Effector Recognition and Activation of the Arabidopsis thaliana NLR Innate Immune Receptors

Plant immunity to coevolved pathogens relies on the perception of pathogenic effectors by nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins—sophisticated intracellular receptors that have both perception and signaling roles in activating defenses. Given the conserved domain architecture of NLRs, a structural biology perspective is particularly relevant to understanding mechanisms of their activation. Here, we summarize our recent findings on the Arabidopsis resistance protein RPP1, a member of the TIR-NBS-LRR family of plant NLRs that specifically recognizes the cognate effector protein ATR1. To study the basis of RPP1 activation, we have taken advantage of a series of RPP1 and ATR1 alleles that differentially condition resistance. In planta association between the LRR domain of RPP1 and ATR1 only occurs in a resistance-activating combination of alleles, suggesting that a direct interaction between RPP1 and ATR1 protein leads to activation of the NLR (Krasileva et al. 2010). Given critical amino acid residues ’ locations on the solved ATR1 crystal structure, and variable amino acid residues on the predicted LRR structure of RPP1, we hypothesize that specific “hotspots ” of the horseshoe-like LRR fold mediate binding to the ATR1 ligand and that polymorphismsmapping to these surfaces condition differences in allelic recognition specificity. We present docking models of a possible co-complex between RPP1 and ATR1, and we propose that ATR1 binding relieves autoinhibition of RPP1 resistance signaling. This is consistent with current models of activation for both TIR- and CC-type plant NLRs, where conformational changes could lead to NLR oligomerization, nucleotide binding, translocation, and other critical downstream events in triggering immunity