Non-canonical activation of the ER stress sensor ATF6 by Legionella pneumophila effectors

The intracellular bacterial pathogen Legionella pneumophila (L.p.) secretes ~330 effector proteins into the host cell to sculpt an Endoplasmic Reticulum (ER)-derived replicative niche. We previously reported five L.p. effectors that inhibit IRE1, a key sensor of the homeostatic unfolded protein response (UPR) pathway. In this study, we discovered a subset of L.p. toxins that selectively activate the UPR sensor ATF6, resulting in its cleavage, nuclear translocation and target gene transcription without affecting other UPR sensors such as PERK. In a deviation from the conventional model, this L.p. dependent activation of ATF6 does not require its transport to the Golgi or its cleavage by the S1P/S2P proteases. We believe that our findings highlight the unique regulatory control that L.p. exerts upon the three UPR sensors and expand the repertoire of bacterial proteins that selectively perturb host homeostatic pathways.