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The structural basis for activation of plant immunity by bacterial effector protein AvrPto


Pathogenic microbes use effectors to enhance susceptibility in host plants. However, plants have evolved a sophisticated immune system to detect these effectors using cognate disease resistance proteins1, a recognition that is highly specific, often elicits rapid and localized cell death, known as a hypersensitive response, and thus potentially limits pathogen growth2,3,4,5. Despite numerous genetic and biochemical studies on the interactions between pathogen effector proteins and plant resistance proteins, the structural bases for such interactions remain elusive. The direct interaction between the tomato protein kinase Pto and the Pseudomonas syringae effector protein AvrPto is known to trigger disease resistance and programmed cell death6,7 through the nucleotide-binding site/leucine-rich repeat (NBS-LRR) class of disease resistance protein Prf8. Here we present the crystal structure of an AvrPto–Pto complex. Contrary to the widely held hypothesis that AvrPto activates Pto kinase activity, our structural and biochemical analyses demonstrated that AvrPto is an inhibitor of Pto kinase in vitro. The AvrPto–Pto interaction is mediated by the phosphorylation-stabilized P+1 loop and a second loop in Pto, both of which negatively regulate the Prf-mediated defences in the absence of AvrPto in tomato plants. Together, our results show that AvrPto derepresses host defences by interacting with the two defence-inhibition loops of Pto.

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Figure 1: Bipartite AvrPto–Pto interfaces.
Figure 2: The active conformation of Pto is important for AvrPto binding.
Figure 3: Mutations in the two AvrPto–Pto interfaces trigger a CGF hypersensitive response in N. benthamiana.
Figure 4: AvrPto inhibits the kinase activity of Pto in vitro.


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We thank R. Innes, S. He and X. Tang for critical reading and comments on our manuscript, and Y. Dong and P. Liu at the BSRF (Beijing, China) beam line 3W1A for assistance with the data collection. We are grateful to X. Liu and L. Ma for help with SPR assay. This research is funded by a Chinese Ministry of Science and Technology grant to J.C. and to J.-M.Z.

Author Contributions W.X. purified, crystallized and determined the structure and performed biochemical assays; Y.Z. performed Agrobacterium-mediated transient expression; Q.L., Q. Huang and Q. Hao determined structure; J.L. and X.L. purified proteins; S.C. performed the mass spectrometry assay; J.-W.W. measured the half-maximal inhibitory concentration; R.B. and L.Z. were involved in the study design; and J.-M.Z. and J.C. designed the study, analysed data and prepared the manuscript.

The atomic coordinates and structure factors of the AvrPto–Pto complex have been deposited in the RCSB Protein Data Bank under accession code 2QKW.

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Correspondence to Jijie Chai.

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Xing, W., Zou, Y., Liu, Q. et al. The structural basis for activation of plant immunity by bacterial effector protein AvrPto. Nature 449, 243–247 (2007).

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