The SERK3 elongated allele defines a role for BIR ectodomains in brassinosteroid signalling


The leucine-rich repeat receptor kinase (LRR-RK) BRASSINOSTEROID INSENSITIVE 1 (BRI1) requires a shape-complementary SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) co-receptor for brassinosteroid sensing and receptor activation1. Interface mutations that weaken the interaction between receptor and co-receptor in vitro reduce brassinosteroid signalling responses2. The SERK3 elongated (elg) allele3,4,5 maps to the complex interface and shows enhanced brassinosteroid signalling, but surprisingly no tighter binding to the BRI1 ectodomain in vitro. Here, we report that rather than promoting the interaction with BRI1, the elg mutation disrupts the ability of the co-receptor to interact with the ectodomains of BRI1-ASSOCIATED-KINASE1 INTERACTING KINASE (BIR) receptor pseudokinases, negative regulators of LRR-RK signalling6. A conserved lateral surface patch in BIR LRR domains is required for targeting SERK co-receptors and the elg allele maps to the core of the complex interface in a 1.25 Å BIR3–SERK1 structure. Collectively, our structural, quantitative biochemical and genetic analyses suggest that brassinosteroid signalling complex formation is negatively regulated by BIR receptor ectodomains.

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Fig. 1: SERK3 elg is a gain of function mutation in vivo but not in vitro.
Fig. 2: BIR ectodomains interact with different SERK co-receptors in vitro.
Fig. 3: The BIR2 ectodomain adopts a SERK-like fold with an additional lateral protein interaction interface.
Fig. 4: A BIR3–SERK1 complex structure provides a mechanism for SERK gain-of-function mutations.


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We thank B. Kemmerling for kindly providing us with BIR2 and BIR3 polyclonal antibodies, N. Geldner for providing seeds, and the staff at beam line PXIII of the Swiss Light Source, Villigen, Switzerland, for technical assistance during data collection, J. Santiago for providing the SERK2 expression plasmid, and K. Lau for help with preparing figures. This work was supported by grant 31003A_176237 from the Swiss National Science Foundation and by an International Research Scholar Award from the Howard Hughes Medical Institute (to M.H.).

Author information

U.H. and M.H. designed research. U.H. performed most of the experiments. J.N. contributed to generation and characterization of transgenic lines and A.M. conducted experiments on the cytoplasmic domains. U.H., A.M., L.A.H. and M.H. analysed data. U.H. and M.H. wrote the manuscript.

Correspondence to Michael Hothorn.

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Supplementary Methods, Supplementary Figures 1–10, Supplementary Tables 1–3 and Supplementary References.

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