Kinase activity of SOBIR1 and BAK1 is required for immune signalling

Abstract

Summary Leucine‐rich repeat‐receptor‐like proteins (LRR‐RLPs) and LRR‐receptor‐like kinases (LRR‐RLKs) trigger immune signalling to promote plant resistance against pathogens. LRR‐RLPs lack an intracellular kinase domain, and several of these receptors have been shown to constitutively interact with the LRR‐RLK Suppressor of BIR1‐1/EVERSHED (SOBIR1/EVR) to form signalling‐competent receptor complexes. Ligand perception by LRR‐RLPs initiates recruitment of the co‐receptor BRI1‐Associated Kinase 1/Somatic Embryogenesis Receptor Kinase 3 (BAK1/SERK3) to the LRR‐RLP/SOBIR1 complex, thereby activating LRR‐RLP‐mediated immunity. We employed phosphorylation analysis of in planta ‐produced proteins, live cell imaging, gene silencing and co‐immunoprecipitation to investigate the roles of SOBIR1 and BAK1 in immune signalling. We show that Arabidopsis thaliana ( At ) SOBIR1, which constitutively activates immune responses when overexpressed in planta , is highly phosphorylated . Moreover, in addition to the kinase activity of SOBIR1 itself, kinase‐active BAK1 is essential for At SOBIR1‐induced constitutive immunity and for the phosphorylation of At SOBIR1. Furthermore, the defence response triggered by the tomato LRR‐RLP Cf‐4 on perception of Avr4 from the extracellular pathogenic fungus Cladosporium fulvum is dependent on kinase‐active BAK1. We argue that, in addition to the trans‐autophosphorylation of SOBIR1, it is likely that SOBIR1 and BAK1 transphosphorylate, and thereby activate the receptor complex. The signalling‐competent cell surface receptor complex subsequently activates downstream cytoplasmic signalling partners to initiate RLP‐mediated immunity.

An EFR‐Cf‐9 chimera confers enhanced resistance to bacterial pathogens by SOBIR1‐ and BAK1‐dependent recognition of elf18

“…Kinase activity of both SOBIR1 and BAK1 is required for the function of Cf‐4 (Liebrand et al ., ; Postma et al ., ; Van Der Burgh et al ., ). The At BAK1‐5 mutant contains a point mutation in its kinase domain, which causes this protein to have a slightly lower kinase activity than BAK1 itself (Schwessinger et al ., ).…”

Crystal structure of the LRR ectodomain of the plant immune receptor kinase SOBIR1

“…Conserved GxxxG motifs in their trans-membrane regions, but neither the SOBIR1 LRR ectodomain nor the kinase domain are required for this interaction to occur in planta (Bi et al, 2016). However, both an active kinase domain and the SOBIR1 LRR ectodomain are required for signalling (Bi et al, 2016;van der Burgh et al, 2019). The 1.55 Å crystal structure reveals that the SOBIR1 ectodomain has five LRRs sandwiched between unusual N-terminal and C-terminal capping domains (Fig.…”

“…Presently, it is known that a GxxxG motif in the SOBIR1 trans-membrane helix is required for the interaction with different RLPs (Bi et al, 2016). It has also been demonstrated that the kinase activity of SOBIR1 is essential for its signalling function (van der Burgh et al, 2019). Here, we present the crystal structure of the SOBIR1 ectodomain from Arabidopsis thaliana and discuss its implications for plant immune signalling.…”

Crystal structure of DRIK1, a stress-responsive receptor-like pseudokinase, reveals the molecular basis for the absence of ATP binding

“…These catalytically inactive kinases retain a high degree of sequence conservation in the kinase domain, suggesting that kinase domain fold and structure are required for signaling activity, while their different biological functions are driven by divergences in the extracellular domains [12,36,37]. Nevertheless, it has recently been shown that the specificity of some pseudokinases can also be determined by their intracellular kinase domains, while the ectodomains allow their binding to other transmembrane proteins [4,26,38,39].…”