Manipulation of the unfolded protein response pathway by Legionella pneumophila
The intracellular bacterial pathogen Legionella pneumophila (L.p.) secretes over 300 bacterial proteins (effectors) to establish its replicative niche within host cells. Through its infectious life cycle, L.p. deploys its effectors to disrupt numerous host cell processes, including endoplasmic reticulum (ER) homeostasis. My dissertation explored the interaction between L.p. and the major homeostatic response pathway in the ER, the unfolded protein response (UPR). The results from my dissertation provide evidence that L.p. infection induces cleavage of the UPR sensor, activating transcription factor-6 (ATF6). Furthermore, I show that L.p.-mediated ATF6 cleavage is independent of proteasomal processing and does not require ER-associated degradation pathways. L.p. infection caused downstream activation of ATF6 target genes which include both lipid metabolism and proteostasis related genes. Interestingly, ATF6 activation during L.p. infection bypassed the requirement of conventional ATF6 pathway components. For example, chemical inhibition of ER to Golgi translocation of ATF6 with Ceapin A7 did not block its activation during L.p. infection. Furthermore, conventional site 1 and 2 protease (S1P and S2P) activities were dispensable during L.p. infection, as were the S1P and S2P cleavage sites on the ATF6 sensor. Additionally, time-lapse and confocal microscopy revealed that L.p.-mediated ATF6 activation bypasses ER to Golgi translocation and showed a direct recruitment of ATF6 to the L.p.-containing vacuole. Next, I compared ATF6 cleavage in the L.p. Philadelphia strain to the L.p. Paris strain, and showed that the Paris strain failed to activate the ATF6 pathway. Bioinformatic comparison between the Philadelphia and Paris strains revealed 17 effectors that were unique to the Philadelphia strain. When individually expressed, L.p. effectors Lpg2131, Lpg0519, Lpg2523, and Lpg2465 could induce expression of an ATF6-specific luciferase reporter. Subsequent analysis showed that Lpg0519 could induce ATF6 processing without affecting other UPR sensors, such as PERK. Thus, my findings highlight the unique regulatory control that L.p. exerts upon the UPR sensors and discovers a novel strategy by which an intracellular bacterium can selectively perturb host homeostatic pathways.