Type III effector AvrPtoB requires intrinsic E3 ubiquitin ligase activity to suppress plant cell death and immunity

Abstract

Microbial pathogens of both plants and animals employ virulence factors that suppress the host immune response. The tomato pathogen Pseudomonas syringae injects the AvrPtoB type III effector protein into the plant cell to suppress programmed cell death (PCD) associated with plant immunity. AvrPtoB also inhibits PCD in yeast, indicating that AvrPtoB manipulates a conserved component of eukaryotic PCD. To identify host targets of AvrPtoB, we performed a yeast two-hybrid screen and identified tomato ubiquitin (Ub) as a strong AvrPtoB interactor. AvrPtoB is ubiquitinated in vitro and exhibits E3 Ub ligase activity in the presence of recombinant E1 activating enzyme and specific E2 Ub-conjugating enzymes. The C terminus of AvrPtoB is sufficient for both anti-PCD and E3 Ub ligase activities, suggesting the two functions are associated. Indeed, mutation of AvrPtoB lysine residues in the C terminus, between K512 and K529, disrupts AvrPtoB-Ub interactions, decreases AvrPtoB-mediated anti-PCD activity, and abrogates P. syringae pathogenesis of susceptible tomato plants. Remarkably, quantitative decreases in AvrPtoB anti-PCD activity are correlated with decreases in AvrPtoB ubiquitination and E3 Ub ligase activity. Overall, these data reveal a unique bacterial pathogenesis strategy, where AvrPtoB manipulates the host Ub system and requires intrinsic E3 Ub ligase activity to suppress plant immunity.pathogenesis ͉ plant disease ͉ programmed cell death ͉ ubiquitin ͉ Pseudomonas

Ralstonia solanacearum requires F-box-like domain-containing type III effectors to promote disease on several host plants

“…These bacteria manage to do so by mimicking components of their eukaryotic hosts (28,29), and some have been shown to target key cell mechanisms: There are examples of T3SS effectors that interfere with the transcription machinery (30,31), whereas others are functional cysteine proteases and can cleave plant proteins (2, 3, 5, 32). Bacterial T3SS effectors that directly interfere with their host ubiquitin͞proteasome pathway have recently been identified and studied in Shigella flexneri (OspG, inhibiting E2 ubiquitin-conjugating enzyme) (33) and Pseudomonas syringae (AvrPtoB, U-box type E3-ubiquitin ligase) (34,35). The work on AvrPtoB highlights that bacterial T3SS effectors can mimic a eukaryotic E3 ubiquitin ligase.…”

Effector Biology of Plant-Associated Organisms: Concepts and Perspectives

“…Some effectors, particularly bacterial T3SS effectors, are enzymes that biochemically modify host molecules, typically impeding their function or eliminating them (Cunnac et al 2009;Deslandes and Rivas 2012). The enzymatic activities of effectors are diverse and include protease, hydrolase, phosphatase, kinase, transferase, and ubiquitin ligase activities (Shao et al 2003;Abramovitch et al 2006b;Janjusevic et al 2006;Fu et al 2007;Lee et al 2012a;Rodriguez-Herva et al 2012;van Damme et al 2012). Other effectors do not carry enzymatic activities and act by binding host proteins to modulate their functions.…”

XopD SUMO Protease Affects Host Transcription, Promotes Pathogen Growth, and Delays Symptom Development inXanthomonas-Infected Tomato Leaves

“…T3S-dependent suppressors of host basal defense responses include HopAI1, a phosphothreonine lyase that inhibits the phosphorylation of mitogen-activated protein kinases, MPK3 and MPK6 (Zhang et al, 2007); HopU1, a mono-ADP-ribosyltransferase that modifies Gly-rich RNA binding protein GRP7 (Fu et al, 2007); and HopM1, an inhibitor of vesicle trafficking via proteasome-dependent degradation of At MIN7 (Arabidopsis thaliana HopM interactor 7), an adenosine diphosphate ribosylation factor guanine nucleotide exchange factor (Nomura et al, 2006). Suppressors of host R proteindependent responses include AvrRpt2, a Cys protease that degrades RIN4 interfering with RPM1-dependent defense signaling (Axtell et al, 2003;Mackey et al, 2003) and the AvrPtoB E3 ligase, a general suppressor of programmed cell death (Abramovitch et al, 2006b;Rosebrock et al, 2007).…”