The immunoprecipitation followed by mass-spectrometry (AP-MS) experiment was performed as follows. The N-terminal Kelch domain, the C-terminal phosphatase domain or a catalytically inactive version of the C-terminal phosphatase domain of the Kelch phosphatase BSU1 from Arabidopsis thaliana were constitutively expressed with a C-terminal Flag tag in the Arabidopsis thaliana bri1-5 brassinosteroid signaling mutants. AP-MS experiments were performed at seedling stage.