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

Plant unique membrane receptor kinases with leucine-rich repeat (LRR) extracellular domains are key regulators of development and immune responses. Here we present the 1.55 [A] resolution crystal structure of the immune receptor kinase SOBIR1 from Arabidopsis. The ectodomain structure reveals the presence of 5 LRRs sandwiched between non-canonical capping domains. The disulphide bond-stabilized N-terminal cap harbors an unusual {beta}-hairpin structure. The C-terminal cap features a highly positively charged linear motif which we find largely disordered in our structure. Size-exclusion chromatography and right-angle light scattering experiments suggest that SOBIR1 is a monomer in solution. The protruding {beta}-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.\n\nSynopsisThe ectodomain structure of a novel plant membrane receptor kinase with unusual capping domains is reported.