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

Abstract

Plants evolved intracellular immune receptors that belong to the NOD-like receptor (NLR) family to recognize the presence of pathogen-derived effector proteins. NLRs possess an N-terminal Toll-like/IL-1 receptor (TIR) or a non-TIR domain [some of which contain coiled coils (CCs)], a central nucleotide-binding (NB-ARC) domain, and a C-terminal leucine-rich repeat (LRR). Activation of NLR proteins results in a rapid and high-amplitude immune response, eventually leading to host cell death at the infection site, the so-called hypersensitive response. Despite their important contribution to immunity, the exact mechanisms of NLR activation and signaling remain unknown and are likely heterogenous. We undertook a detailed structure-function analysis of the plasma membrane (PM)-localized CC NLR Resistance to <i>Pseudomonas syringae pv. maculicola</i> 1 (RPM1) using both stable transgenic <i>Arabidopsis</i> and transient expression in <i>Nicotiana benthamiana</i> We report that immune signaling is induced only by activated full-length PM-localized RPM1. Our interaction analyses demonstrate the importance of a functional P-loop for <i>in planta</i> interaction of RPM1 with the small host protein RPM1-interacting protein 4 (RIN4), for constitutive preactivation and postactivation self-association of RPM1 and for proper PM localization. Our results reveal an additive effect of hydrophobic conserved residues in the CC domain for RPM1 function and RPM1 self-association and their necessity for RPM1-RIN4 interaction. Thus, our findings considerably extend our understanding of the mechanisms regulating NLR activation at, and signaling from, the PM.