Structure-Function Analysis of Immune Receptor AtRLP23 with Its Ligand nlp20 and Coreceptors AtSOBIR1 and AtBAK1

Pattern-triggered immunity is an inherent feature of the plant immune system. Recognition of either microbe-derived surface structures (patterns) or of plant materials released due to the deleterious impact of microbial infection is brought about by plasma membrane pattern recognition receptors (PRRs). PRRs composed of leucine-rich repeat (LRR) ectodomains are thought to mediate sensing of proteinaceous patterns and to initiate signaling cascades culminating in the activation of generic plant defenses. In contrast to LRR receptor kinases, LRR receptor proteins (LRR-RPs) lack a cytoplasmic kinase domain for initiation of downstream signal transduction. LRR-RPs form heteromeric constitutive, ligand-independent complexes with coreceptor SOBIR1. Upon ligand binding to LRR-RPs, recruitment of coreceptor SERK3/BAK1 results in formation of a ternary PRR complex. Structure-function analysis of LRR-RP-type PRRs is missing. AtRLP23 constitutes an LRR-RP PRR that mediates recognition of a peptide motif (nlp20) found in numerous bacterial, fungal, and oomycete necrosis and ethylene-inducing peptide 1-like proteins (NLPs). We here report the use of a series of AtRLP23 variants to decipher subdomains required for ligand binding and interaction with coreceptors AtSOBIR1 and AtBAK1, respectively. Deletion of LRR1 or LRR3 subdomains efficiently abrogated the ability of AtRLP23 receptor variants to confer nlp20 pattern sensitivity, to bind nlp20, and to recruit AtBAK1 into a ternary PRR complex. This suggests that the very N-terminal part of the AtRLP23 ectodomain is crucial for receptor function. Deletion of the intracellular 17-amino-acid tail of AtRLP23 reduced but did not abolish receptor function, suggesting an auxiliary role of this domain in receptor function. We further found that interaction of AtRLP23 and other LRR-RP-type PRRs with AtSOBIR1 does not require the AtRLP23 LRR ectodomain but is brought about by a GxxxG protein dimerization motif in the transmembrane domain and a stretch of negatively charged glutamic acid residues in the outer juxta-membrane domain of the receptor. Further, AtRLP23 levels were found to be unaltered in Atsobir1-1 mutant genotypes, suggesting that AtSOBIR1 does not act as a protein scaffold in stabilizing LRR-RP-type PRRs in Arabidopsis.