Animal NLRs provide structural insights into plant NLR function

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Plants use intracellular immunity receptors, known as nucleotidebinding oligomerization domain-like receptors (NLRs), to recognize specific pathogen effector proteins and induce immune responses. These proteins provide resistance to many of the world's most destructive plant pathogens, yet we have a limited understanding of the molecular mechanisms that lead to defense signaling. We examined the wheat NLR protein, Sr33, which is responsible for strainspecific resistance to the wheat stem rust pathogen, Puccinia graminis f. sp. tritici. We present the solution structure of a coiled-coil (CC) fragment from Sr33, which adopts a four-helix bundle conformation. Unexpectedly, this structure differs from the published dimeric crystal structure of the equivalent region from the orthologous barley powdery mildew resistance protein, MLA10, but is similar to the structure of the distantly related potato NLR protein, Rx. We demonstrate that these regions are, in fact, largely monomeric and adopt similar folds in solution in all three proteins, suggesting that the CC domains from plant NLRs adopt a conserved fold. However, larger C-terminal fragments of Sr33 and MLA10 can self-associate both in vitro and in planta, and this self-association correlates with their cell death signaling activity. The minimal region of the CC domain required for both cell death signaling and self-association extends to amino acid 142, thus including 22 residues absent from previous biochemical and structural protein studies. These data suggest that self-association of the minimal CC domain is necessary for signaling but is likely to involve a different structural basis than previously suggested by the MLA10 crystallographic dimer.plant innate immunity | resistance protein | NLR proteins | effectortriggered immunity | nuclear magnetic resonance spectroscopy

Section: Discussionmentioning

confidence: 71%

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confidence: 99%

The CC domain structure from the wheat stem rust resistance protein Sr33 challenges paradigms for dimerization in plant NLR proteins

Casey

Lavrencic

Bentham

et al. 2016

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160

“…Recent studies of plant and animal NLR proteins support models in which self-and hetero-association of NLRs are key mechanisms of activation and signaling (13,14,31,32) (SI Appendix, Table S1). The tobacco TIR-NLR (TNL) N self-associates only in the presence of its cognate effector, and this self-association is P-loop dependent (33).…”

Section: Significancementioning

confidence: 86%

“…NLRs have a central nucleotide-binding site (NB-ARC) with homology to the animal immune receptors Apaf-1 and CED-4, C-terminal leucine-rich repeats (LRRs), and either a Toll-like/IL-1 receptor (TIR) domain or a non-TIR domain at their N termini. The latter contains a subclass containing N-terminal coiled-coil (CC) domains, subdividing NLRs into TNLs, CNLs, and others, respectively (13,14). The conserved nucleotide-binding site is critical for NLR activation, and negative regulation of this domain, likely via intramolecular interactions, is required to limit ectopic activation, which can result in hyperimmune signaling and ectopic cell death (15,16).…”

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confidence: 99%

Signaling from the plasma-membrane localized plant immune receptor RPM1 requires self-association of the full-length protein

Kasmi

Chung

Anderson

et al. 2017

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Plants evolved intracellular immune receptors that belong to the NOD-like receptor (NLR) family to recognize the presence of pathogenderived effector proteins. NLRs possess an N-terminal Toll-like/IL-1 receptor (TIR) or a non-TIR domain [some of which contain coiled coils (CCs)], a central nucleotide-binding (NB-ARC) domain, and a C-terminal leucine-rich repeat (LRR). Activation of NLR proteins results in a rapid and high-amplitude immune response, eventually leading to host cell death at the infection site, the so-called hypersensitive response. Despite their important contribution to immunity, the exact mechanisms of NLR activation and signaling remain unknown and are likely heterogenous. We undertook a detailed structure-function analysis of the plasma membrane (PM)-localized CC NLR Resistance to Pseudomonas syringae pv. maculicola 1 (RPM1) using both stable transgenic Arabidopsis and transient expression in Nicotiana benthamiana. We report that immune signaling is induced only by activated full-length PM-localized RPM1. Our interaction analyses demonstrate the importance of a functional P-loop for in planta interaction of RPM1 with the small host protein RPM1-interacting protein 4 (RIN4), for constitutive preactivation and postactivation self-association of RPM1 and for proper PM localization. Our results reveal an additive effect of hydrophobic conserved residues in the CC domain for RPM1 function and RPM1 self-association and their necessity for RPM1-RIN4 interaction. Thus, our findings considerably extend our understanding of the mechanisms regulating NLR activation at, and signaling from, the PM.plant immunity | effector-triggered immunity | NLR receptor | oligomerization | Arabidopsis thaliana

“…Homodimerization of the N-terminal domain is thought to activate downstream effector-triggered immunity (ETI). The mechanistic activation of NLRs and how they trigger downstream ETI remain obscure, although recent structural studies of animal analogs have begun to provide important details and support NLR oligomerization as a key feature of activation (7)(8)(9).…”

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confidence: 99%

TIR-only protein RBA1 recognizes a pathogen effector to regulate cell death inArabidopsis

Nishimura

Anderson

Cherkis

et al. 2017

139

131

Detection of pathogens by plants is mediated by intracellular nucleotide-binding site leucine-rich repeat (NLR) receptor proteins. NLR proteins are defined by their stereotypical multidomain structure: an N-terminal Toll-interleukin receptor (TIR) or coiled-coil (CC) domain, a central nucleotide-binding (NB) domain, and a C-terminal leucine-rich repeat (LRR). The plant innate immune system contains a limited NLR repertoire that functions to recognize all potential pathogens. We isolated Response to the bacterial type III effector protein HopBA1 (RBA1), a gene that encodes a TIR-only protein lacking all other canonical NLR domains. RBA1 is sufficient to trigger cell death in response to HopBA1. We generated a crystal structure for HopBA1 and found that it has similarity to a class of proteins that includes esterases, the hemebinding protein ChaN, and an uncharacterized domain of Pasteurella multocida toxin. Self-association, coimmunoprecipitation with HopBA1, and function of RBA1 require two previously identified TIR-TIR dimerization interfaces. Although previously described as distinct in other TIR proteins, in RBA1 neither of these interfaces is sufficient when the other is disrupted. These data suggest that oligomerization of RBA1 is required for function. Our identification of RBA1 demonstrates that "truncated" NLRs can function as pathogen sensors, expanding our understanding of both receptor architecture and the mechanism of activation in the plant immune system. plant immunity | NLR | Toll-interleukin-1 receptor homology domain | oligomerization | type III secretion