The Legionella pneumophila type IVb secretion system effector BinA subverts amino acid transport to sensitize TORC1 signaling in macrophages
Legionella pneumophila is an environmental Gram-negative bacterium that parasitizes unicellular protozoa and can cause severe pulmonary infections when aerosolized bacteria are inhaled by humans. One critical aspect of Legionella pathogenesis is the establishment in the cytosol of infected macrophages of a unique ER-derived vacuole, that requires a sustained supply of host lipids during expansion. Subversion of pro-lipogenic pathways downstream of the metabolic checkpoint kinase mTOR (Mechanistic Target of Rapamycin) are critical for niche expansion. In eukaryotic cells, amino acids sufficiency and growth factor sensory signals converge on mTOR to ensure metabolic processes are coupled to nutrients/energy availability. Legionella can trigger mTOR signaling in infected cells by increasing the intracellular abundance of amino acids through inhibition of host translation. Here, we describe a novel mechanism by which Legionella sensitizes mTOR in infected macrophages. A forward genetic screen identified Lpg0393 protein as a putative bacterial mTOR regulator that contains a VPS9-domain typically found in eukaryotic GEFs (Guanine nucleotide exchange factors) for Rab5 GTPase family members (Rab5/Rab21/Rab22). We uncovered that Lpg0393 lowers the activation threshold for mTOR signaling upon stimulation with arginine or leucine through rewiring of processes upstream of mTOR by removing Rab5-dependency and replacing it with a Rab21/Rab22-dependency. Data from cells expressing either a bacterial or a eukaryotic mTOR sensitizing factor uncovered two distinct intraorganellar Arg/Leu pools that fuel mTOR activation in parallel - one regulated by Rab21/22 and the other by Rab5. Consistent with the role of mTOR in expansion of the Legionella-occupied organelle, deletion of Lpg0393 also resulted in premature vacuolar rupture in a mTOR-dependent manner. All together, we have identified a novel bacterial mTOR regulator and consistent with its reported functions we propose Lpg0393 is named as BinA (Bacterial initiator of TORC1 signaling and an activator of Rab5 family GTPases). Author SummaryLegionella pneumophila - a prototypical vacuolar pathogen - manipulates host lipogenesis to sustain membrane biogenesis, which ensures the integrity of the vacuolar compartment is not compromised as demand for housing capacity increases during bacterial replication. Subversion of the host mTOR kinase signaling, which functions as a central regulatory hub coordinating nutrients sufficiency with metabolic output, is one mechanism by which Legionella increases de novo lipogenesis in infected cells. Here, we report a novel mechanism by which the bacteria sustain mTOR signaling through the actions of the type IV secretion system effector BinA. We found that BinA rewires amino acid transport mechanisms by manipulating Rab5 family GTPase to selectively sensitize mTOR to arginine and leucine but not methionine or glutamine stimulation. mTOR sensitization by BinA revealed a switch in upstream regulators from Rab5 to Rab21/Rab22. Our work provides novel insight into how pathogens exert metabolic control over host cells to maximize intracellular replication.