Department of Veterinary Biosciences, The Ohio State University, Columbus, OH 43210.
Department of Veterinary Preventive Medicine, The Ohio State University, Columbus, OH 43210.
Proc Natl Acad Sci U S A. 2021 Jun 8;118(23). doi: 10.1073/pnas.2026598118.
Iron is essential for survival and proliferation of an obligatory intracellular bacterium that causes an emerging zoonosis, human monocytic ehrlichiosis. However, how acquires iron in the host cells is poorly understood. Here, we found that native and recombinant (cloned into the genome) translocated factor-3 (Etf-3), a previously predicted effector of the type IV secretion system (T4SS), is secreted into the host cell cytoplasm. Secreted Etf-3 directly bound ferritin light chain with high affinity and induced ferritinophagy by recruiting NCOA4, a cargo receptor that mediates ferritinophagy, a selective form of autophagy, and LC3, an autophagosome biogenesis protein. Etf-3-induced ferritinophagy caused ferritin degradation and significantly increased the labile cellular iron pool, which feeds Indeed, an increase in cellular ferritin by ferric ammonium citrate or overexpression of Etf-3 or NCOA4 enhanced proliferation, whereas knockdown of Etf-3 in via transfection with a plasmid encoding an Etf-3 antisense peptide nucleic acid inhibited proliferation. Excessive ferritinophagy induces the generation of toxic reactive oxygen species (ROS), which could presumably kill both and host cells. However, during proliferation, we observed concomitant up-regulation of Fe-superoxide dismutase, which is an integral component of T4SS operon, and increased mitochondrial Mn-superoxide dismutase by cosecreted T4SS effector Etf-1. Consequently, despite enhanced ferritinophagy, cellular ROS levels were reduced in infected cells compared with uninfected cells. Thus, safely robs host cell iron sequestered in ferritin. Etf-3 is a unique example of a bacterial protein that induces ferritinophagy to facilitate pathogen iron capture.
铁对于一种必需的胞内细菌的生存和增殖至关重要，这种细菌会导致一种新兴的人畜共患疾病，即人类单核细胞埃立克体病。然而，目前尚不清楚 是如何在宿主细胞中获取铁的。在这里，我们发现天然和重组（克隆到 基因组中）的转位因子-3（Etf-3），一种先前预测的 IV 型分泌系统（T4SS）效应物，被分泌到宿主细胞质中。分泌的 Etf-3 与铁蛋白轻链直接结合具有高亲和力，并通过招募 NCOA4 诱导铁蛋白自噬，NCOA4 是一种介导铁蛋白自噬的货物受体，铁蛋白自噬是一种选择性的自噬形式，以及 LC3，一种自噬体生物发生蛋白。Etf-3 诱导的铁蛋白自噬导致铁蛋白降解，并显著增加了不稳定的细胞铁池，这为 提供了铁。事实上，通过柠檬酸铁铵增加细胞内铁蛋白或过表达 Etf-3 或 NCOA4 增强了 的增殖，而通过转染编码 Etf-3 反义肽核酸的质粒抑制 Etf-3 在 中的表达则抑制了 的增殖。过量的铁蛋白自噬会诱导产生有毒的活性氧（ROS），这可能会杀死 和宿主细胞。然而，在 增殖过程中，我们观察到 T4SS 操纵子的组成部分 Fe-超氧化物歧化酶以及共分泌的 T4SS 效应物 Etf-1 同时上调了线粒体 Mn-超氧化物歧化酶。因此，尽管铁蛋白自噬增强，但与未感染细胞相比，感染细胞中的细胞 ROS 水平降低。因此， 安全地从铁蛋白中窃取宿主细胞的铁。Etf-3 是一种独特的细菌蛋白的例子，它诱导铁蛋白自噬以促进病原体铁的捕获。
