Regulation of host immunity by a novel Legionella pneumophila E3 ubiquitin ligase

Legionella pneumophila, the causative agent of Legionnaires' disease, exists ubiquitously in natural and artificial water systems. This pathogen poses serious threat to human health. One salient feature of L. pneumophila pathogenesis is the hundreds of effectors delivered into host cells by its Dot/Icm transporter. These virulence factors interfere with multiple hosts signaling pathways to subvert host defense. The ubiquitin network is essential in host signaling involved in immunity and thus is a common target of L. pneumophila effectors. At least thirteen Dot/Icm effectors have been shown to function as E3 ubiquitin ligases that cooperate with the host ubiquitination machinery by distinct mechanisms. In addition, seven deubiquitinases (DUBs) have been characterized. Furthermore, effectors that utilize catalysis mechanisms that are chemically distinct from the canonical one found in eukaryotes have been reported, indicating that hijacking of the host ubiquitin network by L. pneumophila is extensive and complex. Here, we identified ubiquitin interacting proteins with a proximity labeling method and found that the effector Lug14 (Lpg1106) functions as a novel ubiquitin ligase. Lug14 works with the E2 UbcH5c to catalyze ubiquitination with a preference for K11-linked chains by a mechanism that does not require a cysteine residue. Finally, we found that Lug14 targets ARIH2, a member of the host RBR E3 ligase family, leading to increased activation of the NLRP3 inflammasome in macrophages.