Receptor with a dual specificity kinase activity acting on both serine/threonine- and tyrosine-containing substrates. Regulates, in response to brassinosteroid binding, a signaling cascade involved in plant development, including expression of light- and stress-regulated genes, promotion of cell elongation, normal leaf and chloroplast senescence, and flowering. Binds brassinolide (BL), and less effectively castasterone (CS), but not 2,3,22,23-O-tetramethylbrassinolide or ecdysone. May be involved in a feedback regulation of brassinosteroid biosynthesis. Phosphorylates BRI1-associated receptor kinase 1 (BAK1), Transthyretin-Like protein (TTL) and SERK1 on 'Ser-299' and 'Thr-462' in vitro. May have a guanylyl cyclase activity (PubMed:10557222, PubMed:10938344, PubMed:17138891, PubMed:17520012, PubMed:18694562, PubMed:19124768). Phosphorylates BSK1, BSK2 and BSK3 in vitro (PubMed:18653891). Phosphorylates BSK1, BSK3, BSK5, BSK6, BSK8 AND BSK11 in vitro (PubMed:23496207). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated by Ser and Thr phosphorylation. Monomer or homodimer in the plasma membrane. Heterodimer with BAK1 in the endosomes. Interacts with SERK1 and TTL in a kinase-dependent manner. Bind to SERK1 in a brassinolide-dependent manner (PubMed:23929946). Component of the SERK1 signaling complex, composed of KAPP, CDC48A, GRF6 or GRF7, SERK1, SERK2, SERK3/BAK1 and BRI1. Interacts with CDG1 (PubMed:21855796). No interactions with PSKR1 or CNGC17 (PubMed:26071421). Interacts with BIK1 (PubMed:23818580). Interacts with B'ALPHA, B'BETA, B'GAMMA and B'ETA (PubMed:26517938). Interacts with BSK1 and BSK3 (PubMed:18653891). Interacts with BSK5, BSK6 and BSK11 (PubMed:23496207). Expressed ubiquitously. Expressed constitutively in either dark- or light-grown seedlings. Contains one leucine-zipper motif and two pairs of conservatively spaced Cys (Cys pair 1 and 2) involved in forming heterodimers. A 70 amino acid island between the 21th and the 22th LRR is essential for the binding of brassinosteroids. The JM domain (815-883) is a positive regulator of kinase activity and is required for Tyr phosphorylation. A guanylyl cyclase domain (1021-1134) having an in vitro activity is included in the C-terminal kinase domain. Autophosphorylated on Tyr-831, Tyr-956 and maybe Tyr-1072. Phosphorylated on at least 12 sites, with a preference for Ser residues. Transphosphorylated on Ser-887 by SERK1 and on Ser-838, Thr-846, Ser-858 and Ser-1166 by BAK1. Phosphorylation on Ser-1166 enhances the kinase activity. Glycosylated. Dwarf phenotype and aberrant leaf shape. Binding of brassinosteroid induces intramolecular autophosphorylation of BRI1. Interaction with BAK1 activates both receptor kinases and the full activation of either receptor kinase requires transphosphorylation by their partners. Optimum in vitro phosphorylation of the substrate requires Arg or Lys residues at P-3, P-4, and P+5 (relative to the phosphorylated amino acid at P=0). Homodimerizes in the absence of ligand and binds brassinosteroid in the absence of its coreceptor BAK1. The bri1-9 mutation produces a fully active protein with a subtle conformational change that is recognized for reglucosylation by UGGT, resulting in its endoplasmic reticulum retention via Glc(1)Man(9)GlcNAc(2)-calreticulin/calnexin interaction. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.