Constitutive Activation of Leucine-Rich Repeat Receptor Kinase Signaling Pathways by BAK1-INTERACTING RECEPTOR-LIKE KINASE3 Chimera

Abstract

Receptor kinases with extracellular leucine-rich repeat domains (LRR-RKs) form the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecule, peptide, or protein ligands, and may be activated by ligand-induced interaction with a shape complementary SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptor kinase. We have previously shown that SERKs can also form constitutive, ligand-independent complexes with the LRR ectodomains of BAK1-interacting receptor-like kinase 3 (BIR3) receptor pseudokinases, negative regulators of LRR-RK signaling. Here we report that receptor chimera in which the extracellular LRR domain of BIR3 is fused to the cytoplasmic kinase domains of the SERK-dependent LRR-RKs BRASSINOSTEROID INSENSITIVE1, HAESA and ERECTA form tight complexes with endogenous SERK co-receptors in the absence of ligand stimulus. Expression of these chimeras under the control of the endogenous promoter of the respective LRR-RK leads to strong gain-of-function brassinosteroid, floral abscission, and stomatal patterning phenotypes, respectively. Importantly, a BIR3-GSO1/SGN3 chimera can partially complement sgn3 Casparian strip formation phenotypes, suggesting that SERK proteins also mediate GSO1/SGN3 receptor activation. Collectively, our protein engineering approach may be used to elucidate the physiological functions of orphan LRR-RKs and to identify their receptor activation mechanism in single transgenic lines.

Molecular mechanisms of early plant pattern-triggered immune signaling

“…Another layer of complexity in plant PRR complex formation is the role of so-called regulatory or accessory RKs, which can modulate the ligand-induced interaction between PRRs and their co-receptors (Figure 2). For instance, members of the BAK1-IN-TERACTING RECEPTOR-LIKE KINASE (BIR) family such as BIR3 interact with BAK1 in the resting state and are thought to sequester it to prevent the ectopic activation of immune signaling (Hohmann et al, 2020;Imkampe et al, 2017;Ma et al, 2017). FERONIA (FER), however, is a malectin-like RK of the CrRLK1L family, which together with the GPI-anchored protein LORELEIlike-GPI-anchored protein 1 (LLG1) can promote FLS2-BAK1 complex formation (Stegmann et al, 2017;Xiao et al, 2019).…”

Activation Loop Phosphorylation of a Non-Rd Receptor Kinase Initiates Plant Innate Immune Signaling

“…First, EFR could serve as a protein-protein interaction scaffold to define specificity in activating downstream responses. In support of this, studies of chimeric receptor kinases indicate that the cytoplasmic domain of the ligand binding receptor defines signaling specificity (Hohmann et al, 2020(Hohmann et al, , 2018. This suggests that the EFR cytoplasmic domain functions as a scaffold for the components required to execute immunity-specific downstream signaling.…”

“…Of note, plant RK cytoplasmic protein kinase domains share monophyletic ancestry with the well-known INTERLEUKIN-1 RECEPTOR-ASSOCIATED KINASES (IRAKs) that have central roles in innate immune signaling in animals (Shiu and Bleecker, 2001;Su et al, 2020) Among plant RKs, members with leucine-rich repeat (LRR) ectodomains (LRR-RKs) represent the largest sub-family and fulfill critical roles in development and stress response (Couto and Zipfel, 2016;Hohmann et al, 2017). LRR-RKs have thus been the focus of extensive biochemical and structural analyses aimed at understanding how they activate intracellular signaling in response to ligand perception (Hohmann et al, 2020;Hothorn et al, 2011;Okuda et al, 2020;Santiago et al, 2013;Sun et al, 2013b). A common mode of activation among RKs is ligand-induced heterodimerization with co-receptors.…”

“…Following ligand perception, plant LRR-RKs recruit co-receptorswhich are themselves LRR-RKs with short, shape-complementary ectodomainsthat typically form contacts with both the ligand and the ligand-binding receptor (Hohmann et al, 2017). In this context, ligand-dependent receptor/co-receptor heterodimer formation acts as a binary switch to initiate intracellular signaling (Hohmann et al, 2020(Hohmann et al, , 2018Santiago et al, 2013;Sun et al, 2013b). Although structural analysis of receptor ectodomains has provided a detailed understanding of how receptor/coreceptor interactions occur in a ligand-dependent manner (Han et al, 2014;Hohmann et al, 2017;Hothorn et al, 2011;Santiago et al, 2013;Sun et al, 2013aSun et al, , 2013bTang et al, 2015), much less is known mechanistically about how receptor/co-receptor dimerization activates the intracellular protein kinase activities and subsequent downstream signaling.…”

The TIR-NBS-LRR protein CSA1 is required for autoimmune cell death in Arabidopsis pattern recognition co-receptorbak1andbir3mutants

“…The IP-MS interactome of BIR3 revealed about 30 RKs as candidate interactors. This broadens the spectrum of RKs that can be assessed by BIR3 and will allow the functional determination of more orphan RKs (Hohmann et al, 2018; Hohmann et al, 2020). RKs associated with BIR proteins are potentially also guarded by NLRs.…”