Legionella pneumophila exploits the endo-lysosomal network for phagosome biogenesis by co-opting SUMOylated Rab7.

Legionella pneumophila strains harboring wild-type rpsL such as Lp02rpsLWT cannot replicate in mouse bone marrow-derived macrophages (BMDMs) due to induction of extensive lysosome damage and apoptosis. The bacterial factor directly responsible for inducing such cell death and the host factor involved in initiating the signaling cascade that leads to lysosome damage remain unknown. Similarly, host factors that may alleviate cell death induced by these bacterial strains have not yet been investigated. Using a genome-wide CRISPR/Cas9 screening, we identified Hmg20a and Nol9 as host factors important for restricting strain Lp02rpsLWT in BMDMs. Depletion of Hmg20a protects macrophages from infection-induced lysosomal damage and apoptosis, allowing productive bacterial replication. The restriction imposed by Hmg20a was mediated by repressing the expression of several endo-lysosomal proteins, including the small GTPase Rab7. We found that SUMOylated Rab7 is recruited to the bacterial phagosome via SulF, a Dot/Icm effector that harbors a SUMO-interacting motif (SIM). Moreover, overexpression of Rab7 rescues intracellular growth of strain Lp02rpsLWT in BMDMs. Our results establish that L. pneumophila exploits the lysosomal network for the biogenesis of its phagosome in BMDMs.

The Shigella kinase effector OspG modulates host ubiquitin signaling to escape septin-cage entrapment.

Shigella flexneri is a Gram-negative bacterium causing severe bloody dysentery. Its pathogenesis is largely dictated by a plasmid-encoded type III secretion system (T3SS) and its associated effectors. Among these, the effector OspG has been shown to bind to the ubiquitin conjugation machinery (E2~Ub) to activate its kinase activity. However, the cellular targets of OspG remain elusive despite years of extensive efforts. Here we show by unbiased phosphoproteomics that a major target of OspG is CAND1, a regulatory protein controlling the assembly of cullin-RING ubiquitin ligases (CRLs). CAND1 phosphorylation weakens its interaction with cullins, which is expected to impact a large panel of CRL E3s. Indeed, global ubiquitome profiling reveals marked changes in the ubiquitination landscape when OspG is introduced. Notably, OspG promotes ubiquitination of a class of cytoskeletal proteins called septins, thereby inhibiting formation of cage-like structures encircling cytosolic bacteria. Overall, we demonstrate that pathogens have evolved an elaborate strategy to modulate host ubiquitin signaling to evade septin-cage entrapment.

Legionella metaeffector MavL reverses ubiquitin ADP-ribosylation via a conserved arginine-specific macrodomain.

ADP-ribosylation is a reversible post-translational modification involved in various cellular activities. Removal of ADP-ribosylation requires (ADP-ribosyl)hydrolases, with macrodomain enzymes being a major family in this category. The pathogen Legionella pneumophila mediates atypical ubiquitination of host targets using the SidE effector family in a process that involves ubiquitin ADP-ribosylation on arginine 42 as an obligatory step. Here, we show that the Legionella macrodomain effector MavL regulates this pathway by reversing the arginine ADP-ribosylation, likely to minimize potential detrimental effects caused by the modified ubiquitin. We determine the crystal structure of ADP-ribose-bound MavL, providing structural insights into recognition of the ADP-ribosyl group and catalytic mechanism of its removal. Further analyses reveal DUF4804 as a class of MavL-like macrodomain enzymes whose representative members show unique selectivity for mono-ADP-ribosylated arginine residue in synthetic substrates. We find such enzymes are also present in eukaryotes, as exemplified by two previously uncharacterized (ADP-ribosyl)hydrolases in Drosophila melanogaster. Crystal structures of several proteins in this class provide insights into arginine specificity and a shared mode of ADP-ribose interaction distinct from previously characterized macrodomains. Collectively, our study reveals a new regulatory layer of SidE-catalyzed ubiquitination and expands the current understanding of macrodomain enzymes.

Size-selective sampler combined with an immunochromatographic assay for the rapid detection of airborne Legionella pneumophila.

Airborne biological aerosols (also called bioaerosols) are found in various environmental and occupational settings. Among these, pathogenic bioaerosols can cause diseases such as legionellosis, influenza, measles, and tuberculosis. To prevent or minimize people's exposure to these pathogenic bioaerosols in the field, a rapid detection method is required. In this study, a size-selective bioaerosol (SSB) sampler was combined with the immunochromatographic assay (ICA). The SSB sampler can collect bioaerosols on the sampling swab and the lateral flow test kit used in ICA can rapidly detect the pathogens in bioaerosols collected on the swab. Before testing the combined method, the lower limit of detection (LOD) of the lateral flow test kit was determined. Legionella pneumophila (L. pneumophila) was used as a target pathogen. The results show that at least 1.3 × 103L. pneumophila cells are required to be detected by the lateral flow test kit. To test the developed method, L. pneumophila suspension was aerosolized in the sampling chamber and collected using two SSB samplers with different sampling times (10 and 20 min). The developed method could detect aerosolized L. pneumophila and also estimate the concentrations from the lower LOD, sampling time, and formation of a positive line on a test strip. When positive results were obtained from sampling for 10 min and 20 min, concentrations of respirable L. pneumophila were estimated ≥5.2 × 104 CFUresp/m3 and ≥2.6 × 104 CFUresp/m3, respectively. The conventional sampler Andersen impactor with colony counting was also used for comparison. In all cases, the estimated concentrations obtained by the developed method were higher than those obtained by the conventional method. These findings confirm that the developed method can overcome the limitations of conventional methods and eventually benefit environmental and occupational health by providing a better method for risk assessment.

Legionella pneumophila exploits the endo-lysosomal network for phagosome biogenesis by co-opting SUMOylated Rab7.

L. pneumophila strains harboring wild-type rpsL such as Lp02rpsLWT cannot replicate in mouse bone marrow-derived macrophages (BMDMs) due to induction of extensive lysosome damage and apoptosis. The mechanism of this unique infection-induced cell death remains unknown. Using a genome-wide CRISPR/Cas9 screening, we identified Hmg20a and Nol9 as host factors important for restricting strain Lp02rpsLWT in BMDMs. Depletion of Hmg20a protects macrophages from infection-induced lysosomal damage and apoptosis, allowing productive bacterial replication. The restriction imposed by Hmg20a was mediated by repressing the expression of several endo-lysosomal proteins, including the small GTPase Rab7. We found that SUMOylated Rab7 is recruited to the bacterial phagosome via SulF, a Dot/Icm effector that harbors a SUMO-interacting motif (SIM). Moreover, overexpression of Rab7 rescues intracellular growth of strain Lp02rpsLWT in BMDMs. Our results establish that L. pneumophila exploits the lysosomal network for the biogenesis of its phagosome in BMDMs.

Acinetobacter baumannii coordinates central metabolism, plasmid dissemination, and virulence by sensing nutrient availability.

Plasmid conjugation plays an important role in the dissemination of antibiotic-resistance genes. The emergence of multidrug-resistant isolates of Acinetobacter baumannii poses grave challenges in treating infections caused by this notorious nosocomial pathogen. Yet, the composition, functionality, and regulation of conjugative machinery utilized by A. baumannii remain poorly understood. Here, we found that conjugation of the major plasmid pAB3 of A. baumannii is mediated by a type IVB secretion system similar to the Dot/Icm transporter of Legionella pneumophila. Furthermore, the expression of the structural genes of the Dot/Icm-like system is co-regulated with genes involved in central metabolism by the GacS/GacA two-component system in response to various metabolites, including intermediates of the tricarboxylic acid cycle. Loss of GacS/A also severely impaired bacterial virulence. These results establish that A. baumannii coordinates metabolism with plasmid conjugation and virulence by sensing nutrient availability, which may be exploited to develop inhibitory agents for controlling the spread of drug-resistance genes and virulence factors. IMPORTANCE Plasmid conjugation is known to be an energy-expensive process, but our understanding of the molecular linkage between conjugation and metabolism is limited. Our finding reveals that Acinetobacter baumannii utilizes a two-component system to co-regulate metabolism, plasmid transfer, and virulence by sensing reaction intermediates of key metabolic pathways, which suggests that nutrient availability dictates not only bacterial proliferation but also horizontal gene transfer. The identification of Dot/Icm-like proteins as components of a conjugation system involved in the dissemination of antibiotic-resistance genes by A. baumannii has provided important targets for the development of agents capable of inhibiting virulence and the spread of anti-microbial-resistance genes in bacterial communities.

Ubiquitination of Sec22b by a novel Legionella pneumophila ubiquitin E3 ligase.

Legionella pneumophila is a facultative intracellular pathogen that causes legionellosis. The key to its virulence is the delivery of hundreds of effector proteins into host cells via the defective in organelle trafficking/intracellular multiplication type IV secretion system. These effectors modulate numerous host signaling pathways to create a niche called the Legionella-containing vacuole (LCV) permissive for its intracellular replication. Previous investigation revealed that exploitation of the host ubiquitin system is among the most important strategies used by L. pneumophila to coopt host processes for its benefit. Here, we show that the effector Legionella ubiquitin ligase gene 15 (Lug15) (Lpg2327), which has no detectable homology with any enzyme involved in ubiquitin signaling, is an E3 ligase. In L. pneumophila-infected cells, Lug15 is localized on the LCV and impacts its association with polyubiquitinated proteins. We also demonstrate that Sec22b is ubiquitinated and recruited to the LCV by Lug15. Thus, our results establish Lug15 as a novel E3 ligase that functions to recruit a SNARE protein to remodel the L. pneumophila phagosome.IMPORTANCEProtein ubiquitination is one of the most important post-translational modifications that plays critical roles in the regulation of a wide range of eukaryotic signaling pathways. Many successful intracellular bacterial pathogens can hijack host ubiquitination machinery through the action of effector proteins that are injected into host cells by secretion systems. Legionella pneumophila is the etiological agent of legionellosis that is able to survive and replicate in various host cells. The defective in organelle trafficking (Dot)/intracellular multiplication (Icm) type IV secretion system of L. pneumophila injects over 330 effectors into infected cells to create an optimal environment permissive for its intracellular proliferation. To date, at least 26 Dot/Icm substrates have been shown to manipulate ubiquitin signaling via diverse mechanisms. Among these, 14 are E3 ligases that either cooperate with host E1 and E2 enzymes or adopt E1/E2-independent catalytic mechanisms. In the present study, we demonstrate that the L. pneumophila effector Legionella ubiquitin ligase gene 15 (Lug15) is a novel ubiquitin E3 ligase. Lug15 is involved in the remodeling of LCV with polyubiquitinated species. Moreover, Lug15 catalyzes the ubiquitination of host SNARE protein Sec22b and mediates its recruitment to the LCV. Ubiquitination of Sec22b by Lug15 promotes its noncanonical pairing with plasma membrane-derived syntaxins (e.g., Stx3). Our study further reveals the complexity of strategies utilized by L. pneumophila to interfere with host functions by hijacking host ubiquitin signaling.

Phosphoribosyl-linked serine ubiquitination of USP14 by the SidE family effectors of Legionella excludes p62 from the bacterial phagosome.

Xenophagy is an evolutionarily conserved host defensive mechanism to eliminate invading microorganisms through autophagic machinery. The intracellular bacterial pathogen Legionella pneumophila can avoid clearance by the xenophagy pathway via the actions of multiple Dot/Icm effector proteins. Previous studies have shown that p62, an adaptor protein involved in xenophagy signaling, is excluded from Legionella-containing vacuoles (LCVs). Such defects are attributed to the multifunctional SidE family effectors (SidEs) that exhibit classic deubiquitinase (DUB) and phosphoribosyl ubiquitination (PR-ubiquitination) activities, yet the mechanism remains elusive. In the present study, we demonstrate that the host DUB USP14 is PR-ubiquitinated by SidEs at multiple serine residues, which impairs its DUB activity and its interactions with p62. The exclusion of p62 from the bacterial phagosome requires the ubiquitin ligase but not the DUB activity of SidEs. These results reveal that PR-ubiquitination of USP14 by SidEs contributes to the evasion of xenophagic clearance by L. pneumophila.

Legionella longbeachae Regulates the Association of Polyubiquitinated Proteins on Bacterial Phagosome with Multiple Deubiquitinases.

Legionella spp. are the causative agents of a severe pneumonia known as Legionnaires' disease. Upon being engulfed by host cells, these environmental bacteria replicate intracellularly in a plasma membrane-derived niche termed Legionella-containing vacuole (LCV) in a way that requires the defective in organelle trafficking/intracellular multiplication (Dot/Icm) protein transporter. Our understanding of interactions between Legionella and its hosts was mostly based on studies of Legionella pneumophila. In this study, we found that the LCVs created by virulent Legionella longbeachae are similarly decorated by polyubiquitinated proteins to those formed by L. pneumophila and that the ubiquitin-proteasome system (UPS) is required for optimal intracellular growth of L. longbeachae. Furthermore, we utilized bioinformatics methods and the ubiquitin-vinylmethyl ester probe to obtain potential deubiquitinases (DUBs) encoded by L. longbeachae. These efforts led to the identification of 9 L. longbeachae DUBs that displayed distinct specificity toward ubiquitin chain types. Among these, LLO_1014 and LLO_2238 are associated with the LCVs and impact the accumulation of polyubiquitinated species on the bacterial phagosome. Moreover, LLO_1014 and LLO_2238 could fully restore the phenotypes associated with Δceg23 (lotB) and Δlem27 (lotC) mutants of L. pneumophila, indicating that these DUBs have similar functions. Together, these results reveal that L. longbeachae uses multiple DUBs to construct an intracellular niche for its replication. IMPORTANCE Legionella spp. are opportunistic intracellular bacterial pathogens that cause Legionnaires' disease. Legionella utilizes the Dot/Icm type IV secretion system to deliver effector protein into host cells to modulate various cellular functions. At least 26 L. pneumophila effectors are known to hijack the host ubiquitin system via diverse mechanisms. L. longbeachae is the second leading cause of Legionnaires' disease worldwide. However, our knowledge about the interactions between L. longbeachae and its hosts is very limited. Here, we found that, similar to L. pneumophila infection, the host ubiquitin proteasome system is also important for the intracellular replication of L. longbeachae. In addition, the bacterial phagosomes harboring L. longbeachae are enriched with polyubiquitinated proteins in a Dot/Icm system-dependent manner. We further identified 9 L. longbeachae proteins that function as DUBs with distinct ubiquitin chain specificity. Of note, several of the phagosome-associated L. longbeachae DUBs regulate the recruitment of polyubiquitinated proteins to the LCV.

Legionella longbeachae effector protein RavZ inhibits autophagy and regulates phagosome ubiquitination during infection.

Legionella organisms are ubiquitous environmental bacteria that are responsible for human Legionnaires' disease, a fatal form of severe pneumonia. These bacteria replicate intracellularly in a wide spectrum of host cells within a distinct compartment termed the Legionella-containing vacuole (LCV). Effector proteins translocated by the Dot/Icm apparatus extensively modulate host cellular functions to aid in the biogenesis of the LCV and intracellular proliferation. RavZ is an L. pneumophila effector that functions as a cysteine protease to hydrolyze lipidated LC3, thereby compromising the host autophagic response to bacterial infection. In this study, we characterized the RavZ (RavZLP) ortholog in L. longbeachae (RavZLLO), the second leading cause of Legionella infections in the world. RavZLLO and RavZLP share approximately 60% sequence identity and a conserved His-Asp-Cys catalytic triad. RavZLLO is recognized by the Dot/Icm systems of both L. pneumophila and L. longbeachae. Upon translocation into the host, it suppresses autophagy signaling in cells challenged with both species, indicating the functional redundancy of RavZLLO and RavZLP. Additionally, ectopic expression of RavZLLO but not RavZLP in mammalian cells reduces the levels of cellular polyubiquitinated and polyneddylated proteins. Consistent with this process, RavZLLO regulates the accumulation of polyubiquitinated species on the LCV during L. longbeachae infection.

The E3 ligase RNF5 restricts SARS-CoV-2 replication by targeting its envelope protein for degradation.

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a severe global health crisis; its structural protein envelope (E) is critical for viral entry, budding, production, and induction of pathology which makes it a potential target for therapeutics against COVID-19. Here, we find that the E3 ligase RNF5 interacts with and catalyzes ubiquitination of E on the 63rd lysine, leading to its degradation by the ubiquitin-proteasome system (UPS). Importantly, RNF5-induced degradation of E inhibits SARS-CoV-2 replication and the RNF5 pharmacological activator Analog-1 alleviates disease development in a mouse infection model. We also found that RNF5 is distinctively expressed in different age groups and in patients displaying different disease severity, which may be exploited as a prognostic marker for COVID-19. Furthermore, RNF5 recognized the E protein from various SARS-CoV-2 strains and SARS-CoV, suggesting that targeting RNF5 is a broad-spectrum antiviral strategy. Our findings provide novel insights into the role of UPS in antagonizing SARS-CoV-2 replication, which opens new avenues for therapeutic intervention to combat the COVID-19 pandemic.

Acinetobacter baumannii Kills Fungi via a Type VI DNase Effector.

Many Gram-negative bacteria deploy a type VI secretion system (T6SS) to inject toxins into target cells to promote their survival and replication in complex environments. Here, we report that Acinetobacter baumannii uses its T6SS to kill fungi and that the effector TafE (ACX60_15365) is responsible for such killing. Although ectopically expressed TafE is toxic to both Escherichia coli and Saccharomyces cerevisiae, deletion of tafE only affects the antifungal activity of A. baumannii. We demonstrate that TafE is a DNase capable of targeting the nuclei of yeast cells and that an Ntox15 domain is essential for its ability to degrade DNA. Furthermore, our findings show that A. baumannii is protected from the toxicity of TafE by elaborating the immunity protein TaeI (ACX60_15360), which antagonizes the activity of the effector by direct binding. The discovery of A. baumannii T6SS effectors capable of killing multiple taxonomically distinct microbes has shed light on a mechanism of the high-level fitness of this pathogen in environments characterized by scarce nutrients and the potential presence of diverse microorganisms. IMPORTANCE Acinetobacter baumannii is an increasing important nosocomial pathogen that is difficult to combat due to its ability to survive in harsh environments and the emergence of isolates that are resistant to multiple antibiotics. A better understanding of the mechanism underlying the toughness of A. baumannii may identify its Achilles' heel, which will facilitate the development of novel preventive and treatment measures. In this study, our findings show that A. baumannii kills fungi with the DNase effector TafE injected into competitor cells by its type VI secretion system. A. baumannii is protected from the activity of TafE by the immunity protein TaeI, which inactivates the effector by direct binding. Our results suggest that inactivation of its T6SS or effectors may reduce the fitness of A. baumannii and increase the effectiveness of treatment by means such as antibiotics. Furthermore, our finding suggests that targeted degradation of TaeI may be an effective strategy to kill A. baumannii.

Atypical Legionella GTPase effector hijacks host vesicular transport factor p115 to regulate host lipid droplet.

The intracellular bacterial pathogen Legionella pneumophila uses hundreds of effector proteins to manipulate multiple processes of the host cells to establish a replicative niche known as Legionella-containing vacuole (LCV). Biogenesis of the LCV has been known to depend on host small guanosine triphosphatases (GTPases), but whether bacterial effector GTPases are also involved remains unknown. Here, we show that an ankyrin repeat containing effector LegA15 localizes directly in host lipid droplets (LDs), leading to Golgi apparatus fragmentation of the host cells by hijacking the host vesicular transport factor p115. LegA15 is a GTPase with a unique catalytic mechanism, unlike any eukaryotic small GTPases. Moreover, the effector LegA15 co-opts p115 to modulate homeostasis of the host LDs in its GTPase-dependent manner. Together, our data reveal that an atypical GTPase effector regulates the host LDs through impeding the vesicle secretion system of the host cells for intracellular life cycle of Legionella.

Molecular mechanism of toxin neutralization in the HipBST toxin-antitoxin system of Legionella pneumophila.

Toxin-antitoxin (TA) systems are ubiquitous genetic modules in bacteria and archaea. Here, we perform structural and biochemical characterization of the Legionella pneumophila effector Lpg2370, demonstrating that it is a Ser/Thr kinase. Together with two upstream genes, lpg2370 constitutes the tripartite HipBST TA. Notably, the toxin Lpg2370 (HipTLp) and the antitoxin Lpg2369 (HipSLp) correspond to the C-terminus and N-terminus of HipA from HipBA TA, respectively. By determining crystal structures of autophosphorylated HipTLp, its complex with AMP-PNP, and the structure of HipTLp-HipSLp complex, we identify residues in HipTLp critical for ATP binding and those contributing to its interactions with HipSLp. Structural analysis reveals that HipSLp binding induces a loop-to-helix shift in the P-loop of HipTLp, leading to the blockage of ATP binding and inhibition of the kinase activity. These findings establish the L. pneumophila effector Lpg2370 as the HipBST TA toxin and elucidate the molecular basis for HipT neutralization in HipBST TA.

Maintaining home integrity by bacterial pathogens: Disruption for the sake of construction.

Caspases are often considered the final checkpoint for a pathogen to save its replicative niche from collapsing after cell death signaling has been initiated in response to infection. Two recent works (Li et al., 2021; Peng et al., 2022) found that pathogens inhibit host cell death by inactivating multiple caspases with a novel posttranslational modification.

Legionella pneumophila regulates host cell motility by targeting Phldb2 with a 14-3-3ζ-dependent protease effector.

The cytoskeleton network of eukaryotic cells is essential for diverse cellular processes, including vesicle trafficking, cell motility, and immunity, thus is a common target for bacterial virulence factors. A number of effectors from the bacterial pathogen Legionella pneumophila have been shown to modulate the function of host actin cytoskeleton to construct the Legionella-containing vacuole (LCV) permissive for its intracellular replication. In this study, we found that the Dot/Icm effector Lem8 (Lpg1290) is a protease whose activity is catalyzed by a Cys-His-Asp motif known to be associated with diverse biochemical activities. Intriguingly, we found that Lem8 interacts with the host regulatory protein 14-3-3ζ, which activates its protease activity. Furthermore, Lem8 undergoes self-cleavage in a process that requires 14-3-3ζ. We identified the Pleckstrin homology-like domain-containing protein Phldb2 involved in cytoskeleton organization as a target of Lem8 and demonstrated that Lem8 plays a role in the inhibition of host cell migration by attacking Phldb2.

Legionella pneumophila modulates host energy metabolism by ADP-ribosylation of ADP/ATP translocases.

The intracellular pathogen Legionella pneumophila delivers more than 330 effectors into host cells by its Dot/Icm secretion system. Those effectors direct the biogenesis of the Legionella-containing vacuole (LCV) that permits its intracellular survival and replication. It has long been documented that the LCV is associated with mitochondria and a number of Dot/Icm effectors have been shown to target to this organelle. Yet, the biochemical function and host cell target of most of these effectors remain unknown. Here, we found that the Dot/Icm substrate Ceg3 (Lpg0080) is a mono-ADP-ribosyltransferase that localizes to the mitochondria in host cells where it attacks ADP/ATP translocases by ADP-ribosylation, and blunts their ADP/ATP exchange activity. The modification occurs on the second arginine residue in the -RRRMMM- element, which is conserved among all known ADP/ATP carriers from different organisms. Our results reveal modulation of host energy metabolism as a virulence mechanism for L. pneumophila.

Coxiella burnetii inhibits host immunity by a protein phosphatase adapted from glycolysis.

Coxiella burnetii is a bacterial pathogen that replicates within host cells by establishing a membrane-bound niche called the Coxiella-containing vacuole. Biogenesis of this compartment requires effectors of its Dot/Icm type IV secretion system. A large cohort of such effectors has been identified, but the function of most of them remain elusive. Here, by a cell-based functional screening, we identified the effector Cbu0513 (designated as CinF) as an inhibitor of NF-κB signaling. CinF is highly similar to a fructose-1,6-bisphosphate (FBP) aldolase/phosphatase present in diverse bacteria. Further study reveals that unlike its ortholog from Sulfolobus tokodaii, CinF does not exhibit FBP phosphatase activity. Instead, it functions as a protein phosphatase that specifically dephosphorylates and stabilizes IκBα. The IκBα phosphatase activity is essential for the role of CinF in C. burnetii virulence. Our results establish that C. burnetii utilizes a protein adapted from sugar metabolism to subvert host immunity.

A bacterial kinase phosphorylates OSK1 to suppress stomatal immunity in rice.

The Xanthomonas outer protein C2 (XopC2) family of bacterial effectors is widely found in plant pathogens and Legionella species. However, the biochemical activity and host targets of these effectors remain enigmatic. Here we show that ectopic expression of XopC2 promotes jasmonate signaling and stomatal opening in transgenic rice plants, which are more susceptible to Xanthomonas oryzae pv. oryzicola infection. Guided by these phenotypes, we discover that XopC2 represents a family of atypical kinases that specifically phosphorylate OSK1, a universal adaptor protein of the Skp1-Cullin-F-box ubiquitin ligase complexes. Intriguingly, OSK1 phosphorylation at Ser53 by XopC2 exclusively increases the binding affinity of OSK1 to the jasmonate receptor OsCOI1b, and specifically enhances the ubiquitination and degradation of JAZ transcription repressors and plant disease susceptibility through inhibiting stomatal immunity. These results define XopC2 as a prototypic member of a family of pathogenic effector kinases and highlight a smart molecular mechanism to activate jasmonate signaling.