在军团菌感染的细胞中，由SidI激活的翻译暂停引发了模拟核糖体毒性应激反应的应激反应特征，该应激反应被诱导核糖体碰撞的延伸抑制剂激活。SidI介导的核糖体效应激活应激激酶ZAKα和p38，进而驱动蛋白激活转录因子3 (ATF3)的积累。

有趣的是，ATF3逃脱了SidI施加的翻译阻滞，易位到细胞核并协调促进细胞死亡的应激诱导基因的转录，揭示了ATF3在应对碰撞核糖体应激中的主要作用。总之，他们的发现阐明了病原细菌利用tRNA模拟劫持调节细胞命运的核糖体到核信号通路的新机制。

据了解，病原体建立感染的一种普遍策略是抑制宿主细胞的蛋白质合成。嗜肺军团菌是一种细胞内细菌病原体和军团病的致病生物，它向宿主细胞分泌一种抑制翻译伸长的蛋白质效应器亚群。关于细菌如何靶向翻译延伸的机制见解仍然不明确。

附：英文原文

Title: A Legionella toxin exhibits tRNA mimicry and glycosyl transferase activity to target the translation machinery and trigger a ribotoxic stress response

Author: Subramanian, Advait, Wang, Lan, Moss, Tom, Voorhies, Mark, Sangwan, Smriti, Stevenson, Erica, Pulido, Ernst H., Kwok, Samentha, Chalkley, Robert J., Li, Kathy H., Krogan, Nevan J., Swaney, Danielle L., Burlingame, Alma L., Floor, Stephen N., Sil, Anita, Walter, Peter, Mukherjee, Shaeri

Issue&Volume: 2023-10-19

Abstract: A widespread strategy employed by pathogens to establish infection is to inhibit host-cell protein synthesis. Legionella pneumophila, an intracellular bacterial pathogen and the causative organism of Legionnaires’ disease, secretes a subset of protein effectors into host cells that inhibit translation elongation. Mechanistic insights into how the bacterium targets translation elongation remain poorly defined. We report here that the Legionella effector SidI functions in an unprecedented way as a transfer-RNA mimic that directly binds to and glycosylates the ribosome. The 3.1 cryo-electron microscopy structure of SidI reveals an N-terminal domain with an ‘inverted L’ shape and surface-charge distribution characteristic of tRNA mimicry, and a C-terminal domain that adopts a glycosyl transferase fold that licenses SidI to utilize GDP–mannose as a sugar precursor. This coupling of tRNA mimicry and enzymatic action endows SidI with the ability to block protein synthesis with a potency comparable to ricin, one of the most powerful toxins known. In Legionella-infected cells, the translational pausing activated by SidI elicits a stress response signature mimicking the ribotoxic stress response, which is activated by elongation inhibitors that induce ribosome collisions. SidI-mediated effects on the ribosome activate the stress kinases ZAKα and p38, which in turn drive an accumulation of the protein activating transcription factor 3 (ATF3). Intriguingly, ATF3 escapes the translation block imposed by SidI, translocates to the nucleus and orchestrates the transcription of stress-inducible genes that promote cell death, revealing a major role for ATF3 in the response to collided ribosome stress. Together, our findings elucidate a novel mechanism by which a pathogenic bacterium employs tRNA mimicry to hijack a ribosome-to-nuclear signalling pathway that regulates cell fate.

DOI: 10.1038/s41556-023-01248-z

Source: https://www.nature.com/articles/s41556-023-01248-z