BKI1 (BRI1 KINASE INHIBITOR 1, At5g42750) is a small, largely intrinsically disordered protein of the plant-specific BKI1/MAKR family. In resting cells it is tethered to the inner face of the plasma membrane by a reiterated polybasic [KR][KR] motif, where it binds the cytoplasmic kinase domain of the brassinosteroid receptor BRI1 and prevents BRI1 from associating with its coreceptor BAK1, keeping the receptor inactive. Brassinosteroid binding activates BRI1, which phosphorylates BKI1 on a tyrosine within the membrane-targeting motif and on serines in its C-terminal region; phosphorylated BKI1 is released into the cytosol, allowing formation of the active BRI1-BAK1 complex. Cytosolic phospho-BKI1 binds 14-3-3 proteins, which may relieve 14-3-3-mediated cytoplasmic retention of the BZR1/BES1 transcription factors. BKI1 therefore acts as a receptor-level negative regulator (and, after release, a modest positive modulator) of brassinosteroid signalling.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004860 protein kinase inhibitor activity | IDA PMID:22075146 Dual role of BKI1 and 14-3-3 s in brassinosteroid signaling ... | ACCEPT | Summary: BKI1 directly inhibits the BRI1 receptor kinase; this is its defining molecular activity. Reason: Direct in vitro/in vivo evidence that BKI1 keeps BRI1 quiescent; 14-3-3 binding of phosphorylated BKI1 relieves the inhibition. Supporting Evidence: PMID:22075146 is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1) PMID:22075146 The 14-3-3 proteins can interact with the phosphorylated BKI1 through a motif that contains the two phosphorylation sites to release inhibition of BRI1 by BKI1. |
| GO:0005515 protein binding | IPI PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | MODIFY | Summary: IPI with BRL1 (Q9ZWC8), a BRI1-like receptor kinase; reflects BKI1 binding to the cytoplasmic kinase domain of BRI1-family receptors. Reason: Generic protein binding is uninformative. BKI1 is an established inhibitor of BRI1, but inhibition of BRL1 specifically is not shown in the cached record, so the interaction with this receptor kinase is captured as protein kinase binding rather than protein kinase inhibitor activity (the BRI1 row carries the inhibitor term). The BRL1 interaction data are in the full text, which is not cached. Proposed replacements: protein kinase binding Supporting Evidence: PMID:16857903 We report a BRI1-interacting protein, BKI1, which is a negative regulator of brassinosteroid signaling. |
| GO:0005515 protein binding | IPI PMID:22075146 Dual role of BKI1 and 14-3-3 s in brassinosteroid signaling ... | REMOVE | Summary: IPI with the 14-3-3 protein GRF8 (AT5G65430); phosphorylated BKI1 is a 14-3-3 client. Reason: Generic protein binding is uninformative. Removal does not mean the interaction is false: BKI1 is a phosphorylation-dependent 14-3-3 client, which is recorded in the description rather than as a molecular function of BKI1. Supporting Evidence: PMID:22075146 The 14-3-3 proteins can interact with the phosphorylated BKI1 through a motif that contains the two phosphorylation sites to release inhibition of BRI1 by BKI1. |
| GO:0005634 nucleus | ISM GO_REF:0000122 | REMOVE | Summary: AtSubP sequence-based prediction of nuclear localization. Reason: No experimental evidence places BKI1 in the nucleus; it is plasma-membrane associated and released into the cytosol upon BR perception. The polybasic [KR][KR] membrane-targeting motif is a likely source of a spurious NLS-like prediction. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location mapping (cytoplasm). Reason: BKI1 is released into the cytosol after BRI1-mediated tyrosine phosphorylation. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005829 cytosol | IDA PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | ACCEPT | Summary: BKI1 relocates from the plasma membrane to the cytosol upon BR perception. Reason: Direct imaging evidence of BR-induced cytosolic release. Supporting Evidence: PMID:16857903 Brassinosteroids cause the rapid dissociation of BKI1-yellow fluorescent protein from the plasma membrane in a process that is dependent on BRI1-kinase. |
| GO:0005829 cytosol | IDA PMID:21289069 Tyrosine phosphorylation controls brassinosteroid receptor a... | ACCEPT | Summary: BKI1 relocates from the plasma membrane to the cytosol upon BR perception. Reason: Direct imaging evidence of BR-induced cytosolic release. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005829 cytosol | IDA PMID:26296798 The Intrinsically Disordered Protein BKI1 Is Essential for I... | ACCEPT | Summary: BKI1 relocates from the plasma membrane to the cytosol upon BR perception. Reason: Direct imaging evidence of BR-induced cytosolic release. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: BKI1 is anchored to the inner leaflet of the plasma membrane in resting cells. Reason: Consistent with direct localization data. Supporting Evidence: PMID:16857903 Brassinosteroids cause the rapid dissociation of BKI1-yellow fluorescent protein from the plasma membrane in a process that is dependent on BRI1-kinase. PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005886 plasma membrane | IDA PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | ACCEPT | Summary: BKI1 is associated with the plasma membrane in the absence of BR, where it binds BRI1. Reason: Direct localization evidence; plasma membrane is the site of BKI1 inhibitory action. Supporting Evidence: PMID:16857903 Brassinosteroids cause the rapid dissociation of BKI1-yellow fluorescent protein from the plasma membrane in a process that is dependent on BRI1-kinase. |
| GO:0005886 plasma membrane | IDA PMID:21289069 Tyrosine phosphorylation controls brassinosteroid receptor a... | ACCEPT | Summary: BKI1 is associated with the plasma membrane in the absence of BR, where it binds BRI1. Reason: Direct localization evidence; plasma membrane is the site of BKI1 inhibitory action. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005886 plasma membrane | IDA PMID:26296798 The Intrinsically Disordered Protein BKI1 Is Essential for I... | ACCEPT | Summary: BKI1 is associated with the plasma membrane in the absence of BR, where it binds BRI1. Reason: Direct localization evidence; plasma membrane is the site of BKI1 inhibitory action. Supporting Evidence: PMID:21289069 Phosphorylation occurs within a reiterated [KR][KR] membrane targeting motif, releasing BKI1 into the cytosol and enabling formation of an active signaling complex. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Automated plasma membrane annotation. Reason: Consistent with experimental localization. Supporting Evidence: PMID:16857903 Brassinosteroids cause the rapid dissociation of BKI1-yellow fluorescent protein from the plasma membrane in a process that is dependent on BRI1-kinase. |
| GO:0010423 negative regulation of brassinosteroid biosynthetic process | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA propagating negative regulation of BR biosynthesis (involved_in) to BKI1 from the BKI1 PAINT node. Reason: The objection is to the scope of the term, not to the phylogenetic transfer. BKI1's own activity is inhibition of the BRI1 receptor kinase; BR-biosynthesis regulation is a downstream feedback output of the BR signalling pathway (via BZR1/BES1 control of biosynthetic genes), not a process in which BKI1 performs a step. Under the involved_in qualifier the IBA asserts participation, which overstates BKI1's role. This is reconciled with the IMP row for the same term, which is kept as non-core because it carries the acts_upstream_of_or_within qualifier that correctly describes an upstream, indirect effect; the two rows differ in what they assert, not in the biology. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: ROLE CONFLATION Sources checked: PANTHER:PTN004993342 · BKI1 PAINT node SUPPORTS TRANSFER The transfer within the BKI1 clade is not in question; the problem is that a biosynthesis-regulation term read as involved_in conflates an upstream signalling inhibitor with the feedback output of the pathway. AGI_LocusCode:AT5G42750 · BKI1 (self) SUPPORTS TRANSFER BKI1 appearing among the node's sources is expected (its own IMP, PMID:16857903). That IMP is an acts_upstream_of_or_within phenotype annotation, which is the appropriate way to record an indirect feedback effect on biosynthesis. Supporting Evidence: PMID:16857903 We report a BRI1-interacting protein, BKI1, which is a negative regulator of brassinosteroid signaling. |
| GO:0010423 negative regulation of brassinosteroid biosynthetic process | IMP PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | KEEP AS NON CORE | Summary: Phenotype-based link between BKI1 and negative regulation of BR biosynthesis (acts_upstream_of_or_within). Reason: The acts_upstream_of_or_within qualifier correctly records an indirect effect, which is why this row is kept while the involved_in IBA for the same term is marked over-annotated. Deferred to the curator, who read the full text (the cached record is abstract-only; Europe PMC has no open-access full text). BKI1 is a direct inhibitor of the BRI1 receptor kinase, not a biosynthetic component, so any effect on BR biosynthesis is an indirect feedback output of BR signalling and is non-core. Caveat: the sign is questionable. A web search (Wang et al. 2011 Dev Cell, PMID:22075146) reports that BKI1-YFP overexpression increases CPD and DWF4 expression, which would put BKI1 upstream of positive, not negative, regulation of biosynthetic gene expression. The underlying data were not seen directly, so the row is kept as non-core and the sign is flagged in suggested_experiments. Supporting Evidence: PMID:16857903 We report a BRI1-interacting protein, BKI1, which is a negative regulator of brassinosteroid signaling. |
| GO:0019210 kinase inhibitor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference of kinase inhibitor activity for the BKI1/MAKR family node. Reason: Consistent with the experimentally demonstrated BRI1 inhibition (GO:0004860 is a child term). The target is among the experimental sources of the PAINT node, as expected. Supporting Evidence: PMID:22075146 is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1) |
| GO:0019210 kinase inhibitor activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (IPR039620, BKI1/MAKR family) mapping to kinase inhibitor activity. Reason: Correct but general; GO:0004860 protein kinase inhibitor activity is the more specific experimentally supported term. Supporting Evidence: PMID:22075146 is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1) |
| GO:0046982 protein heterodimerization activity | IPI PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | MODIFY | Summary: IPI with BRI1 (AT4G39400) captured as protein heterodimerization activity. Reason: BKI1 is not a dimerizing partner of BRI1 in the sense of GO:0046982; it binds the BRI1 kinase domain as an inhibitor and substrate. The interaction is better represented by protein kinase inhibitor activity. Proposed replacements: protein kinase inhibitor activity Supporting Evidence: PMID:16857903 BKI1 is a substrate of BRI1 kinase and limits the interaction of BRI1 with its proposed coreceptor, BAK1, suggesting that BKI1 prevents the activation of BRI1. |
| GO:1900458 negative regulation of brassinosteroid mediated signaling pathway | IMP PMID:16857903 Brassinosteroids regulate dissociation of BKI1, a negative r... | NEW | Summary: BKI1 directly inhibits the BRI1 receptor kinase and limits BRI1-BAK1 association, so it negatively regulates the BR signalling pathway at the receptor. Reason: BKI1 itself performs the inhibitory step (binding the BRI1 kinase domain and blocking coreceptor recruitment), so this is participation-level regulation, not a necessity inference. The regulation term is currently absent from GOA, which instead carries a questionable biosynthesis-regulation term. Supporting Evidence: PMID:16857903 We report a BRI1-interacting protein, BKI1, which is a negative regulator of brassinosteroid signaling. PMID:16857903 BKI1 is a substrate of BRI1 kinase and limits the interaction of BRI1 with its proposed coreceptor, BAK1, suggesting that BKI1 prevents the activation of BRI1. PMID:22075146 is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1) |
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Download this section (compressed HTML)Q: Does the 14-3-3-binding (positive) role of released BKI1 make a quantitatively important contribution to BZR1/BES1 nuclear accumulation in vivo, relative to BIN2 inactivation?
Q: How redundant is BKI1 with MAKR family members (e.g. MAKR2-MAKR5) in restraining BRI1 and related LRR receptor kinases?
Experiment: Measure BR-biosynthetic feedback gene expression (CPD, DWF4) in bki1 loss-of-function and BKI1 overexpression lines side by side, to resolve the sign of BKI1's indirect effect on BR biosynthesis.
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