Two-faced TIRs trip the immune switch.

Both Plant and Animal Immune Receptors Can Carry a TIR Domain Mammals, in addition to their adaptive immune system based on somatic evolution of antibodies, carry an innate immune system based on both cell surface and intracellular immune receptors (1). In animals ranging from insects tomammals, Toll-like receptors (TLRs), with extracellular leucine-rich repeats (LRRs) and an intracellular Toll/interleukin-1 receptor/resistance protein (TIR) domain, recognize extracellular ligands such as bacterial lipopolysaccharide or flagellin, and then activate immunity (2). Ligand perception triggers receptor oligomerization, leading to engagement of TIR domain-containing adapter proteins, activation of protein kinases, and stimulation of activity of NF-κB or related transcription factors that induce defense-related genes (2). Plants also rely on innate immunity, and both plants and animals carry intracellular receptors of the nucleotide-binding, leucine-rich repeat (NLR) class (3, 4). In many plants, such NLRs can carry a TIR domain at their N termini (TNL proteins). Plant and animal TIR domains share a common structure comprising a flavodoxinlike fold with a central parallel β-sheet surrounded by α-helices, although plant TIRs have a longer αD helical region (5). For plant and animal TIR domains, interaction with other TIR domain molecules is key to activation, and thus the interfaces between these domains are in principle key to understanding mechanism (5). However, in the last few years, analysis of plant TIR domain structure and function, has revealed an apparent inconsistency. The flax L6 gene encodes a TNL that confers resistance to flax rust. The structure of the L6 TIR domain revealed an interface involved in TIR–TIR self-association that involves the αD and αE helices (6). In a separate study, the TIR domains of the NLR gene pair RPS4 and RRS1, both of which are TNLs, interact via a different interface that involves helices αA and αE (7). Does this mean different plant TNL TIR domains function in different ways? In two papers published in PNAS (8, 9), signaling mediated via several different TIR domains is shown to require the function of both αD/αE and αA/αE interfaces. Two-Faced Plant TIRs In one of these papers, Zhang et al. (8) report that, for TIR domains from Arabidopsis TNLs SNC1 and RPP1, and for the RPS4 and L6 TIR domains, both αD/αE and αA/αE interfaces contribute to TIR–TIR interactions and are required for function. To show this, the authors first solved a crystal structure of TIR, which revealed three molecules of the protein bound through both αD/αE and αA/αE interfaces. This demonstrated the capacity for the same TIR protein to oligomerize via both of these two interfaces. To explore their biological relevance, they exploited the fact that transient expression of TIR domains in Nicotiana benthamiana or Nicotiana tabacum leaves often activates a strong necrotic phenotype that may mimic the hypersensitive cell death response (HR) typically involved in NLR-mediated immune signaling. Mutations that perturb either the αD/αE or αA/αE interface of the SNC1, L6, or RPS4 abrogate this HR, and also appear to attenuate TIR–TIR interactions in solution (although these interactions are weak and transient in the wild-type proteins). A crystal structure of the TIR domain of the Arabidopsis RPP1 protein, which confers race-specific downy mildew resistance, also revealed a trimer involving both interfaces, although the interfaces are subtly different. Again, mutations in either of these interfaces abrogate the HR triggered upon transient expression of the RPP1-TIR in N. benthamiana. Interestingly, it appears that, in certain TIR–TIR interactions, the αA/αE surface forms the primary interface for interaction. Both αD/αE and αA/ αE interfaces are also required for TIR domain function in the context of the full-length L6 protein that recognizes the flax rust effector AvrL567. A mutation in either interface abrogates AvrL567-dependent HR. The same is true for mutations in the αD/αE and αA/αE interfaces of RPS4, both for RRS1-dependent effector recognition and for transducing the activity of RRS1 autoactive alleles. In a parallel study, Nishimura et al. (9) report a similar conclusion for a different and remarkable TIR domain protein from Arabidopsis, RBA1. They surveyed Arabidopsis diversity for recognition of an extensive set of