Orientia tsutsugamushi Modulates Endoplasmic Reticulum-Associated Degradation To Benefit Its Growth

Abstract

<i>Orientia tsutsugamushi</i>, an obligate intracellular bacterium that is auxotrophic for the aromatic amino acids and histidine, causes scrub typhus, a potentially deadly infection that threatens 1 billion people. <i>O. tsutsugamushi</i> growth is minimal during the first 24 to 48 h of infection but its growth becomes logarithmic thereafter. How the pathogen modulates cellular functions to support its growth is poorly understood. The unfolded protein response (UPR) is a cytoprotective pathway that relieves endoplasmic reticulum (ER) stress by promoting ER-associated degradation (ERAD) of misfolded proteins. Here, we show that <i>O. tsutsugamushi</i> invokes the UPR in the first 48 h and benefits from ER stress in an amino acid-dependent manner. <i>O. tsutsugamushi</i> also impedes ERAD during this time period. By 72 h, ER stress is alleviated and ERAD proceeds unhindered. Sustained inhibition of ERAD using RNA interference results in an <i>O. tsutsugamushi</i> growth defect at 72 h that can be rescued by amino acid supplementation. Thus, <i>O. tsutsugamushi</i> temporally stalls ERAD until ERAD-derived amino acids are needed to support its growth. The <i>O. tsutsugamushi</i> effector Ank4 is linked to this phenomenon. Ank4 interacts with Bat3, a eukaryotic chaperone that is essential for ERAD, and is transiently expressed by <i>O. tsutsugamushi</i> during the infection period when it inhibits ERAD. Ectopically expressed Ank4 blocks ERAD to phenocopy <i>O. tsutsugamushi</i> infection. Our data reveal a novel mechanism by which an obligate intracellular bacterial pathogen modulates ERAD to satisfy its nutritional virulence requirements.