SARM1 (sterile alpha and TIR motif containing 1) is responsible for depletion of nicotinamide adenine dinucleotide in its oxidized form (NAD+) during Wallerian degeneration associated with neuropathies. Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors recognize pathogen effector proteins and trigger localized cell death to restrict pathogen infection. Both processes depend on closely related Toll/interleukin-1 receptor (TIR) domains in these proteins, which, as we show, feature self-association–dependent NAD+ cleavage activity associated with cell death signaling. We further show that SARM1 SAM (sterile alpha motif) domains form an octamer essential for axon degeneration that contributes to TIR domain enzymatic activity. The crystal structures of ribose and NADP+ (the oxidized form of nicotinamide adenine dinucleotide phosphate) complexes of SARM1 and plant NLR RUN1 TIR domains, respectively, reveal a conserved substrate binding site. NAD+ cleavage by TIR domains is therefore a conserved feature of animal and plant cell death signaling pathways.

Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll-interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.

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

The self-association of Toll/interleukin-1 receptor/resistance protein (TIR) domains has been implicated in signaling in plant and animal immunity receptors. Structure-based studies identified different TIRdomain dimerization interfaces required for signaling of the plant nucleotide-binding oligomerization domain-like receptors (NLRs) L6 from flax and disease resistance protein RPS4 from Arabidopsis. Here we show that the crystal structure of the TIR domain from the Arabidopsis NLR suppressor of npr1-1, constitutive 1 (SNC1) contains both an L6-like interface involving helices αD and αE (DE interface) and an RPS4-like interface involving helices αA and αE (AE interface). Mutations in either the AE-or DE-interface region disrupt cell-death signaling activity of SNC1, L6, and RPS4 TIR domains and full-length L6 and RPS4. Selfassociation of L6 and RPS4 TIR domains is affected by mutations in either region, whereas only AE-interface mutations affect SNC1 TIR-domain self-association. We further show two similar interfaces in the crystal structure of the TIR domain from the Arabidopsis NLR recognition of Peronospora parasitica 1 (RPP1). These data demonstrate that both the AE and DE self-association interfaces are simultaneously required for self-association and cell-death signaling in diverse plant NLRs.plant immunity | NLR | TIR domain | plant disease resistance | signaling by cooperative assembly formation P lants have evolved a sophisticated innate immune system to detect pathogens, in which plant resistance (R) proteins recognize pathogen proteins (effectors) in a highly specific manner. This recognition leads to the effector-triggered immunity (ETI) response that often induces a localized cell death known as the hypersensitive response (1). Most R proteins belong to the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family. NLRs are prevalent in the immune systems of plants and animals and provide resistance to a broad range of pathogens, including fungi, oomycetes, bacteria, viruses, and insects (2, 3). NLRs contain a central nucleotide-binding (NB) domain, often referred to as the nucleotide-binding adaptor shared by APAF-1, resistance proteins, and CED-4 (NB-ARC domain) (4) and a C-terminal leucine-rich repeat (LRR) domain. Plant NLRs can be further classified into two main subfamilies, depending on the presence of either a Toll/interleukin-1 receptor domain (TIR-NLR) or a coiled-coil domain (CC-NLR) at their N termini (5).The CC and TIR domains of many plant NLRs can autonomously signal cell-death responses when expressed ectopically in planta, and mutations in these domains within full-length proteins also compromise signaling, suggesting that these domains are responsible for propagating the resistance signal after activation of the receptor (6-14). Self-association of both TIR (8, 9, 11, 15) and CC (10,13,16,17) domains has been shown to be important for the signaling function. In animal NLRs, the formation of postactivation oligomeric complexes, such as the NLRC4/NAIP inflammasome or...

Background: TcpB is a TIR domain-containing protein form Brucella. Results: TcpB interacts with the host Toll-like receptor and adaptors, and its structure reveals a dimer essential for activity. Conclusion: TcpB forms a nonfunctional complex with host molecules, thus suppressing signaling. Significance: The work explains the structural and functional basis of immune suppression by the protein TcpB from a pathogenic bacterium.