Structural and biochemical basis of activation of an NLR immune receptor network

This poster was presented at the British Society of Plant Pathology meeting held in Oxford University, UK in September 2024.

Abstract

Plant immune systems utilize intracellular nucleotide-binding domain and leucine-rich repeat (NLR) receptors to perceive and respond to pathogen-secreted proteins, termed effectors. In recent years, pivotal insights into NLR activation mechanisms stemmed from structural studies on the inactive and active Arabidopsis NLR ZAR1. To date, our understanding of NLR structure and function has predominantly been shaped by a limited set of “singleton” NLRs that function individually. Structural and biochemical insights on NLRs that activate immunity by working in higher-order configurations, such as pairs or networks, are lacking. We have leveraged a Nicotiana benthamiana protein purification pipeline, which we termed IP-EM, for accelerated structural biology to obtain structural insights on the NRC network — a hierarchical solanaceous NLR immune receptor network that mediates resistance against diverse pathogens. This network links sensor NLRs and cell-surface receptors to helper NLRs of the NRC clade. We report that, unlike ZAR1, NRC helpers exist as homodimers at resting state. Pathogen perception triggers conformational rearrangements in upstream sensors that allow them to signal via their central nucleotide-binding (NB) domain. The sensor NB domain is perceived by NRC homodimers and ultimately leads to their conversion into hexameric resistosomes. The activated NRC resistosomes then accumulate at the host plasma membrane, separate from the sensors that activated them, to initiate immune signaling. This activation mechanism is, biochemically and structurally distinct from the previously published ZAR1 example, highlighting the need to extend NLR mechanistic frameworks beyond the singleton model. As we uncover more NLR structural and functional diversity, revising the way in which we conceptualize NLRs, their domains and their roles in immune activation is crucial.