| Topic | Key findings | Key molecules/residues | Evidence type | Year | DOI/URL |
|---|---|---|---|---|---|
| Identity/domains | Arabidopsis thaliana BRI1 (UniProt O22476; At4g39400) is consistently identified as a plasma-membrane leucine-rich repeat receptor-like kinase (LRR-RLK) with an extracellular LRR domain containing an island domain for steroid perception and an intracellular kinase domain; this matches the UniProt annotation. (pqac-00000003, pqac-00000005, pqac-00000009) | BRI1; LRR ectodomain; island domain; kinase domain | Review + primary | 2024, 2008 | https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.1016/j.devcel.2008.06.011 |
| Ligand perception | BRI1 perceives brassinosteroids, especially brassinolide (BL), through the extracellular LRR/island domain; BL binding creates a SERK-binding surface and the island domain together with LRR22 directly forms the steroid-binding motif. Recent structural work further mapped BL-contacting chemistry important for co-receptor recruitment. (pqac-00000001, pqac-00000009, pqac-00000014) | BL; island domain; LRR22; Tyr642; Lys601; His62 | Primary | 2025, 2008, 2005 | https://doi.org/10.1101/2025.08.08.669299 ; https://doi.org/10.1016/j.devcel.2008.06.011 ; https://doi.org/10.1016/j.devcel.2005.05.001 |
| Co-receptor complex | BR binding promotes BRI1 association with SERK co-receptors, especially BAK1/AtSERK3; the complex undergoes sequential transphosphorylation, and BRI1 can phosphorylate BAK1 before reciprocal activation further boosts signaling. BL also increases BAK1-GFP co-immunoprecipitation with BRI1, whereas BRZ reduces association. (pqac-00000002, pqac-00000006, pqac-00000009, pqac-00000011) | BAK1/AtSERK3; SERK1; BRZ; BL | Review + primary | 2025, 2024, 2008, 2005 | https://doi.org/10.3390/ijms26104502 ; https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.1016/j.devcel.2008.06.011 ; https://doi.org/10.1105/tpc.105.031393 |
| Kinase activity | BRI1 is a dual-specificity kinase: historically classified as Ser/Thr, but later shown to autophosphorylate on tyrosine as well. Autophosphorylation appears hierarchical (Ser > Thr > Tyr), and the juxtamembrane domain activates kinase output. (pqac-00000005, pqac-00000006) | Ser/Thr/Tyr phosphorylation; juxtamembrane domain | Review + primary | 2024, 2012, 2009 | https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.3389/fpls.2012.00175 ; https://doi.org/10.1073/pnas.0810249106 |
| Kinase activity/phosphosites | In vivo phosphosites identified on BRI1 include S838, S858, T872, T880, T982, and S1168, with additional activation-loop sites detected by MS. Activation-loop residues are functionally critical: T1049 and S1044/T1045 are essential for kinase activity/signaling; T1039A and S1042A show intermediate rescue. T872A increased Vmax ~10-fold and lowered Km ~2-fold. (pqac-00000010, pqac-00000013, pqac-00000015) | S838; S858; T872; T880; T982; S1168; T1039; S1042; S1044; T1045; T1049 | Primary | 2005 | https://doi.org/10.1105/tpc.105.031393 |
| Receptor regulation | BRI1 activation includes release of the inhibitor BKI1 via tyrosine phosphorylation; BKI1 dissociation is an early step linking receptor activation to downstream signaling. (pqac-00000002, pqac-00000005, pqac-00000008) | BKI1; Tyr phosphorylation | Review + primary | 2025, 2024, 2011 | https://doi.org/10.3390/ijms26104502 ; https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.1016/j.molcel.2011.05.037 |
| Early downstream signaling | Activated BRI1 phosphorylates RLCKs including BSK1 and CDG1; BSK1 is phosphorylated at Ser230, and CDG1 at Ser44, Ser47, and Ser234, with Ser234 important for CDG1 activation. (pqac-00000008) | BSK1 Ser230; CDG1 Ser44/Ser47/Ser234 | Primary | 2011 | https://doi.org/10.1016/j.molcel.2011.05.037 |
| Core phosphorylation cascade | Activated CDG1 phosphorylates BSU1 at Ser764, enhancing BSU1-mediated dephosphorylation of BIN2 at Tyr200; BIN2 inactivation releases BES1/BZR1 to accumulate in the nucleus and regulate BR-responsive genes. (pqac-00000002, pqac-00000006, pqac-00000008, pqac-00000014) | BSU1 Ser764; BIN2 Tyr200; BES1; BZR1/BZR2 | Review + primary | 2025, 2024, 2011, 2005 | https://doi.org/10.3390/ijms26104502 ; https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.1016/j.molcel.2011.05.037 ; https://doi.org/10.1016/j.devcel.2005.05.001 |
| Localization/nanodomains | BRI1 is primarily plasma-membrane localized and enriched in distinct PM nanodomains separate from FLS2; BRI1-BAK1-BIK1 complexes align with cortical microtubules, supporting spatially organized signaling. (pqac-00000000, pqac-00000003, pqac-00000016) | PM nanodomains; BIK1; cortical microtubules | Review | 2024 | https://doi.org/10.1038/s41477-024-01621-2 |
| Trafficking/endocytosis | BRI1 abundance is dynamically controlled by endocytosis and recycling. Internalization occurs via AP-2/T-PLATE-dependent clathrin-mediated endocytosis and a clathrin-independent FLOT1 pathway, followed by trafficking through TGN/EE and vacuolar degradation. (pqac-00000003, pqac-00000016) | AP-2; T-PLATE; FLOT1; TGN/EE; vacuole | Review | 2024 | https://doi.org/10.1038/s41477-024-01621-2 |
| PTM control of receptor abundance | K63-polyubiquitination by PUB12/13 with E2s UBC35/36 promotes BRI1 endocytosis; deubiquitination by UBP12/13 stabilizes PM BRI1. Mutation of 25 cytosolic Lys residues abolishes internalization/degradation and causes hyperactive BR signaling. SUMOylation stabilizes BRI1, whereas deSUMOylation by DeSi3a promotes internalization. (pqac-00000000, pqac-00000003, pqac-00000016) | PUB12/13; UBC35/36; UBP12/13; 25 Lys residues; SUMO; DeSi3a | Review | 2024 | https://doi.org/10.1038/s41477-024-01621-2 |
| Functional/phenotypic evidence | Exogenous BR decreases root stele area in wild type, whereas perturbed BR signaling increases stele area; epidermis-specific BRI1 expression in bri1 mutants can rescue root growth and limit stele area. Recent chemical-biology assays used 100 nM BL in root growth inhibition tests (n = 30-35). (pqac-00000004, pqac-00000001) | Root stele area; bri1 mutants; 100 nM BL; n=30-35 | Review + primary | 2024, 2025 | https://doi.org/10.1038/s41477-024-01621-2 ; https://doi.org/10.1101/2025.08.08.669299 |


*Table: This table summarizes experimentally supported functional annotation evidence for Arabidopsis thaliana BRI1 (UniProt O22476), covering identity, ligand perception, receptor-complex formation, kinase activity, trafficking, and downstream signaling. It is useful as a compact evidence map linking molecular features to specific publications and residues.*