RavN is a member of a previously unrecognized group of Legionella pneumophila E3 ubiquitin ligases.

The eukaryotic ubiquitylation machinery catalyzes the covalent attachment of the small protein modifier ubiquitin to cellular target proteins in order to alter their fate. Microbial pathogens exploit this post-translational modification process by encoding molecular mimics of E3 ubiquitin ligases, eukaryotic enzymes that catalyze the final step in the ubiquitylation cascade. Here, we show that the Legionella pneumophila effector protein RavN belongs to a growing class of bacterial proteins that mimic host cell E3 ligases to exploit the ubiquitylation pathway. The E3 ligase activity of RavN was located within its N-terminal region and was dependent upon interaction with a defined subset of E2 ubiquitin-conjugating enzymes. The crystal structure of the N-terminal region of RavN revealed a U-box-like motif that was only remotely similar to other U-box domains, indicating that RavN is an E3 ligase relic that has undergone significant evolutionary alteration. Substitution of residues within the predicted E2 binding interface rendered RavN inactive, indicating that, despite significant structural changes, the mode of E2 recognition has remained conserved. Using hidden Markov model-based secondary structure analyses, we identified and experimentally validated four additional L. pneumophila effectors that were not previously recognized to possess E3 ligase activity, including Lpg2452/SdcB, a new paralog of SidC. Our study provides strong evidence that L. pneumophila is dedicating a considerable fraction of its effector arsenal to the manipulation of the host ubiquitylation pathway.

Host Cell-catalyzed S-Palmitoylation Mediates Golgi Targeting of the Legionella Ubiquitin Ligase GobX.

The facultative intracellular pathogen Legionella pneumophila, the causative agent of Legionnaires disease, infects and replicates within human alveolar macrophages. L. pneumophila delivers almost 300 effector proteins into the besieged host cell that alter signaling cascades and create conditions that favor intracellular bacterial survival. In order for the effectors to accomplish their intracellular mission, their activity needs to be specifically directed toward the correct host cell protein or target organelle. Here, we show that the L. pneumophila effector GobX possesses E3 ubiquitin ligase activity that is mediated by a central region homologous to mammalian U-box domains. Furthermore, we demonstrate that GobX exploits host cell S-palmitoylation to specifically localize to Golgi membranes. The hydrophobic palmitate moiety is covalently attached to a cysteine residue at position 175, which is part of an amphipathic α-helix within the C-terminal region of GobX. Site-directed mutagenesis of cysteine 175 or residues on the hydrophobic face of the amphipathic helix strongly attenuated palmitoylation and Golgi localization of GobX. Together, our study provides evidence that the L. pneumophila effector GobX exploits two post-translational modification pathways of host cells, ubiquitination and S-palmitoylation.