Autoacetylation of the Ralstonia solanacearum Effector PopP2 Targets a Lysine Residue Essential for RRS1-R-Mediated Immunity in Arabidopsis

Céline Tasset,Susana Rivas,Laurent Deslandes,Sylvie Kieffer-Jacquinod,Maud Bernoux,Cécile Pouzet,Christian Brière,Alain Jauneau,Yves Marco,Jeffery L. Dangl

doi:10.1371/journal.ppat.1001202

Abstract

Type III effector proteins from bacterial pathogens manipulate components of host immunity to suppress defence responses and promote pathogen development. In plants, host proteins targeted by some effectors called avirulence proteins are surveyed by plant disease resistance proteins referred to as “guards”. The Ralstonia solanacearum effector protein PopP2 triggers immunity in Arabidopsis following its perception by the RRS1-R resistance protein. Here, we show that PopP2 interacts with RRS1-R in the nucleus of living plant cells. PopP2 belongs to the YopJ-like family of cysteine proteases, which share a conserved catalytic triad that includes a highly conserved cysteine residue. The catalytic cysteine mutant PopP2-C321A is impaired in its avirulence activity although it is still able to interact with RRS1-R. In addition, PopP2 prevents proteasomal degradation of RRS1-R, independent of the presence of an integral PopP2 catalytic core. A liquid chromatography/tandem mass spectrometry analysis showed that PopP2 displays acetyl-transferase activity leading to its autoacetylation on a particular lysine residue, which is well conserved among all members of the YopJ family. These data suggest that this lysine residue may correspond to a key binding site for acetyl-coenzyme A required for protein activity. Indeed, mutation of this lysine in PopP2 abolishes RRS1-R-mediated immunity. In agreement with the guard hypothesis, our results favour the idea that activation of the plant immune response by RRS1-R depends not only on the physical interaction between the two proteins but also on its perception of PopP2 enzymatic activity.

Highlights

To defend themselves against pathogen infection, plants have evolved a bipartite, inducible innate immune system that efficiently recognizes and wards off pathogens [1,2,3]
Plant and animal bacterial pathogens have evolved to produce virulence factors, called type III effectors, which are injected into host cells to suppress host defences and provide an environment beneficial for pathogen growth
In addition to demonstrating PopP2 ability to stabilize the expression of its cognate Arabidopsis RRS1-R resistance protein and physically interact with it, we investigated the enzymatic activity of PopP2

Summary

Introduction

To defend themselves against pathogen infection, plants have evolved a bipartite, inducible innate immune system that efficiently recognizes and wards off pathogens [1,2,3]. Plants can perceive specific effectors [previously referred to as avirulence (Avr) proteins] through additional receptors –typically nucleotide binding site-leucine rich repeat (NB-LRR) resistance (R) proteins. This second layer of defence is called effector-triggered immunity (ETI). To explain the lack of evidence for direct interaction for most of known Avr-R pairs, the guard model was proposed [11,12] This model, validated in the case of several RAvr interactions, postulates that R proteins guard effector targets or ‘‘guardees’’ from effector-triggered manipulation in host cells [13,14,15,16]. Several effector targets have been identified and shown to play crucial a role in the establishment of plant resistance [17,18]

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Results

Discussion

Conclusion