PBL27 (AVRPPHB SUSCEPTIBLE1-LIKE 27; At5g18610) is a receptor-like cytoplasmic kinase of subfamily VII (RLCK VII-1) in Arabidopsis thaliana and the counterpart of rice OsRLCK185. It is a catalytically active serine/threonine protein kinase held at the inner face of the plasma membrane, where it associates with the chitin receptor kinase CERK1 in the LYK5-CERK1 receptor complex. After chitin perception CERK1 phosphorylates PBL27 in its activation-loop region, and PBL27 in turn phosphorylates downstream targets. In the chitin-to-MAPK route it phosphorylates the C-terminal domain of the MAP kinase kinase kinase MAPKKK5 (strongest at Ser-622), with which it is pre-associated at the plasma membrane and from which it dissociates upon chitin treatment, contributing to activation of the MKK4/MKK5-MPK3/MPK6 cascade; independent genetic work indicates that RLCK VII-4 members act redundantly or predominantly in this step. In guard cells PBL27 phosphorylates the S-type anion channel SLAH3 at Ser-127 and Ser-189, activating anion efflux required for chitin-induced stomatal closure. PBL27 is not needed for the chitin-induced ROS burst, which runs through BIK1-type kinases, and contributes little to flagellin signalling. Loss of PBL27 reduces resistance to the necrotrophic fungus Alternaria brassicicola and to non-virulent Pseudomonas syringae, and PBL27 also acts with the receptor kinase HAK1 in responses to insect oral-secretion polysaccharides.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004672 protein kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA protein kinase activity from the RLCK VII PANTHER node; PBL27 is an active kinase with demonstrated substrates (MAPKKK5, SLAH3). Reason: Phylogenetically sound and experimentally confirmed on PBL27 itself by in vitro kinase assays. The more specific Ser/Thr kinase terms are also annotated. Supporting Evidence: PMID:27679653 PBL27 directly phosphorylated only the C‐terminal domain of MAPKKK5 PMID:31524595 We conclude that PBL27 is the primary kinase that directly phosphorylates SLAH3 for the release of anions and functions in an activation status-dependent manner regulated by CERK1. |
| GO:0004672 protein kinase activity | IDA PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | ACCEPT | Summary: IDA from Yamada et al. 2016: PBL27 directly phosphorylates the MAPKKK5 C-terminal domain in vitro; mapped acceptor sites are serines and a threonine. Reason: Directly supported by in vitro kinase assays with MAPKKK5. The paper also identifies Ser/Thr acceptor residues on MAPKKK5 (S617, S622, S658, S660, T677, S685), so the more specific protein serine/threonine kinase activity (already annotated via EC mapping) is the term used in core functions. Supporting Evidence: PMID:27679653 PBL27 directly phosphorylated only the C‐terminal domain of MAPKKK5 PMID:27679653 The S622A mutation strongly reduces phosphorylation by PBL27. |
| GO:0004672 protein kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based IEA for protein kinase activity (kinase domain, active site signatures). Reason: Correct; PBL27 has an intact, catalytically active kinase domain. Less specific than the Ser/Thr terms but not wrong. Supporting Evidence: PMID:27679653 PBL27 directly phosphorylated only the C‐terminal domain of MAPKKK5 |
| GO:0004674 protein serine/threonine kinase activity | IEA GO_REF:0000003 | ACCEPT | Summary: EC 2.7.11.1 mapping to protein serine/threonine kinase activity. Reason: UniProt records both serine and threonine phosphorylation with experimental evidence; PBL27 phosphorylates Ser and Thr residues of MAPKKK5 and Ser residues of SLAH3. This is the core molecular function. Supporting Evidence: PMID:27679653 The S622A mutation strongly reduces phosphorylation by PBL27. PMID:31524595 PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. |
| GO:0005515 protein binding | IPI PMID:24750441 Selective regulation of the chitin-induced defense response ... | REMOVE | Summary: IPI with CERK1 (At3g21630) from Shinya et al. 2014; CERK1 phosphorylates PBL27 and associates with it at the plasma membrane. Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. Supporting Evidence: PMID:24750441 PBL27, an Arabidopsis ortholog of OsRLCK185, is an immediate downstream component of the chitin receptor CERK1 PMID:24750441 CERK1 preferentially phosphorylated PBL27 in comparison to BIK1 |
| GO:0005515 protein binding | IPI PMID:24750441 Selective regulation of the chitin-induced defense response ... | REMOVE | Summary: IPI with CERK1 (A8R7E6) from Shinya et al. 2014. Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. Supporting Evidence: PMID:24750441 PBL27, an Arabidopsis ortholog of OsRLCK185, is an immediate downstream component of the chitin receptor CERK1 |
| GO:0005515 protein binding | IPI PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | REMOVE | Summary: IPI with CERK1 (A8R7E6) from Yamada et al. 2016 (CERK1-PBL27 complex at the plasma membrane). Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. Supporting Evidence: PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0005515 protein binding | IPI PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | REMOVE | Summary: IPI with MAPKKK3 (F4HRJ4, At1g53570) from a yeast two-hybrid screen in Yamada et al. 2016; the interaction was weak and mapkkk3 did not affect chitin-induced MAPK activation. Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. The MAPKKK3 interaction was weak in yeast and was not followed up. Supporting Evidence: PMID:27679653 PBL27 interacted strongly with MAPKKK5 (At5g66850) and weakly with MAPKKK3 (At1g53570) |
| GO:0005515 protein binding | IPI PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | REMOVE | Summary: IPI with MAPKKK5 (Q9C5H5) from Yamada et al. 2016 (Y2H, co-IP, BiFC at the plasma membrane); MAPKKK5 is the PBL27 substrate. Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. Supporting Evidence: PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex PMID:27679653 PBL27 phosphorylates MAPKKK5 in a CERK1‐dependent manner. |
| GO:0005515 protein binding | IPI PMID:32385340 Soy and Arabidopsis receptor-like kinases respond to polysac... | REMOVE | Summary: IPI with the receptor kinase HAK1 (At1g06840) from Uemura et al. 2020 (AlphaScreen, BiFC, co-IP) in the herbivore oral-secretion polysaccharide response. Reason: Bare protein binding is uninformative. The interaction itself is real and removal does not dispute it; it is an enzyme-substrate relation (PBL27 is phosphorylated by CERK1, or phosphorylates the partner) that is already captured by the protein serine/threonine kinase activity and the signalling process terms, and no more informative binding term is warranted. Removed, consistent with the CERK1 review. No downstream substrate of PBL27 was identified in this herbivore pathway. Supporting Evidence: PMID:32385340 immunoprecipitation assays also confirmed in vivo interactions between AtHAK1 molecules themselves or AtHAK1 and AtPBL27 in N. benthamiana leaves following their transient expression |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: IEA ATP binding from the protein kinase domain signature. Reason: PBL27 is a catalytically active kinase and therefore binds ATP; the kinase-dead K112E variant is used as a negative control in the substrate assays. Supporting Evidence: PMID:27679653 PBL27 directly phosphorylated only the C‐terminal domain of MAPKKK5 |
| GO:0005886 plasma membrane | HDA PMID:14506206 Large-scale analysis of in vivo phosphorylated membrane prot... | ACCEPT | Summary: HDA from a large-scale IMAC phosphoproteomic analysis of Arabidopsis plasma membrane fractions. Reason: Consistent with the direct imaging evidence and the predicted N-terminal palmitoylation that anchors RLCK VII proteins to the plasma membrane. Abstract only cached; the high-throughput identification is corroborative. Supporting Evidence: PMID:14506206 identification of plasma membrane phosphoproteins of Arabidopsis PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: IBA is_active_in plasma membrane for the RLCK VII clade. Reason: PBL27 acts at the plasma membrane, where it forms complexes with CERK1, MAPKKK5 and SLAH3. Supporting Evidence: PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0005886 plasma membrane | IDA PMID:24750441 Selective regulation of the chitin-induced defense response ... | ACCEPT | Summary: IDA plasma membrane from Shinya et al. 2014 (abstract only cached); UniProt records cell membrane localization and lipid anchor from this paper. Reason: Consistent with all other localization evidence; curators read the full text. Supporting Evidence: PMID:24750441 PBL27, an Arabidopsis ortholog of OsRLCK185, is an immediate downstream component of the chitin receptor CERK1 PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0005886 plasma membrane | IDA PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | ACCEPT | Summary: IDA plasma membrane from Yamada et al. 2016 (BiFC with plasmolysis places the PBL27-MAPKKK5 complex at the plasma membrane). Reason: Direct evidence of the site of action. Supporting Evidence: PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: ARBA-based IEA plasma membrane. Reason: Correct; agrees with experimental localization. Supporting Evidence: PMID:27679653 PBL27 interacts with MAPKKK5 at the PM where CERK1 and PBL27 form the complex |
| GO:0043410 positive regulation of MAPK cascade | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | ACCEPT | Summary: IMP from Shinya et al. 2014: pbl27 knockouts show reduced chitin-induced MPK3/6 activation. Yamada et al. 2016 supplied the mechanism: PBL27 phosphorylates MAPKKK5, the MAP3K upstream of MKK4/MKK5-MPK3/MPK6. Reason: PBL27 performs a step in the activation of the MAPK cascade (phosphorylation of MAPKKK5), so positive regulation of MAPK cascade is an appropriate process term and matches the module annoton. The quantitative genetic requirement is disputed: Rao et al. 2018 and Bi et al. 2018 attribute chitin-induced MAPKKK5 phosphorylation and MAPK activation mainly to RLCK VII-4 members (e.g. PBL19 at MAPKKK5 Ser-599). Those papers are abstract-only in the cache and do not contradict the biochemical activity, so the curator-made annotation is kept, with the dispute recorded. Supporting Evidence: PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. PMID:27679653 PBL27 phosphorylates MAPKKK5 in a CERK1‐dependent manner. PMID:29907700 RLCK VII-4 members were required for the chitin-triggered activation of MAPK, demonstrating that these kinases link a PRR to MAPK activation PMID:29871986 directly phosphorylate MAPKKK5 Ser-599, which is required for pattern-triggered MPK3/6 activation file:ARATH/PBL27/PBL27-deep-research-falcon.md The claim that PBL27 is the uniquely required chitin-to-MAPK connector remains contested. |
| GO:0045087 innate immune response | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | MODIFY | Summary: IMP innate immune response (acts upstream of or within) from Shinya et al. 2014, based on loss of chitin-induced defence responses in pbl27. Reason: Correct in direction but very broad. PBL27 is a relay kinase within the CERK1 pattern recognition receptor pathway: it is phosphorylated by CERK1 and phosphorylates MAPKKK5 and SLAH3. The more precise term cell surface pattern recognition receptor signaling pathway (GO:0002752) describes the work PBL27 does and matches the CERK1 review and the chitin perception module. Proposed replacements: cell surface pattern recognition receptor signaling pathway Supporting Evidence: PMID:24750441 PBL27, an Arabidopsis ortholog of OsRLCK185, is an immediate downstream component of the chitin receptor CERK1 PMID:27679653 PBL27 phosphorylates MAPKKK5 in a CERK1‐dependent manner. |
| GO:0045088 regulation of innate immune response | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | KEEP AS NON CORE | Summary: IMP regulation of innate immune response from Shinya et al. 2014. Reason: Not wrong, since PBL27 positively relays chitin-triggered immune signalling, but it is a general regulatory restatement of the phenotype; the core process is the receptor signalling pathway itself. Supporting Evidence: PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. |
| GO:0050832 defense response to fungus | IDA PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | KEEP AS NON CORE | Summary: IDA defense response to fungus from Yamada et al. 2016. The full text cites, rather than re-tests, the pbl27 Alternaria brassicicola phenotype (Shinya et al. 2014) and tests mapkkk5 mutants directly. Reason: Reduced resistance to A. brassicicola in pbl27 shows that PBL27 is needed for full antifungal defence, but PBL27 acts upstream as a signalling kinase rather than carrying out a defence response itself; the mechanistic terms are the receptor signalling pathway and the kinase activity. Liu et al. 2019 also found that pbl27 did not show increased Botrytis lesions, suggesting redundancy. Kept as a non-core phenotype-level process. Supporting Evidence: PMID:27679653 As reported previously, lyk5, cerk1, and pbl27 mutations reduce resistance to the fungal pathogen Alternaria brassicicola PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. |
| GO:0071323 cellular response to chitin | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | ACCEPT | Summary: IMP cellular response to chitin from Shinya et al. 2014. Reason: PBL27 is a CERK1 substrate and a component of chitin signalling, required for chitin-induced MAPK activation, callose deposition and stomatal closure; it relays the chitin signal rather than merely being required for it. Supporting Evidence: PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. PMID:31524595 consistently stomata of pbl27 mutants showed no closure to chitin either |
| GO:0106310 protein serine kinase activity | EXP PMID:27679653 The Arabidopsis CERK1-associated kinase PBL27 connects chiti... | ACCEPT | Summary: EXP protein serine kinase activity from Yamada et al. 2016 (phosphorylation of MAPKKK5 serine residues). Reason: Directly supported by mapped phosphoserine sites on MAPKKK5 and by SLAH3 serine phosphorylation. Supporting Evidence: PMID:27679653 The S622A mutation strongly reduces phosphorylation by PBL27. PMID:31524595 PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. |
| GO:0106310 protein serine kinase activity | IEA GO_REF:0000116 | ACCEPT | Summary: Rhea-based IEA protein serine kinase activity. Reason: Consistent with the experimental data. Supporting Evidence: PMID:27679653 The S622A mutation strongly reduces phosphorylation by PBL27. |
| GO:1900150 regulation of defense response to fungus | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | KEEP AS NON CORE | Summary: IMP regulation of defense response to fungus from Shinya et al. 2014. Reason: Phenotype-level consequence of PBL27 acting in chitin signalling; retained as non-core. Supporting Evidence: PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. |
| GO:1900426 positive regulation of defense response to bacterium | IMP PMID:24750441 Selective regulation of the chitin-induced defense response ... | KEEP AS NON CORE | Summary: IMP positive regulation of defense response to bacterium from Shinya et al. 2014 (reduced resistance to P. syringae pv. tomato DC3000 hrcC according to UniProt). Reason: Supported by the knockout phenotype; PBL27 contributes indirectly to antibacterial defence (bacteria also release chitin-like/peptidoglycan signals perceived via CERK1). Non-core. Supporting Evidence: PMID:24750441 Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. |
| GO:0090333 regulation of stomatal closure | IDA PMID:31524595 Anion channel SLAH3 is a regulatory target of chitin recepto... | NEW | Summary: PBL27 directly phosphorylates the S-type anion channel SLAH3 at Ser-127 and Ser-189, activating anion currents; pbl27 stomata do not close in response to chitin, and SLAH3 S127A/S189A variants fail to restore chitin-induced closure. Reason: Participation test: PBL27 performs the regulatory step itself by phosphorylating and activating the channel that drives closure, rather than being merely required. Comparator check: the related RLCK VII kinase BIK1, which phosphorylates the calcium channel OSCA1.3 in stomatal immunity, carries regulation of stomatal movement. No GOA annotation captures this experimentally established PBL27 function. Supporting Evidence: PMID:31524595 PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. PMID:31524595 We conclude that PBL27 is the primary kinase that directly phosphorylates SLAH3 for the release of anions and functions in an activation status-dependent manner regulated by CERK1. PMID:31524595 consistently stomata of pbl27 mutants showed no closure to chitin either |
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Download this section (compressed HTML)Q: How large is PBL27's contribution to chitin-induced MAPKKK5 phosphorylation and MPK3/MPK6 activation relative to RLCK VII-4 kinases such as PBL19, given that Shinya et al. 2014 and Rao et al. 2018 / Bi et al. 2018 reached different conclusions with pbl27 alleles?
Suggested experts: Plant immune signalling researchers
Q: Do the PBL27 sites on MAPKKK5 (Ser-622 region) and the RLCK VII-4 site (Ser-599) act additively or in distinct cell types?
Suggested experts: Plant kinase biochemists
Q: What substrate(s) does PBL27 phosphorylate in the HAK1 herbivore oral-secretion pathway?
Experiment: Combine pbl27 with rlck vii-4 higher-order mutants and quantify chitin-induced MAPKKK5 phosphorylation (Ser-599 and Ser-622 phospho-specific antibodies) and MPK3/MPK6 activation side by side with independent pbl27 alleles.
Hypothesis: PBL27 and RLCK VII-4 kinases act redundantly upstream of MAPKKK5.
Experiment: Express wild-type or kinase-dead PBL27 from a guard-cell-specific promoter in pbl27 and measure chitin-induced stomatal closure and SLAH3 Ser-127/Ser-189 phosphorylation.
Hypothesis: PBL27 activity in guard cells is sufficient to restore chitin-induced stomatal closure.
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