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Proc Natl Acad Sci U S A. 2016 Nov 8;113(45):12856-12861. doi: 10.1073/pnas.1609922113. Epub 2016 Oct 17.

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

Author information

1
School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience and Australian Infectious Diseases Research Centre, University of Queensland, Brisbane, QLD 4072, Australia.
2
Centre for Advanced Imaging, University of Queensland, Brisbane, QLD 4072, Australia.
3
School of Biological Sciences, Flinders University, Adelaide, SA 5001, Australia.
4
Commonwealth Scientific and Industrial Research Organization Agriculture, Canberra, ACT 2601, Australia.
5
Macromolecular Crystallography (MX) Beamlines, Australian Synchrotron, Melbourne, VIC 3168, Australia.
6
School of Biomedical Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia.
7
Centre of Excellence for Integrated Approaches in Chemistry and Biology of Proteins, International Postgraduate School Jozef Stefan, Jozef Stefan Institute, 1000 Ljubljana, Slovenia.
8
Centre for Advanced Imaging, University of Queensland, Brisbane, QLD 4072, Australia; s.williams8@uq.edu.au m.mobli@uq.edu.au b.kobe@uq.edu.au.
9
School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience and Australian Infectious Diseases Research Centre, University of Queensland, Brisbane, QLD 4072, Australia; s.williams8@uq.edu.au m.mobli@uq.edu.au b.kobe@uq.edu.au.
10
Plant Sciences Division, Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia.

Abstract

Plants use intracellular immunity receptors, known as nucleotide-binding 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 strain-specific 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.

KEYWORDS:

NLR proteins; effector-triggered immunity; nuclear magnetic resonance spectroscopy; plant innate immunity; resistance protein

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