Structural Determinants at the Interface of the ARC2 and LRR Domains Control the Activation of the NB-LRR Plant Immune Receptors Rx1 and Gpa2

Many plant and animal immune receptors have a modular NB-LRR architecture in which a nucleotide-binding switch domain (NB-ARC) is tethered to a leucine-rich repeat sensor domain (LRR). The cooperation between the switch and sensor domains, which regulates the activation of these proteins, is poorly understood. Here we report structural determinants governing the interaction between the NB-ARC and LRR in the highly homologous plant immune receptors Gpa2 and Rx1, which recognize the potato cyst nematode Globodera pallida and Potato Virus X, respectively. Systematic shuffling of polymorphic sites between Gpa2 and Rx1 showed that a minimal region in the ARC2 and the N-terminal repeats of the LRR domain coordinate the activation state of the protein. We identified two closely spaced amino acid residues in this region of the ARC2 (position 401 and 403) that distinguish between autoactivation and effector-triggered activation. Furthermore, a highly acidic loop region in the ARC2 domain and basic patches in the N-terminal end of the LRR domain were demonstrated to be required for the physical interaction between the ARC2 and LRR. The NB-ARC and LRR domains dissociate upon effector-dependent activation and the complementary charged regions are predicted to mediate a fast re-association enabling multiple rounds of activation. Finally, we present a mechanistic model showing how the ARC2, NB and N-terminal half of the LRR form a clamp, which regulates the dissociation and re-association of the switch and sensor domains in NB-LRR proteins.