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

Orientia tsutsugamushi, 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. O. tsutsugamushi 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 O. tsutsugamushi invokes the UPR in the first 48 h and benefits from ER stress in an amino acid-dependent manner. O. tsutsugamushi 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 O. tsutsugamushi growth defect at 72 h that can be rescued by amino acid supplementation. Thus, O. tsutsugamushi temporally stalls ERAD until ERAD-derived amino acids are needed to support its growth. The O. tsutsugamushi effector Ank4 is linked to this phenomenon. Ank4 interacts with Bat3, a eukaryotic chaperone that is essential for ERAD, and is transiently expressed by O. tsutsugamushi during the infection period when it inhibits ERAD. Ectopically expressed Ank4 blocks ERAD to phenocopy O. tsutsugamushi infection. Our data reveal a novel mechanism by which an obligate intracellular bacterial pathogen modulates ERAD to satisfy its nutritional virulence requirements.