Crystal structure of the leucine-rich repeat ectodomain of the plant immune receptor kinase SOBIR1.

Ulrich Hohmann,Michael Hothorn

doi:10.1107/s2059798319005291

Abstract

Plant-unique membrane receptor kinases with leucine-rich repeat (LRR) extracellular domains are key regulators of development and immune responses. Here, the 1.55 Å resolution crystal structure of the immune receptor kinase SOBIR1 from Arabidopsis is presented. The ectodomain structure reveals the presence of five LRRs sandwiched between noncanonical capping domains. The disulfide-bond-stabilized N-terminal cap harbours an unusual β-hairpin structure. The C-terminal cap features a highly positively charged linear motif which was found to be largely disordered in this structure. Size-exclusion chromatography and right-angle light-scattering experiments suggest that SOBIR1 is a monomer in solution. The protruding β-hairpin, a set of highly conserved basic residues at the inner surface of the SOBIR LRR domain and the presence of a genetic missense allele in LRR2 together suggest that the SOBIR1 ectodomain may mediate protein-protein interaction in plant immune signalling.

Highlights

Plants have evolved a unique set of membrane receptor kinases (LRR-RKs) that are composed of a leucine-rich repeat ectodomain, a transmembrane helix and a dualspecificity kinase domain in the cytoplasm (Shiu & Bleecker, 2001)
The solution in space group P65 comprises a dimer in the asymmetric unit, with the nine putative sulfur sites corresponding to a disulfide bridge in the N-terminal LRR capping domain, to a free cysteine and a methionine residue in the LRR core and to a free ion, which we interpreted as a chlorine anion originating from the crystallization buffer (Fig. 2b)
Utilization of the native signal peptide (AtNat), the baculoviral glycoprotein 67 signal peptide (GP67) or the signal peptide from Drosophila melanogaster binding protein (DmBiP; Fig. 6a) all lead to accumulation and secretion of RLP23, but we found that the protein either aggregated or degraded in analytical size-exclusion chromatography assays (Fig. 6b)

Summary

Introduction

Plants have evolved a unique set of membrane receptor kinases (LRR-RKs) that are composed of a leucine-rich repeat ectodomain, a transmembrane helix and a dualspecificity kinase domain in the cytoplasm (Shiu & Bleecker, 2001). The ectodomains of LRR-RKs show a bimodal size distribution (Fig. 1a). Members with large ectodomains (15–30 LRRs) represent ligand-binding receptors (Hohmann et al, 2017). Ligand binding to large LRR-RKs promotes their association with shape-complementary SERKs at the cell surface, which in turn enables their cytoplasmic kinase domains to interact and to transphosphorylate each other (Santiago et al, 2013, 2016; Hohmann, Santiago et al, 2018). SERKs represent only five of the $60 small LRR-RKs in Arabidopsis (Fig. 1a; Dufayard et al, 2017), but genetic evidence suggests that sequence-related NIK/CIK/CLERK proteins may fulfil similar functions (Hu et al, 2018; Cui et al, 2018; Anne et al, 2018)

Methods

Results

Conclusion