Dual specificity kinase acting on both serine/threonine- and tyrosine-containing substrates. Controls the expression of genes associated with innate immunity in the absence of pathogens or elicitors. Involved in brassinosteroid (BR) signal transduction. Phosphorylates BRI1. May be involved in changing the equilibrium between plasma membrane-located BRI1 homodimers and endocytosed BRI1-BAK1 heterodimers. Interaction with MSBP1 stimulates the endocytosis of BAK1 and suppresses brassinosteroid signaling. Acts in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) via its interactions with FLS2 and EFR, and the phosphorylation of BIK1. Involved in programmed cell death (PCD) control. Positively regulates the BR-dependent plant growth pathway and negatively regulates the BR-independent cell-death pathway (PubMed:17583510, PubMed:17600708, PubMed:18667726, PubMed:18694562, PubMed:19124768, PubMed:20018402, PubMed:20404519, PubMed:21693696). Phosphorylates BIR2 and thus promotes interaction with BIR2 (PubMed:24388849, PubMed:24556575). This interaction prevents interaction with FLS2 in the absence of pathogen-associated molecular patterns (PAMP) (PubMed:24388849, PubMed:24556575). Component of the RLP23-SOBIR1-BAK1 complex that mediates NLP-triggered immunity (PubMed:27251392). Required for PSK promotion of seedling growth and protoplast expansion (PubMed:26071421). CNGC17 and AHAs form a functional cation-translocating unit that is activated by PSKR1/BAK1 and possibly other BAK1/RLK complexes (PubMed:26071421). Probably required during small peptide (e.g. RGF1) signaling (Probable). 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] Interacts constitutively with BIR2, thereby preventing interaction with the ligand-binding LRR-RLK FLS2. Upon infection, pathogen-associated molecular patterns (PAMP) perception leads to BIR2 release from the BAK1 complex and enables the recruitment of BAK1 into the FLS2 complex (PubMed:24388849, PubMed:24556575). Heterodimer with FLS2 (PubMed:24114786). Monomer. Heterodimer with BRI1 in the endosomes. Component of the SERK1 signaling complex, composed of KAPP, CDC48A, GRF6 or GRF7, SERK1, SERK2, SERK3/BAK1 and BRI1. Interacts with the P.syringae AvrPto and hopAB2/AvrPtoB, ERD13, PEPR1 and PEPR2. Interacts (via extracellular region) with MSBP1. Interacts with the EF-Tu receptor EFR and FLS2 in a specific ligand-induced manner. Interacts with TMK4/BARK1. Part of a functional complex containing PSKR1, BAK1, CNGC17, and AHA (PubMed:26071421). Interacts with CNGC17 and PSKR1 (PubMed:26071421). Binds to IOS1 which triggers FLS2-BAK1 complex formation upon microbe-associated molecular patterns (MAMPs) treatment (PubMed:27317676). Interacts with ERECTA in a EPF2-induced manner. Interacts with ERL1 in a EPF1-induced manner. Interacts with TMM (PubMed:26320950). Component of a trimeric complex composed of RLP23, SOBIR1 and BAK1. BAK1 is recruited into a pre-formed RLP23-SOBIR1 complex in a ligand-dependent manner (PubMed:27251392). In the presence of the signal peptide RGF1, interacts with RGI3/RGFR1 and RGI4/RGFR2/SKM2 (PubMed:27229311). Endocytosis enhanced upon interaction with MSBP1. A number of isoforms are produced. According to EST sequences. Expressed ubiquitously. Up-regulated by flagellin and harpin. Contains one leucine-zipper motif and one pair of conservatively spaced Cys (Cys pair) involved in forming heterodimers. Autophosphorylated on Ser-290, Thr-312, Thr-446, Thr-449, Thr-455 and Tyr-610. Probable autophosphorylation on additional Tyr residues. Transphosphorylated by BRI1. It is not sure whether Thr-589 or Ser-595 is the target of the phosphorylation. The phosphorylations on Thr and Tyr are induced by brassinolide. Phosphorylation at Ser-286, Ser-290 Thr-446 or Thr-449 is not critical for flagellin signaling. Semi-dwarfed phenotype, altered leaf morphology and reduced sensitivity to brassinolide and flagellin. Enhanced chlorosis and lesion formation upon pathogen infection. Bak1 and bkk1 double mutants are seedling lethal. Interaction with BRI1 activates both receptor kinases and the full activation of either receptor kinase requires transphosphorylation by their partners. This interaction in vitro is magnesium dependent. Instantaneous heteromeric complex formation between FLS2 and BAK1 and reciprocal transphosphorylation after binding of the flagellin flg22 ligand to FLS2. The kinase activity is not required for the complex formation. The interaction with the bacterial effectors AvrPto and AvrPtoB/hopAB2 interfers with FLS2 binding and plant immunity. Phosphorylated residues T-450 and T-455 have stronger functional effects than other phosphorylated residues by interacting with both the catalytic and activation loops to achieve a conformational stability, locking BAK1 kinase in the active conformation. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. An article reported the role of autophosphorylation of Tyr-610 in brassinosteroid signaling; however, this paper was later retracted. A second article from the same group reported the role of phosphorylation; however, this paper was also retracted.