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The University of Geneva team used protein engineering to reveal the activation of the leucine-rich kinase signaling pathway by the BAK1 chimera

Jul 19, 2021

Recently, the internationally renowned Plant Cell published an online study titled "Constitutive Activation of Leucine-Rich Repeat Receptor Kinase Signaling Pathways by BAK1-Interacting Receptor-Like Kinase 3 Chimera" by Michael Hothorn's team from the Department of Botany and Plant Biology, University of Geneva, Switzerland. Sexual thesis. The constitutive activation mechanism of the BAK1 interaction receptor kinase 3 chimera on the leucine-rich repeat receptor kinase signaling pathway was revealed.

The plant's unique mechanism of co-receptor-dependent activation of SERK is conserved in many LRR-RKs, such as LRR-RK HAESA, whose function includes controlling the shedding of floral organs in Arabidopsis thaliana by interacting with the peptide hormone IDA. The SERK-dependent mitogen-activated protein kinase (MAPK) signaling pathway involves LRR-RK ERECTA (ER) and its homologous genes ERECTA-LIKE1 (ERL1) and ERL2, and plays different roles in plant development. The identification of the constitutive and ligand-independent interactions between the LRR extracellular domains of two plant membrane signal proteins prompted us to investigate whether the protein chimera between the BIR3 extracellular domain and the cytoplasmic domains of various receptor kinases is possible Leads to a constitutively active signal complex. Although there are significant structural differences between the LRR-RKSERK and BIR-SERK complexes, the researchers’ data indicate that a wide range of oBIR3-iLRR-RK chimeras are functional in plants (Figure 1).

Figure 1

The receptor kinases of leucine-rich extracellular repeat domains (LRR-RKs) are the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecules, peptides or protein ligands, and can be activated by ligand-induced interaction with somatic embryogenesis receptor-like kinase (SERK) co-receptor kinase. Our previous studies have shown that SERK can also interact with BAK1 as receptor kinase 3 (BIR3) to form a constitutive, ligand-independent complex. Here, the researchers report a receptor chimera, in which the extracellular LRR domain of BIR3 is fused with the cytoplasmic kinase domain of SERK-dependent LRR-RKs. In the absence of ligand stimulation, BRASSINOSTEROID INSENSITIVE1, HAESA, and ERECTA Source-derived SERK co-receptors form tight complexes. These chimeras are expressed under the control of the endogenous promoters of the respective LRR-RKs and lead to effective functions to obtain the phenotype of brassinosteroids, flower shedding, and stomatal patterns. Importantly, the BIR3-GSO1/SGN3 chimera can partially complement the phenotype formed by the sgn3 Karnofsky band, which indicates that the SERK protein also mediates the activation of the GSO1/SGN3 receptor. In general, our protein engineering methods can be used to clarify the physiological functions of LRR-RKs and determine the receptor activation mechanism in a single transgenic strain (Figure 2).

Figure 2

In summary, our availability of simple, Lego-type assembly and suitable control lines for BIR3 chimera now allows the genetic characterization of orphan LRR-RKs with unknown/unclear loss-of-function phenotypes and the analysis of their underlying activation mechanisms . BIR3 protein chimeras can also be used for biochemical or genetic interaction screening, where the constitutively active form of the receptor is desirable.