Plant Receptor-like proteins (RLPs): Structural features enabling versatile immune recognition

Plant immune recognition of pests and pathogens relies on a germline-encoded repertoire of innate immune receptors. In recent years, many examples of cell surface receptors in the large gene family of receptor-like proteins (RLPs) have been shown to govern recognition specificity. RLPs lack the canonical intracellular kinase domain associated with receptor kinases, but instead transduce immune signaling through interactions with co-receptor and adaptor kinases including BAK1 and SOBIR1. We review recognition and signaling functions mediated by subdomains of leucine-rich repeat (LRR) type RLPs. LRR-RLP extracellular domains, transmembrane motifs, and intracellular tails can mediate ligand binding, co-receptor recruitment, and immune signaling specificity. A recently reported cryo-EM structure of an LRR-RLP, RXEG1, now links domain architecture and sequence features of LRR-RLPs to key molecular functions. Finally, we propose a new motif-based classification of LRR-RLPs based on shared sequence features in the island domain (ID). In both dicot and monocot species, receptors encoding the Y-x8-KG and the newly defined K-x5-Y motif in their IDs correspond with conserved, sister clades of LRR-RLP genes. We propose that the conservation of distinct Y-x8-KG and K-x5-Y groups of LRR-RLPs implies that the motif is a useful classifier of immunity-related LRR-RLPs, distinguishing them from receptors with similar domain architecture but roles in growth and development. Deeply conserved structural features of LRR-RLPs likely provide a versatile platform for diversification of receptor repertoires and recognition specificities.