BSU1 (BRI1 SUPPRESSOR 1, At1g03445) is a plant-specific phosphoprotein phosphatase of the PPP family with an N-terminal Kelch-repeat domain and a C-terminal PP1-like catalytic domain; it is the founding member of a small family (BSU1, BSL1, BSL2, BSL3) with overlapping functions. In brassinosteroid signalling, BRI1-phosphorylated BSK1 binds BSU1, and the RLCK CDG1 phosphorylates BSU1 on Ser764, enhancing its activity. Activated BSU1 dephosphorylates the conserved phospho-tyrosine (pTyr200) in the activation loop of the GSK3-like kinase BIN2, inactivating BIN2 so that the BZR1 and BES1 transcription factors accumulate in their unphosphorylated, active form in the nucleus. Its phosphatase domain is Mn2+-dependent and acts on both phospho-Ser/Thr and phospho-Tyr residues; BSU1 raises the steady-state level of dephosphorylated BES1 in planta, but it does not bind or effectively dephosphorylate BES1/BZR1 in vitro, so its effect on these transcription factors is mediated through BIN2 inactivation. It was identified as the dominant bsu1-1D suppressor of bri1, is preferentially expressed in elongating cells, and is found in nuclear and plasma-membrane-associated pools.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004721 phosphoprotein phosphatase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based phosphoprotein phosphatase activity. Reason: Correct parent of the experimentally supported Ser/Thr and Tyr phosphatase activities. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. PMID:19734888 BSU1 inactivates the GSK3-like kinase BIN2 (BR-insensitive 2) by dephosphorylating a conserved phospho-tyrosine residue (pTyr 200) |
| GO:0004722 protein serine/threonine phosphatase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference of Ser/Thr phosphatase activity (BSU1/BSL clade of PPP phosphatases). Reason: BSU1 is itself an experimental source for this node, as expected for a well-characterised member. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0004722 protein serine/threonine phosphatase activity | IDA PMID:14977918 Nuclear protein phosphatases with Kelch-repeat domains modul... | ACCEPT | Summary: BSU1 is a PPP-family protein phosphatase with Ser/Thr phosphatase activity. Reason: Mora-Garcia et al. identified BSU1 as a Ser/Thr phosphatase (the curator read the full text, which is not cached). Kim et al. 2009 independently showed Mn2+-dependent phosphatase activity of BSU1 on phospho-Ser/Thr and phospho-Tyr substrates. The 2004 interpretation that BES1 is a direct substrate is not carried forward: Kim et al. found that BSU1 does not interact with or effectively dephosphorylate BES1/BZR1 in vitro, and acts on them through BIN2 (see the finding_review on PMID:14977918). Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. PMID:19734888 BSU1 contains an N-terminal Kelch-repeat domain and a C-terminal phosphatase domain14, which dephosphorylates both phospho-Ser/Thr and phospho-Tyr residues PMID:19734888 show manganese-dependent phosphatase activities (Supplementary Information, Fig. S2b, c) |
| GO:0004722 protein serine/threonine phosphatase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated EC 3.1.3.16 mapping. Reason: Consistent with direct evidence. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0005515 protein binding | IPI PMID:19452453 Proteomic profiling of tandem affinity purified 14-3-3 prote... | REMOVE | Summary: BSU1 recovered in tandem-affinity-purified 14-3-3 omega (GRF2) complexes. Reason: High-throughput co-purification captured by the uninformative protein binding term; removal does not imply that the association is false. Supporting Evidence: PMID:19452453 Proteomic profiling of tandem affinity purified 14-3-3 protein complexes |
| GO:0005515 protein binding | IPI PMID:21855796 The CDG1 kinase mediates brassinosteroid signal transduction... | MODIFY | Summary: IPI with the receptor-like cytoplasmic kinases CDG1 / CDL1 (PBL7), which phosphorylate BSU1 at Ser764 to activate it. Reason: Generic protein binding is uninformative; the partners are protein kinases that bind and phosphorylate BSU1, which is better captured as protein kinase binding. Proposed replacements: protein kinase binding Supporting Evidence: PMID:21855796 CDG1 in turn phosphorylates S764 to activate BSU1, which inactivates BIN2 by dephosphorylating Y200 of BIN2. |
| GO:0005515 protein binding | IPI PMID:21855796 The CDG1 kinase mediates brassinosteroid signal transduction... | MODIFY | Summary: IPI with the receptor-like cytoplasmic kinases CDG1 / CDL1 (PBL7), which phosphorylate BSU1 at Ser764 to activate it. Reason: Generic protein binding is uninformative; the partners are protein kinases that bind and phosphorylate BSU1, which is better captured as protein kinase binding. Proposed replacements: protein kinase binding Supporting Evidence: PMID:21855796 CDG1 in turn phosphorylates S764 to activate BSU1, which inactivates BIN2 by dephosphorylating Y200 of BIN2. |
| GO:0005634 nucleus | IDA PMID:14977918 Nuclear protein phosphatases with Kelch-repeat domains modul... | ACCEPT | Summary: BSU1 nuclear localization. Reason: Nuclear localization was shown directly for BSU1-YFP fusions. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: BSU1 nuclear localization. Reason: Nuclear localization was shown directly for BSU1-YFP fusions. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0005634 nucleus | ISM GO_REF:0000122 | ACCEPT | Summary: BSU1 nuclear localization. Reason: Nuclear localization was shown directly for BSU1-YFP fusions. Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0005886 plasma membrane | IDA PMID:21855796 The CDG1 kinase mediates brassinosteroid signal transduction... | ACCEPT | Summary: BSU1 association with the plasma membrane, where it is phosphorylated by membrane-anchored CDG1 and binds BSK1. Reason: Consistent with BSU1 being activated by plasma-membrane-anchored RLCKs (BSK1, CDG1); the curator read the full text. BSU1 is therefore present in both plasma membrane and nuclear pools. Supporting Evidence: PMID:21855796 CDG1 in turn phosphorylates S764 to activate BSU1, which inactivates BIN2 by dephosphorylating Y200 of BIN2. PMID:19734888 We show that phosphorylation of BSK1 (BR-signalling kinase 1) by the BR receptor kinase BRI1 (BR-insensitive 1) promotes BSK1 binding to the BSU1 (BRI1 suppressor 1) phosphatase |
| GO:0009742 brassinosteroid mediated signaling pathway | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based BR signalling annotation for the BSU1 family. Reason: BSU1 is a core relay component between BSK1/CDG1 and BIN2. Supporting Evidence: PMID:19734888 BSU1 inactivates the GSK3-like kinase BIN2 (BR-insensitive 2) by dephosphorylating a conserved phospho-tyrosine residue (pTyr 200) |
| GO:0009966 regulation of signal transduction | IBA GO_REF:0000033 | MODIFY | Summary: Phylogenetic inference of regulation of signal transduction. Reason: Correct but very general; BSU1's experimentally defined role is positive regulation of BR-mediated signalling through BIN2 inactivation. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN005303947 · BSU1 clade PAINT node SUPPORTS TRANSFER Transfer is sound; the term is simply more general than the experimentally defined BR-specific role. Proposed replacements: positive regulation of brassinosteroid mediated signaling pathway Supporting Evidence: PMID:19734888 BSU1 inactivates the GSK3-like kinase BIN2 (BR-insensitive 2) by dephosphorylating a conserved phospho-tyrosine residue (pTyr 200) |
| GO:0016787 hydrolase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro-based hydrolase activity. Reason: Too general; BSU1 is an experimentally characterised protein phosphatase. Proposed replacements: protein serine/threonine phosphatase activity Supporting Evidence: PMID:14977918 BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. |
| GO:0032880 regulation of protein localization | IDA PMID:17873094 Nucleocytoplasmic shuttling of BZR1 mediated by phosphorylat... | KEEP AS NON CORE | Summary: BSU1 activity promotes rapid nuclear accumulation of BZR1 (acts upstream of or within regulation of protein localization). Reason: Real but indirect: BSU1 controls BZR1 localization by inactivating BIN2 (BSU1 does not effectively dephosphorylate BZR1/BES1 in vitro), which changes 14-3-3 binding and nuclear export. Supporting Evidence: PMID:17873094 BR-activated phosphatase mediates rapid nuclear localization of BZR1. |
| GO:1900459 positive regulation of brassinosteroid mediated signaling pathway | IMP PMID:21855796 The CDG1 kinase mediates brassinosteroid signal transduction... | ACCEPT | Summary: BSU1 positively regulates BR signalling by dephosphorylating and inactivating BIN2. Reason: Core process supported genetically and biochemically. Supporting Evidence: PMID:21855796 CDG1 in turn phosphorylates S764 to activate BSU1, which inactivates BIN2 by dephosphorylating Y200 of BIN2. PMID:19734888 BSU1 inactivates the GSK3-like kinase BIN2 (BR-insensitive 2) by dephosphorylating a conserved phospho-tyrosine residue (pTyr 200) |
| GO:0004725 protein tyrosine phosphatase activity | IDA PMID:19734888 Brassinosteroid signal transduction from cell-surface recept... | NEW | Summary: BSU1 dephosphorylates the conserved phospho-Tyr200 in the activation loop of BIN2 (and of other GSK3-like kinases), which inactivates BIN2. Reason: BSU1 itself catalyses the dephosphorylation of a phosphotyrosine on its substrate BIN2; this is an enzymatic activity of BSU1, directly shown in vitro and confirmed genetically (bin2-Y200F, bsu1 quadruple mutants). GOA currently carries only the serine/threonine phosphatase activity. Supporting Evidence: PMID:19734888 BSU1 inactivates the GSK3-like kinase BIN2 (BR-insensitive 2) by dephosphorylating a conserved phospho-tyrosine residue (pTyr 200) PMID:19734888 These results indicate that BSU1-mediated tyrosine dephosphorylation is a common mechanism shared by at least two of six GSK3-like kinases that are likely involved in BR signaling. PMID:21855796 CDG1 in turn phosphorylates S764 to activate BSU1, which inactivates BIN2 by dephosphorylating Y200 of BIN2. |
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Download this section (compressed HTML)Q: How does a PPP-family phosphatase achieve selectivity for the phosphotyrosine of BIN2, and is pTyr dephosphorylation a general property of the Kelch-phosphatase (BSL) family?
Q: Which of BSU1's two pools (plasma membrane vs nucleus) carries out BIN2 inactivation in vivo?
Experiment: Phosphatase assays of recombinant BSU1, BSL1-3 against phospho-Tyr and phospho-Ser/Thr peptides from GSK3-like kinases and BZR1/BES1, to define substrate preference quantitatively.
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