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

Abstract

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

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

“…Selfassociation and/or dimerization of NLR proteins or their N-terminal domains has been demonstrated to be necessary for immune signaling in animals and plants (31,32,45,48,49) (SI Appendix, Table S1). Therefore, we asked whether RPM1 also self-associates, and whether such self-association is necessary for RPM1 function.…”

Multiple strategies for pathogen perception by plant immune receptors

“…Several plant TIR domains have also been structurally characterized revealing oligomerization by two distinct interfaces that are each important for both self-association and defense signaling (Williams et al, 2014;Nishimura et al, 2017;Zhang et al, 2017) (Fig. 3a).…”

Autoimmunity and effector recognition in Arabidopsis thaliana can be uncoupled by mutations in the RRS1‐R immune receptor

“…The downstream signaling cascade that culminates in HR is initiated on correct TIR-TIR domain self-association, such as in flax L6 (Bernoux et al, 2011). Interestingly, it appears that TIR domains from different NLRs may employ a common selfassociation mechanism via multiple interfaces (Nishimura et al, 2017;Zhang et al, 2017b).…”