Translationally Controlled Tumor Protein-Mediated Stabilization of Host Antiapoptotic Protein MCL-1 Is Critical for Establishment of Infection by Intramacrophage Parasite Leishmania donovani .

In the early phase of infection, the intramacrophage pathogen Leishmania donovani protects its niche with the help of the antiapoptotic protein myeloid cell leukemia-1 (MCL-1). Whether Leishmania could exploit MCL-1, an extremely labile protein, at the late phase is still unclear. A steady translational level of MCL-1 observed up to 48 h postinfection and increased caspase-3 activity in MCL-1-silenced infected macrophages documented its importance in the late hours of infection. The transcript level of MCL-1 showed a sharp decline at 6 h postinfection, and persistent MCL-1 expression in cyclohexamide-treated cells negates the possibility of de novo protein synthesis, thereby suggesting infection-induced stability. Increased ubiquitination, a prerequisite for proteasomal degradation of MCL-1, was also found to be absent in the late hours of infection. Lack of interaction with its specific E3 ubiquitin ligase MULE (MCL-1 ubiquitin ligase E3) and specific deubiquitinase USP9X prompted us to search for blockade of the ubiquitin-binding site in MCL-1. To this end, TCTP (translationally controlled tumor protein), a well-known binding partner of MCL-1 and antiapoptotic regulator, was found to be strongly associated with MCL-1 during infection. Phosphorylation of TCTP, a requirement for MCL-1 binding, was also increased in infected macrophages. Knockdown of TCTP decreased MCL-1 expression and short hairpin RNA-mediated silencing of TCTP in an infected mouse model of visceral leishmaniasis showed decreased parasite burden and induction of liver cell apoptosis. Collectively, our investigation revealed a key mechanism of how L. donovani exploits TCTP to establish infection within the host.
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