Iron robbery by intracellular pathogen via bacterial effector-induced ferritinophagy

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.