Abstract
Upon recognition of pathogen virulence effectors, plant nucleotide-binding leucine-rich repeat (NLR) proteins induce defense responses including localized host cell death. In an effort to understand the molecular mechanisms leading to this response, we examined the Arabidopsis thaliana NLR protein RECOGNITION OF PERONOSPORA PARASITICA1 (RPP1), which recognizes the Hyaloperonospora arabidopsidis effector ARABIDOPSIS THALIANA RECOGNIZED1 (ATR1). Expression of the N-terminus of RPP1, including the Toll/interleukin-1 receptor (TIR) domain (“N-TIR”), elicited an effector-independent cell death response, and we used allelic variation in TIR domain sequences to define the key residues that contribute to this phenotype. Further biochemical characterization indicated that cell death induction was correlated with N-TIR domain self-association. In addition, we demonstrated that the nucleotide-binding (NB)-ARC1 region of RPP1 self-associates and plays a critical role in cell death activation, likely by facilitating TIR:TIR interactions. Structural homology modeling of the NB subdomain allowed us to identify a putative oligomerization interface that was shown to influence NB-ARC1 self-association. Significantly, full-length RPP1 exhibited effector-dependent oligomerization and, although mutations at the NB-ARC1 oligomerization interface eliminated cell death induction, RPP1 self-association was unaffected, suggesting that additional regions contribute to oligomerization. Indeed, the leucine-rich repeat domain of RPP1 also self-associates, indicating that multiple interaction interfaces exist within activated RPP1 oligomers. Finally, we observed numerous intramolecular interactions that likely function to negatively regulate RPP1, and present a model describing the transition to an active NLR protein.
Highlights
Through interactions with a plethora of phytopathogenic organisms, plants have evolved a sophisticated molecular surveillance system that involves nucleotide-binding leucine-rich repeat (NLR) immune receptors that recognize pathogen-derived virulence effector proteins
Plants have evolved receptors, described as nucleotide-binding leucine-rich repeat (NLR) proteins, which recognize the activity of pathogen effectors and stimulate defense responses
We focus on RECOGNITION OF PERONOSPORA PARASITICA1 (RPP1), an NLR from Arabidopsis that recognizes the oomycete effector ARABIDOPSIS THALIANA RECOGNIZED1 (ATR1)
Summary
Through interactions with a plethora of phytopathogenic organisms, plants have evolved a sophisticated molecular surveillance system that involves nucleotide-binding leucine-rich repeat (NLR) immune receptors that recognize pathogen-derived virulence effector proteins. Plant NLR proteins are generally modular in structure, with a C-terminal leucine-rich repeat (LRR) domain often being responsible for effector recognition, either directly or indirectly. Immune responses are thought to signal through the N-terminal region of the NLR protein, which may comprise an N-terminal coiled-coil (CC) or Toll/ interleukin-1 receptor (TIR)-like domain. Effector recognition by the LRR likely causes a conformational change that increases the accessibility of the NB-ARC domain to allow nucleotide exchange and subsequent NLR protein activation, the specific structural changes that lead to this active state are unclear
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