PBL1 (PBS1-LIKE 1, also called BLK or CCE5) is a receptor-like cytoplasmic kinase of subfamily VII in Arabidopsis thaliana and the closest paralog of BIK1. It is a myristoylated serine/threonine protein kinase anchored at the cytoplasmic face of the plasma membrane, where it associates with cell-surface pattern recognition receptors including FLS2, EFR, PEPR1 and MIK2, and becomes phosphorylated within minutes of ligand perception. Acting partly redundantly with BIK1, PBL1 relays receptor activation to early immune outputs: it is required for full cytosolic calcium elevations triggered by flg22, elf18 and AtPep1 (but not by chitin), contributes to the RBOHD-dependent oxidative burst and is the main RLCK for flg22-induced root growth inhibition. Direct substrates include the cytosolic loops of the PHT1;1 and PHT1;4 phosphate transporters, linking immune signalling to inhibition of phosphate uptake, and the autoinhibitory serine cluster of the vacuolar Ca2+/H+ exchangers CAX1 and CAX3. PBL1 is also targeted for cleavage by the Pseudomonas syringae effector protease AvrPphB.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002221 pattern recognition receptor signaling pathway | IBA GO_REF:0000033 | ACCEPT | Summary: PBL1 relays signalling from several cell-surface PRRs (FLS2, EFR, PEPR1, MIK2) as a BIK1-like RLCK; the PAINT node covering RLCK-VII members fits the experimental data. Reason: PBL1 associates with PRRs, is phosphorylated upon ligand perception and is needed for PRR-triggered Ca2+ elevation; it does the work of relaying the signal by phosphorylating downstream substrates. The IBA is at an appropriate level and agrees with the BIK1 review. Supporting Evidence: PMID:20413097 In unstimulated plants, BIK1 and PBL1 interact with FLS2 and are rapidly phosphorylated upon FLS2 activation by its ligand flg22. PMID:23431184 PEPR1 specifically interacts with receptor-like cytoplasmic kinases botrytis-induced kinase 1 (BIK1) and PBS1-like 1 (PBL1) to mediate Pep1-induced defenses. PMID:25522736 Using these lines, a survey of different MAMPs/DAMPs showed reduced calcium responses to flg22, elf18 and AtPep1 but a normal response to chitin octamers (ch8) in cce5 (Figure 2). PMID:34535661 Nevertheless, the MIK2-BAK1 receptor complex activation by SCOOPs triggers phosphorylation of BIK1 and PBL1 in transducing signaling to downstream events. file:ARATH/PBL1/PBL1-deep-research-falcon.md These findings place it immediately downstream of plasma-membrane pattern recognition |
| GO:0002237 response to molecule of bacterial origin | IMP PMID:25522736 Microbe-associated molecular pattern-induced calcium signali... | ACCEPT | Summary: pbl1/cce5 mutants show reduced cytosolic Ca2+ responses to flg22 and elf18 and reduced flg22-induced root growth inhibition. Reason: Mutant phenotype in responses to the bacterial MAMPs flg22 and elf18 in the paper cited (full text available). PBL1 acts as a signalling kinase in these responses (receptor-proximal phosphorylation relay), so this is a direct role rather than an indirect effect. Treated as core, consistent with the BIK1 review. Supporting Evidence: PMID:25522736 Using these lines, a survey of different MAMPs/DAMPs showed reduced calcium responses to flg22, elf18 and AtPep1 but a normal response to chitin octamers (ch8) in cce5 (Figure 2). PMID:25522736 PBL1 plays a more important role than BIK1 in the late root growth inhibition response to flg22 |
| GO:0004672 protein kinase activity | IEA GO_REF:0000002 | MODIFY | Summary: Generic protein kinase activity from InterPro kinase-domain mapping. Correct but less specific than the serine/threonine kinase activity supported for PBL1. Reason: PBL1 has demonstrated autophosphorylation and directly phosphorylates protein substrates (PHT1 loops, CAX S-cluster); serine/threonine kinase activity (GO:0004674) is the appropriate specific term, already supported by IBA. Proposed replacements: protein serine/threonine kinase activity Supporting Evidence: PMID:25522736 Taken together, these three cce5 mutations led to the loss of PBL1 kinase activity PMID:38418878 the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca2+ signals in immunity |
| GO:0004674 protein serine/threonine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Serine/threonine kinase activity inherited across the RLCK-VII clade; supported in PBL1 by autophosphorylation and in vitro phosphorylation of transporter substrates. Reason: Core molecular function. cce5 kinase-domain alleles abolish autophosphorylation and loss of function; recombinant PBL1 but not kinase-dead PBL1 phosphorylates PHT1;4/PHT1;1 cytosolic loops, and PBL1 phosphorylates the CAX1/CAX3 serine cluster. Supporting Evidence: PMID:25522736 Taken together, these three cce5 mutations led to the loss of PBL1 kinase activity PMID:34919806 Indeed, maltose-binding protein (MBP)-fused BIK1 and PBL1, but not their kinase-dead variants, phosphorylated the glutathione S-transferase (GST)-fused cytosolic loop of PHT1;4 PMID:38418878 the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca2+ signals in immunity |
| GO:0004674 protein serine/threonine kinase activity | IEA GO_REF:0000003 | ACCEPT | Summary: EC 2.7.11.1 mapping; consistent with experimental kinase evidence. Reason: Agrees with the IBA and with direct kinase assays on PBL1. Supporting Evidence: PMID:25522736 Taken together, these three cce5 mutations led to the loss of PBL1 kinase activity PMID:34919806 Indeed, maltose-binding protein (MBP)-fused BIK1 and PBL1, but not their kinase-dead variants, phosphorylated the glutathione S-transferase (GST)-fused cytosolic loop of PHT1;4 |
| GO:0005515 protein binding | IPI PMID:20413097 Receptor-like cytoplasmic kinases integrate signaling from m... | REMOVE | Summary: Co-immunoprecipitation of PBL1 with the flagellin receptor FLS2 (Q9FL28). Bare protein binding is uninformative. Reason: Removal does not mean the interaction is false: the receptor association is real and biologically central (PBL1 is a PRR-associated RLCK that is phosphorylated upon receptor activation), but bare GO:0005515 conveys no function; the relationship is captured by the kinase MF and PRR signalling BP. Treated as in the BIK1 review. Supporting Evidence: PMID:20413097 In unstimulated plants, BIK1 and PBL1 interact with FLS2 and are rapidly phosphorylated upon FLS2 activation by its ligand flg22. |
| GO:0005515 protein binding | IPI PMID:23431184 BIK1 interacts with PEPRs to mediate ethylene-induced immuni... | REMOVE | Summary: Interaction of PBL1 with the Pep1 receptor PEPR1 (Q9SSL9). Bare protein binding is uninformative. Reason: Removal does not mean the interaction is false: the receptor association is real and biologically central (PBL1 is a PRR-associated RLCK that is phosphorylated upon receptor activation), but bare GO:0005515 conveys no function; the relationship is captured by the kinase MF and PRR signalling BP. Treated as in the BIK1 review. Supporting Evidence: PMID:23431184 PEPR1 specifically interacts with receptor-like cytoplasmic kinases botrytis-induced kinase 1 (BIK1) and PBS1-like 1 (PBL1) to mediate Pep1-induced defenses. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: ATP binding inferred from the protein kinase domain and ATP-binding site signature. Reason: PBL1 is an active kinase (autophosphorylation abolished by kinase-domain mutations), so ATP binding by the canonical kinase cleft is expected. Supportive rather than informative. Supporting Evidence: PMID:25522736 Taken together, these three cce5 mutations led to the loss of PBL1 kinase activity |
| GO:0005829 cytosol | HDA PMID:28887381 Global Analysis of Membrane-associated Protein Oligomerizati... | KEEP AS NON CORE | Summary: Detected in a proteomic membrane/cytosol fractionation study (protein correlation profiling). Reason: PBL1 is a myristoylated peripheral membrane protein whose functional location is the cytoplasmic face of the plasma membrane; a soluble/cytosolic pool is plausible for a lipid-anchored RLCK and the high-throughput observation is not contradicted, but the cytosol is not where its signalling function has been demonstrated. Supporting Evidence: PMID:25522736 Hence, myristoylation and proper targeting of PBL1 to plasma membrane is essential for signaling function of PBL1. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Plasma membrane localization inherited across RLCK-VII; experimentally supported for PBL1. Reason: Myristoylation-dependent plasma membrane targeting is required for PBL1 signalling; the G2A variant fails to complement. Supporting Evidence: PMID:25522736 Hence, myristoylation and proper targeting of PBL1 to plasma membrane is essential for signaling function of PBL1. |
| GO:0005886 plasma membrane | IDA PMID:25522736 Microbe-associated molecular pattern-induced calcium signali... | ACCEPT | Summary: Plasma membrane location supported by G2A myristoylation-site analysis in the cited paper. Reason: The paper shows the G2A variant is not phosphorylated after flg22 and fails to complement calcium and root phenotypes, concluding that plasma membrane targeting is essential; consistent with the IBA and with BIK1. Supporting Evidence: PMID:25522736 Hence, myristoylation and proper targeting of PBL1 to plasma membrane is essential for signaling function of PBL1. |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location mapping (cell membrane, lipid anchor). Reason: Consistent with experimental evidence. Supporting Evidence: PMID:25522736 Hence, myristoylation and proper targeting of PBL1 to plasma membrane is essential for signaling function of PBL1. |
| GO:0009507 chloroplast | ISM GO_REF:0000122 | REMOVE | Summary: AtSubP sequence-based prediction of chloroplast localization. Reason: PBL1 is an N-myristoylated kinase that functions at the plasma membrane; no experimental evidence places it in plastids, and the single-method prediction conflicts with the experimentally supported location. Supporting Evidence: PMID:25522736 Hence, myristoylation and proper targeting of PBL1 to plasma membrane is essential for signaling function of PBL1. |
| GO:0106310 protein serine kinase activity | IEA GO_REF:0000116 | ACCEPT | Summary: Rhea-based protein serine kinase activity; a child-like specification of the Ser/Thr kinase activity. Reason: Correct for a Ser/Thr kinase that phosphorylates serine clusters (CAX S-cluster); redundant with GO:0004674 but not wrong. Supporting Evidence: PMID:38418878 the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca2+ signals in immunity |
| GO:1900424 regulation of defense response to bacterium | IMP PMID:25522736 Microbe-associated molecular pattern-induced calcium signali... | MARK AS OVER ANNOTATED | Summary: Assigned from the calcium-signalling mutant study; that paper did not test PBL1 in bacterial infection and notes that PBL1 is not needed for flg22-induced resistance to Pseudomonas syringae. Reason: The phenotypes in PMID:25522736 (full text read) are reduced MAMP-induced Ca2+ elevation and root growth inhibition, which are captured by GO:0002237 and PRR signalling. The paper explicitly states that BIK1, not PBL1, is important for flg22-mediated resistance to P. syringae. The term is not entirely wrong (PTI outputs contribute to antibacterial defense, and AvrPphB targets PBL1), but bacterial-defense regulation is not shown for PBL1 itself. Supporting Evidence: PMID:25522736 BIK1, but not PBL1, has been shown to play an important role in flg22-mediated resistance to subsequent Pseudomonas syringae infection PMID:25522736 Using these lines, a survey of different MAMPs/DAMPs showed reduced calcium responses to flg22, elf18 and AtPep1 but a normal response to chitin octamers (ch8) in cce5 (Figure 2). |
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Download this section (compressed HTML)Q: Does PBL1 itself phosphorylate RBOHD? PBL1 binds the RBOHD N-terminus, but RBOHD phosphosite loss has only been measured in bik1 pbl1 double mutants and direct site-resolved phosphorylation was shown for BIK1.
Q: Does PBL1 contribute to chitin/CERK1 signalling at all, given that cce5/pbl1 mutants show a normal chitin-induced Ca2+ response?
Q: Should the direct PBL1-PHT1 kinase relationship be captured as a process annotation (e.g. negative regulation of phosphate transport) or only via has_input on the kinase activity?
Experiment: In vitro kinase assay of recombinant PBL1 versus kinase-dead PBL1 on the RBOHD N-terminus with phosphosite mapping, and chitin-induced ROS/RBOHD phosphorylation in pbl1 single and bik1 pbl1 double mutants.
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