Structure of the Arabidopsis receptor kinase SRF6 ectodomain determined from crystals obtained using the LRR crystallization screen.

Plant-specific membrane receptor kinases with structurally diverse extracellular domains regulate key processes in plant growth, development, immunity and symbiosis. Structural studies of these glycoproteins are often hampered by the limited quantities in which they can be obtained. Here, we describe the leucine-rich repeat (LRR) crystallization screen, which has enabled the successful crystallization and structure determination of multiple receptor kinase ectodomains, including ligand- and co-receptor-bound complexes. As an example, we report the 1.5 Å resolution crystal structure of the LRR domain of STRUBBELIG-RECEPTOR FAMILY 6 (SRF6) from Arabidopsis thaliana. The SRF6 ectodomain contains seven LRRs and a disulfide-bond-stabilized N-terminal capping domain but lacks the canonical C-terminal cap and the N-glycosylation pattern typically found in other family members. Previously reported protein-protein interactions between the SRF6 and SRF7 ectodomains and the receptor kinases BRI1, BRL1, BRL3, SERK3 and BIR1-BIR3 could not be confirmed by quantitative isothermal titration calorimetry and grating-coupled interferometry assays, suggesting that these structurally conserved LRR receptor kinases may have signalling functions outside the brassinosteroid pathway.