The SET7 protein of Leishmania donovani moderates the parasite’s response to a hostile oxidative environment

Abstract

SET domain proteins methylate specific lysines on proteins, triggering stimulation or repression of downstream processes. Twenty-nine SET domain proteins have been identified in Leishmania donovani through sequence annotations. This study initiates the first investigation into these proteins. We find LdSET7 is predominantly cytosolic. While set7 is not essential, its deletion slows promastigote growth and hypersensitizes the parasite to hydroxyurea-induced G1/S arrest. Intriguingly, set7-nulls survive more proficiently than set7+/+ parasites within host macrophages, suggesting that LdSET7 moderates the parasite’s response to the inhospitable intracellular environment. set7-null in vitro promastigote cultures are highly tolerant to H2O2-induced stress, reflected in their growth pattern and almost complete absence of detectable DNA damage at H2O2 concentrations tested. This is linked to ROS levels remaining virtually unperturbed in set7-nulls in response to H2O2 exposure, contrasting to increased ROS in set7+/+ cells under similar conditions. In analyzing the cell’s ability to scavenge hydroperoxides we find peroxidase activity is not upregulated in response to H2O2 exposure in set7-nulls. Rather, constitutive basal levels of peroxidase activity are significantly higher in these cells, implicating this to be a factor contributing to the parasite’s high tolerance to H2O2. Higher levels of peroxidase activity in set7-nulls are coupled to upregulation of tryparedoxin peroxidase transcripts. Rescue experiments using an LdSET7 mutant suggest that LdSET7 methylation activity is critical to the modulation of the cell’s response to oxidative environment. Thus, LdSET7 tunes the parasite’s behavior within host cells, enabling establishment and persistence of infection without eradicating the host cell population it needs for survival.