Inhibition of eukaryotic translation by tetratricopeptide-repeat proteins of Orientia tsutsugamushi.

Orientia tsutsugamushi, an obligate intracellular bacterium, is the causative agent of scrub typhus. The genome of Orientia tsutsugamushi has revealed multiple ORFs encoding tetratricopeptide-repeat (TPR) proteins. The TPR protein family has been shown to be involved in a diverse spectrum of cellular functions such as cell cycle control, transcription, protein transport, and protein folding, especially in eukaryotic cells. However, little is known about the function of the TPR proteins in O. tsutsugamushi. To investigate the potential role of TPR proteins in host-pathogen interaction, two oriential TPR proteins were expressed in E. coli and applied for GSTpull down assay. DDX3, a DEAD-box containing RNA helicase, was identified as a specific eukaryotic target of the TPR proteins. Since the RNA helicase is involved in multiple RNA-modifying processes such as initiation of translation reaction, we performed in vitro translation assay in the presence of GST-TPR fusion proteins by using rabbit reticulocyte lysate system. The TPR proteins inhibited in vitro translation of a reporter luciferase in a dose dependent manner whereas the GST control proteins did not. These results suggested TPR proteins of O. tsutsugamushi might be involved in the modulation of eukaryotic translation through the interaction with DDX3 RNA helicase after secretion into host cytoplasm.