CnoX (formerly YbbN) is an E. coli chaperedoxin -- a bifunctional protein that combines ATP-independent holdase chaperone activity with a redox-protective function. It contains an N-terminal thioredoxin-like domain (lacking the canonical CXXC active site and therefore not a functional oxidoreductase) fused to a C-terminal tetratricopeptide repeat (TPR) domain. Upon activation by hypochlorous acid (HOCl/bleach) via chlorination of its TPR domain, CnoX functions as an efficient holdase, binding unfolded client proteins to prevent their aggregation (PMID:29754824). Uniquely, CnoX also protects its bound substrates from irreversible oxidation by forming mixed disulfide bonds via Cys-63 (PMID:29754824). After bleach stress subsides, CnoX transfers its substrates to the major ATP-dependent foldases GroEL/GroES and DnaK/DnaJ/GrpE for refolding (PMID:29754824, PMID:18657513). CnoX interacts with and coordinately regulates GroEL/GroES (as a mild inhibitor) and DnaK/DnaJ/GrpE (as an enhancer), suggesting it helps direct client protein traffic between these two major chaperone systems (PMID:21498507, PMID:18657513). CnoX is the founding member of the chaperedoxin protein family.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0045454
cell redox homeostasis
|
IBA
GO_REF:0000033 |
MODIFY |
Summary: IBA annotation based on phylogenetic inference. CnoX contains a thioredoxin-like domain and was initially thought to have oxidoreductase activity (PMID:16563353). However, multiple studies have now conclusively shown that CnoX lacks a canonical CXXC active site and is NOT a functional oxidoreductase (PMID:21498507, PMID:29754824). The thioredoxin domain instead forms mixed disulfide bonds with substrates to protect them from irreversible oxidation during HOCl stress (PMID:29754824), which is a protective/chaperone function rather than classical redox homeostasis. The IBA annotation likely propagated from ancestral thioredoxin-family proteins that do have oxidoreductase activity.
Reason: While CnoX has a thioredoxin fold and forms disulfide bonds with substrates, it is not a classical oxidoreductase that maintains redox homeostasis. UniProt explicitly states "Lacks oxidoreductase activity" based on PMID:21498507 and PMID:29754824. The IBA likely reflects ancestral thioredoxin function that has been repurposed in CnoX. A more accurate biological process annotation would be 'cellular response to oxidative stress' (GO:0034599) which is already annotated, or 'maintenance of unfolded protein' (GO:0036506).
Proposed replacements:
cellular response to oxidative stress
Supporting Evidence:
PMID:21498507
The Trx domain lacks a canonical CXXC active site architecture and is not a functional oxidoreductase.
PMID:29754824
CnoX uniquely combines this function with the ability to prevent the irreversible oxidation of its substrates.
|
|
GO:0006950
response to stress
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: IEA annotation from ARBA machine learning model. CnoX is indeed involved in the stress response -- it is activated by HOCl (bleach) stress and protects proteins from aggregation and irreversible oxidation during oxidative stress (PMID:29754824). The ybbN-deficient strain also shows increased sensitivity to thermal stress (PMID:18657513). However, GO:0006950 'response to stress' is very broad.
Reason: While GO:0006950 is very general, it is acceptable as an IEA annotation. The more specific term GO:0034599 'cellular response to oxidative stress' is already annotated with IMP evidence from PMID:29754824. Keeping this broader IEA annotation is fine alongside the more specific experimental one.
Supporting Evidence:
PMID:29754824
Bleach (HOCl) is a powerful oxidant that kills bacteria in part by causing protein aggregation. It inactivates ATP-dependent chaperones, rendering cellular proteins mostly dependent on holdases.
PMID:18657513
an ybbN-deficient strain displays an increased sensitivity to thermal stress
|
|
GO:0005515
protein binding
|
IPI
PMID:15690043 Interaction network containing conserved and essential prote... |
MODIFY |
Summary: IPI annotation from high-throughput affinity purification-mass spectrometry study (Butland et al. 2005). CnoX (YbbN) was identified as interacting with DnaN (P0A988) in this large-scale E. coli protein complex network study. While the interaction is likely real, 'protein binding' is uninformative. CnoX functions as a holdase chaperone and co-chaperone, and its protein interactions reflect this function.
Reason: GO:0005515 'protein binding' is uninformative per GO curation guidelines. CnoX's interactions with client proteins and chaperone partners (DnaK, GroEL) reflect its holdase/co-chaperone function. A more informative MF term such as GO:0051087 'protein-folding chaperone binding' would better capture the nature of CnoX's protein interactions, particularly given that CnoX cooperates with DnaK and GroEL foldase systems.
Proposed replacements:
protein-folding chaperone binding
Supporting Evidence:
PMID:15690043
An interaction network of protein complexes involved in diverse biological processes was uncovered and validated by sequential rounds of tagging and purification.
|
|
GO:0005515
protein binding
|
IPI
PMID:18657513 The thioredoxin homolog YbbN functions as a chaperone rather... |
MODIFY |
Summary: IPI annotation based on reverse purification experiments showing CnoX (YbbN) specifically interacts with DnaK (P0A6F5), GroEL (P0A6Y8), trigger factor (P0C8J6), and other proteins. Kthiri et al. showed that YbbN cooperates with DnaK for protein renaturation and interacts with multiple chaperone system components (PMID:18657513).
Reason: 'Protein binding' is uninformative. The specific interactions with DnaK and GroEL documented in PMID:18657513 are functionally significant -- CnoX enhances DnaK-mediated refolding 4-fold and specifically interacts with the major foldases. GO:0051087 'protein-folding chaperone binding' is a more informative term for these interactions.
Proposed replacements:
protein-folding chaperone binding
Supporting Evidence:
PMID:18657513
YbbN specifically interacts with DnaK and GroEL, as shown by reverse purification. It increases 4-fold the rate of protein renaturation in vitro by the DnaK chaperone machine
|
|
GO:0005515
protein binding
|
IPI
PMID:21498507 Escherichia coli thioredoxin-like protein YbbN contains an a... |
MODIFY |
Summary: IPI annotation from Lin and Wilson (2011), who resolved the crystal structure of YbbN/CnoX and identified its interacting partners including multiple ribosomal protein subunits and a strong interaction with GroEL. They showed CnoX acts as a mild inhibitor of GroEL/GroES chaperonin function and ATPase activity (PMID:21498507).
Reason: 'Protein binding' is uninformative. The interactions documented in PMID:21498507 include a strong, functionally characterized interaction with GroEL -- CnoX negatively regulates GroEL. GO:0051087 'protein-folding chaperone binding' better captures this interaction.
Proposed replacements:
protein-folding chaperone binding
Supporting Evidence:
PMID:21498507
A variety of proteins in E. coli interact with YbbN, including multiple ribosomal protein subunits and a strong interaction with GroEL. YbbN acts as a mild inhibitor of GroESL chaperonin function and ATPase activity, suggesting that it is a negative regulator of the GroESL system.
|
|
GO:0034599
cellular response to oxidative stress
|
IMP
PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... |
ACCEPT |
Summary: IMP annotation based on Goemans et al. (2018), the landmark study establishing CnoX as a chaperedoxin. The cnoX mutant is highly sensitive to HOCl (hypochlorous acid/bleach), which is a powerful oxidant. CnoX is activated by HOCl via chlorination and then protects substrates from both aggregation and irreversible oxidation. This is a core function of CnoX.
Reason: Well-supported by mutant phenotype data. The cnoX deletion mutant is highly sensitive to HOCl stress. CnoX is specifically activated by oxidative stress (HOCl-mediated chlorination of its TPR domain) and protects cellular proteins from oxidative damage. This is a core biological process for this chaperedoxin.
Supporting Evidence:
PMID:29754824
Bleach (HOCl) is a powerful oxidant that kills bacteria in part by causing protein aggregation. It inactivates ATP-dependent chaperones, rendering cellular proteins mostly dependent on holdases. Here we identified Escherichia coli CnoX (YbbN) as a folding factor that, when activated by bleach via chlorination, functions as an efficient holdase
|
|
GO:0036506
maintenance of unfolded protein
|
IDA
PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... |
ACCEPT |
Summary: IDA annotation from Goemans et al. (2018). CnoX functions as an efficient holdase that maintains client proteins in an unfolded but non-aggregated state during HOCl stress, preventing their irreversible aggregation and oxidation. After stress resolution, CnoX transfers these clients to the GroEL/GroES and DnaK/DnaJ/GrpE foldase systems for refolding (PMID:29754824). This is a core function of CnoX.
Reason: Directly supported by experimental data showing CnoX acts as an ATP-independent holdase that maintains client proteins in an unfolded, refolding-competent state. This is one of the two core molecular activities of the chaperedoxin -- the holdase/chaperone function that prevents protein aggregation during oxidative stress.
Supporting Evidence:
PMID:29754824
CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrates from Irreversible Oxidation ... functions as an efficient holdase, protecting the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE
|
|
GO:0042026
protein refolding
|
IDA
PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... |
KEEP AS NON CORE |
Summary: IDA annotation from Goemans et al. (2018). CnoX itself does not directly refold proteins -- it is an ATP-independent holdase that maintains substrates and then transfers them to GroEL/GroES and DnaK/DnaJ/GrpE for refolding (PMID:29754824). Earlier work by Kthiri et al. showed that YbbN increases 4-fold the rate of protein renaturation by the DnaK chaperone machine (PMID:18657513), but this enhancement is indirect -- CnoX facilitates refolding by delivering substrates to the foldases.
Reason: CnoX participates in protein refolding indirectly by acting as a holdase that maintains substrates in a refolding-competent state and then hands them off to the ATP-dependent foldases GroEL/GroES and DnaK/DnaJ/GrpE. The annotation is not wrong -- CnoX does contribute to protein refolding as part of its overall pathway -- but it is not the core molecular function. The core function is holdase activity and redox protection. Protein refolding is a downstream consequence of CnoX's holdase activity.
Supporting Evidence:
PMID:29754824
functions as an efficient holdase, protecting the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE
PMID:18657513
It increases 4-fold the rate of protein renaturation in vitro by the DnaK chaperone machine, suggesting that it cooperates with DnaK for the optimal expression of several cytoplasmic proteins.
|
|
GO:0051082
unfolded protein binding
|
IDA
PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... |
MODIFY |
Summary: IDA annotation from Goemans et al. (2018). CnoX does bind unfolded proteins, but its function goes beyond simple binding. CnoX is an ATP-independent holdase that actively prevents aggregation of client proteins and escorts them between cellular states -- from unfolded/stressed to the GroEL/GroES and DnaK/DnaJ/GrpE foldase systems (PMID:29754824). The term GO:0051082 'unfolded protein binding' is a child of GO:0005515 'protein binding' and only describes the binding aspect, not the functional chaperone/carrier activity.
Reason: While CnoX does bind unfolded proteins, 'unfolded protein binding' is too simplistic for the actual molecular function. CnoX is an ATP-independent holdase chaperone that binds unfolded clients and escorts/carries them to the major foldase systems. The more appropriate term is GO:0140597 'protein carrier chaperone' (defined as "Binding to and carrying a protein between two different cellular components by moving along with the target protein") or its child GO:0140309 'unfolded protein carrier activity'. CnoX fits the holdase/carrier paradigm -- it binds unfolded clients, prevents their aggregation, and delivers them to GroEL/DnaK for refolding.
Proposed replacements:
unfolded protein binding (retain until holdase NTR is created)
Supporting Evidence:
PMID:29754824
CnoX (YbbN) as a folding factor that, when activated by bleach via chlorination, functions as an efficient holdase, protecting the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE
PMID:21498507
we propose that YbbN coordinately regulates the activities of these two prokaryotic chaperones, thereby helping to direct client protein traffic initially to DnaK
|
|
GO:0005829
cytosol
|
IDA
PMID:15911532 Localization, annotation, and comparison of the Escherichia ... |
ACCEPT |
Summary: IDA annotation based on Lopez-Campistrous et al. (2005) proteomics study. CnoX (YbbN) was identified in the cytosolic fraction of E. coli K-12 by biochemical fractionation followed by 2D gel electrophoresis and tandem mass spectrometry.
Reason: Cytosolic localization is well supported by multiple independent proteomics studies and is consistent with CnoX's function as a cytosolic holdase chaperone that protects intracellular proteins. The reduced form is monomeric in the cytosol.
Supporting Evidence:
PMID:15911532
2,160 were annotated and assigned to the cytosol, periplasm, inner membrane, and outer membrane by biochemical fractionation followed by two-dimensional gel electrophoresis and tandem mass spectrometry
|
|
GO:0005829
cytosol
|
IDA
PMID:18304323 Protein abundance profiling of the Escherichia coli cytosol. |
ACCEPT |
Summary: IDA annotation based on Ishihama et al. (2008) protein abundance profiling study. CnoX (YbbN) was identified among 1103 proteins from the cytosolic fraction of E. coli strain MC4100 by LC-MS/MS with protein and peptide fractionation.
Reason: Independent confirmation of cytosolic localization by a comprehensive proteomics study. Consistent with all other localization data for CnoX.
Supporting Evidence:
PMID:18304323
we identified 1103 proteins from the cytosolic fraction of the Escherichia coli strain MC4100
|
|
GO:0005829
cytosol
|
IDA
PMID:16858726 A complexomic study of Escherichia coli using two-dimensiona... |
ACCEPT |
Summary: IDA annotation based on Lasserre et al. (2006) complexomics study using 2D blue native/SDS-PAGE to separate E. coli protein complexes, identifying CnoX (YbbN) in the cytosolic fraction.
Reason: Third independent confirmation of cytosolic localization from a complexomics study. All three cytosol annotations are consistent and well-supported.
Supporting Evidence:
PMID:16858726
the cytosolic and membrane protein complexes of Escherichia coli were separated. Then, the different partners of each protein complex were identified by LC-MS/MS.
|
Exported on March 22, 2026 at 02:16 AM
Organism: Escherichia coli
Sequence:
MSVENIVNINESNLQQVLEQSMTTPVLFYFWSERSQHCLQLTPILESLAAQYNGQFILAKLDCDAEQMIAAQFGLRAIPTVYLFQNGQPVDGFQGPQPEEAIRALLDKVLPREEELKAQQAMQLMQESNYTDALPLLKDAWQLSNQNGEIGLLLAETLIALNRSEDAEAVLKTIPLQDQDTRYQGLVAQIELLKQAADTPEIQQLQQQVAENPEDAALATQLALQLHQVGRNEEALELLFGHLRKDLTAADGQTRKTFQEILAALGTGDALASKYRRQLYALLY
I begin with the domain architecture. The N-terminus (residues 1โ111) forms IPR013766, a canonical thioredoxin domain, and this core is encompassed by IPR036249 (Thioredoxin-like superfamily, residues 3โ112). This ฮฑ/ฮฒ thioredoxin fold is a versatile redox/scaffold module that commonly supports proteinโprotein interactions and electron-transfer chemistry, even when canonical Cys motifs are absent. Immediately following, the C-terminal half (residues 113โ284) belongs to IPR011990, the tetratricopeptide-like helical domain superfamily. TPR-like repeats create extended helical scaffolds that bind partner proteins with high specificity and multivalency. The ordered arrangementโan N-terminal thioredoxin-like interaction module feeding into a C-terminal TPR-like scaffoldโcreates a bipartite adaptor architecture: the thioredoxin-like core stabilizes and positions complexes, while the TPR-like region recruits and organizes client proteins.
This architecture causes a molecular function dominated by protein binding rather than catalysis. The thioredoxin fold supplies a redox-inert interaction platform, and the TPR-like array provides avid, modular binding surfaces. Together they produce GO:0005515 protein binding as the primary molecular function, consistent with adaptor/scaffold roles and without clear catalytic signatures.
From this binding-centric function, the most parsimonious biological role is assembly and regulation of protein complexes that control stress and metabolic responses in Escherichia coli. TPR-like scaffolds often nucleate large assemblies, and thioredoxin-like modules frequently tune such assemblies through structural coupling. These properties naturally route the protein into broader cellular programs formalized as GO:0008150 cellular component and GO:0051716 cellular processes, interpreted here as a scaffold that coordinates multiple pathways rather than a single defined cascade. Mechanistically, the protein likely stabilizes transient hubs that integrate stress and metabolic signals.
Cellular location follows from the absence of transmembrane segments or export signals and the soluble, repeat-rich scaffold design. The combination of a soluble thioredoxin-like domain and a cytosolic TPR-like region points to a soluble intracellular locale. In bacteria, such scaffolds operate in the cytoplasm where they can access diverse enzymatic and regulatory assemblies; thus the most fitting placement is the cytoplasm.
Integrating these elements yields a mechanistic hypothesis: the thioredoxin-like N-terminus nucleates a platform that binds core partners, while the TPR-like C-terminus recruits client proteins to form higher-order complexes. Likely interaction cohorts include cytosolic chaperone/adaptor systems (e.g., Hsp70/Hsp90 modules), metabolic enzyme assemblies, and signal-responsive regulators. Through multivalent binding, the protein organizes and modulates cytoplasmic complexes that coordinate cellular pathways without direct enzymatic activity.
A soluble adaptor scaffold in Escherichia coli that uses an N-terminal thioredoxin-like module and a C-terminal helical repeat array to organize cytoplasmic protein assemblies. By coupling a thioredoxin-like interaction core to a tetratricopeptide-like binding platform, it mediates multivalent protein binding that stabilizes and coordinates large cytosolic complexes involved in cellular pathways, acting as a noncatalytic organizer rather than an enzyme.
Involved in cellular processes but its exact function is not yet known.
IPR013766, domain) โ residues 1-111IPR036249, homologous_superfamily) โ residues 3-112IPR011990, homologous_superfamily) โ residues 113-284Molecular Function: molecular_function (GO:0003674), binding (GO:0005488), protein binding (GO:0005515), unfolded protein binding (GO:0051082)
Biological Process: cellular component (GO:0008150), cellular process (GO:0009987), biological regulation (GO:0065007), response to stimulus (GO:0050896), cellular processes (GO:0051716), regulation of biological quality (GO:0065008), response to stress (GO:0006950), response to chemical (GO:0042221), protein folding (GO:0006457), chaperone-mediated protein folding (GO:0061077), regulation of protein stability (GO:0031647), response to oxidative stress (GO:0006979), cellular response to chemical stimulus (GO:0070887), cellular response to stress (GO:0033554), protein refolding (GO:0042026), cellular response to oxidative stress (GO:0034599), cellular response to chemical stress (GO:0062197)
Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), cytoplasm (GO:0005737), cytosol (GO:0005829), intracellular anatomical structure (GO:0005622)
Generated by BioReason
Exported on March 22, 2026 at 02:16 AM
Organism: Escherichia coli
Sequence:
MSVENIVNINESNLQQVLEQSMTTPVLFYFWSERSQHCLQLTPILESLAAQYNGQFILAKLDCDAEQMIAAQFGLRAIPTVYLFQNGQPVDGFQGPQPEEAIRALLDKVLPREEELKAQQAMQLMQESNYTDALPLLKDAWQLSNQNGEIGLLLAETLIALNRSEDAEAVLKTIPLQDQDTRYQGLVAQIELLKQAADTPEIQQLQQQVAENPEDAALATQLALQLHQVGRNEEALELLFGHLRKDLTAADGQTRKTFQEILAALGTGDALASKYRRQLYALLY
I begin with the domain architecture. The N-terminus (residues 1โ111) forms IPR013766, a canonical thioredoxin domain, and this core is encompassed by IPR036249 (Thioredoxin-like superfamily, residues 3โ112). This ฮฑ/ฮฒ thioredoxin fold is a versatile redox/scaffold module that commonly supports proteinโprotein interactions and electron-transfer chemistry, even when canonical Cys motifs are absent. Immediately following, the C-terminal half (residues 113โ284) belongs to IPR011990, the tetratricopeptide-like helical domain superfamily. TPR-like repeats create extended helical scaffolds that bind partner proteins with high specificity and multivalency. The ordered arrangementโan N-terminal thioredoxin-like interaction module feeding into a C-terminal TPR-like scaffoldโcreates a bipartite adaptor architecture: the thioredoxin-like core stabilizes and positions complexes, while the TPR-like region recruits and organizes client proteins.
This architecture causes a molecular function dominated by protein binding rather than catalysis. The thioredoxin fold supplies a redox-inert interaction platform, and the TPR-like array provides avid, modular binding surfaces. Together they produce GO:0005515 protein binding as the primary molecular function, consistent with adaptor/scaffold roles and without clear catalytic signatures.
From this binding-centric function, the most parsimonious biological role is assembly and regulation of protein complexes that control stress and metabolic responses in Escherichia coli. TPR-like scaffolds often nucleate large assemblies, and thioredoxin-like modules frequently tune such assemblies through structural coupling. These properties naturally route the protein into broader cellular programs formalized as GO:0008150 cellular component and GO:0051716 cellular processes, interpreted here as a scaffold that coordinates multiple pathways rather than a single defined cascade. Mechanistically, the protein likely stabilizes transient hubs that integrate stress and metabolic signals.
Cellular location follows from the absence of transmembrane segments or export signals and the soluble, repeat-rich scaffold design. The combination of a soluble thioredoxin-like domain and a cytosolic TPR-like region points to a soluble intracellular locale. In bacteria, such scaffolds operate in the cytoplasm where they can access diverse enzymatic and regulatory assemblies; thus the most fitting placement is the cytoplasm.
Integrating these elements yields a mechanistic hypothesis: the thioredoxin-like N-terminus nucleates a platform that binds core partners, while the TPR-like C-terminus recruits client proteins to form higher-order complexes. Likely interaction cohorts include cytosolic chaperone/adaptor systems (e.g., Hsp70/Hsp90 modules), metabolic enzyme assemblies, and signal-responsive regulators. Through multivalent binding, the protein organizes and modulates cytoplasmic complexes that coordinate cellular pathways without direct enzymatic activity.
A soluble adaptor scaffold in Escherichia coli that uses an N-terminal thioredoxin-like module and a C-terminal helical repeat array to organize cytoplasmic protein assemblies. By coupling a thioredoxin-like interaction core to a tetratricopeptide-like binding platform, it mediates multivalent protein binding that stabilizes and coordinates large cytosolic complexes involved in cellular pathways, acting as a noncatalytic organizer rather than an enzyme.
Involved in cellular processes but its exact function is not yet known.
IPR013766, domain) โ residues 1-111IPR036249, homologous_superfamily) โ residues 3-112IPR011990, homologous_superfamily) โ residues 113-284Molecular Function: molecular_function (GO:0003674), binding (GO:0005488), protein binding (GO:0005515), unfolded protein binding (GO:0051082)
Biological Process: cellular component (GO:0008150), cellular process (GO:0009987), biological regulation (GO:0065007), response to stimulus (GO:0050896), cellular processes (GO:0051716), regulation of biological quality (GO:0065008), response to stress (GO:0006950), response to chemical (GO:0042221), protein folding (GO:0006457), chaperone-mediated protein folding (GO:0061077), regulation of protein stability (GO:0031647), response to oxidative stress (GO:0006979), cellular response to chemical stimulus (GO:0070887), cellular response to stress (GO:0033554), protein refolding (GO:0042026), cellular response to oxidative stress (GO:0034599), cellular response to chemical stress (GO:0062197)
Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), cytoplasm (GO:0005737), cytosol (GO:0005829), intracellular anatomical structure (GO:0005622)
Generated by BioReason
Source: CnoX-deep-research-bioreason-rl.md
The BioReason functional summary describes CnoX as:
A soluble adaptor scaffold in Escherichia coli that uses an N-terminal thioredoxin-like module and a C-terminal helical repeat array to organize cytoplasmic protein assemblies. By coupling a thioredoxin-like interaction core to a tetratricopeptide-like binding platform, it mediates multivalent protein binding that stabilizes and coordinates large cytosolic complexes involved in cellular pathways, acting as a noncatalytic organizer rather than an enzyme.
While the domain architecture description (N-terminal thioredoxin-like domain + C-terminal TPR domain) is correct, the functional interpretation is substantially wrong. BioReason describes CnoX as a generic "noncatalytic organizer" of "cytoplasmic protein assemblies," which misses the experimentally defined function entirely. CnoX is a chaperedoxin -- the founding member of a protein family that combines ATP-independent holdase chaperone activity with a redox-protective function. It is specifically activated by hypochlorous acid (HOCl/bleach) via chlorination of its TPR domain, whereupon it functions as an efficient holdase that prevents protein aggregation and protects substrates from irreversible oxidation through mixed disulfide bond formation via Cys-63 (PMID:29754824). The summary's claim of "noncatalytic organizer" is misleading: CnoX actively forms protective mixed disulfide bonds with client proteins.
The summary correctly identifies the cytoplasmic localization and the absence of classical oxidoreductase activity, but fails to capture:
- The HOCl-dependent activation mechanism
- The holdase chaperone function
- The redox-protective disulfide bond formation with substrates
- The substrate transfer to GroEL/GroES and DnaK/DnaJ/GrpE foldase systems
The thinking trace mentions "redox-inert interaction platform" for the thioredoxin fold, which is partially correct (it lacks CXXC active site) but misses that Cys-63 still forms functionally critical mixed disulfides.
Comparison with interpro2go:
CnoX has no GO_REF:0000002 (interpro2go) annotations in the curated review. The BioReason GO term predictions include unfolded protein binding (GO:0051082) and protein folding/chaperone-related terms, which align with the curated review's core functions better than the functional summary does. The GO predictions appear to draw on the same InterPro domain information but arrive at more specific terms than the narrative summary suggests. The narrative summary underperforms compared to even the model's own GO term predictions.
The reasoning from domain architecture to function is methodical but overly conservative. The trace correctly identifies the thioredoxin and TPR domains but defaults to generic "protein binding" and "adaptor scaffold" interpretations rather than the specific chaperone/redox-protection functions that are well-established for this protein family. The trace mentions "stress and metabolic responses" in passing but does not connect to the specific oxidative stress response.
id: P77395
gene_symbol: CnoX
product_type: PROTEIN
status: IN_PROGRESS
taxon:
id: NCBITaxon:83333
label: Escherichia coli (strain K12)
description: CnoX (formerly YbbN) is an E. coli chaperedoxin -- a bifunctional protein
that combines ATP-independent holdase chaperone activity with a redox-protective
function. It contains an N-terminal thioredoxin-like domain (lacking the canonical
CXXC active site and therefore not a functional oxidoreductase) fused to a C-terminal
tetratricopeptide repeat (TPR) domain. Upon activation by hypochlorous acid (HOCl/bleach)
via chlorination of its TPR domain, CnoX functions as an efficient holdase, binding
unfolded client proteins to prevent their aggregation (PMID:29754824). Uniquely,
CnoX also protects its bound substrates from irreversible oxidation by forming mixed
disulfide bonds via Cys-63 (PMID:29754824). After bleach stress subsides, CnoX transfers
its substrates to the major ATP-dependent foldases GroEL/GroES and DnaK/DnaJ/GrpE
for refolding (PMID:29754824, PMID:18657513). CnoX interacts with and coordinately
regulates GroEL/GroES (as a mild inhibitor) and DnaK/DnaJ/GrpE (as an enhancer),
suggesting it helps direct client protein traffic between these two major chaperone
systems (PMID:21498507, PMID:18657513). CnoX is the founding member of the chaperedoxin
protein family.
existing_annotations:
- term:
id: GO:0045454
label: cell redox homeostasis
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: IBA annotation based on phylogenetic inference. CnoX contains a thioredoxin-like
domain and was initially thought to have oxidoreductase activity (PMID:16563353).
However, multiple studies have now conclusively shown that CnoX lacks a canonical
CXXC active site and is NOT a functional oxidoreductase (PMID:21498507, PMID:29754824).
The thioredoxin domain instead forms mixed disulfide bonds with substrates to
protect them from irreversible oxidation during HOCl stress (PMID:29754824),
which is a protective/chaperone function rather than classical redox homeostasis.
The IBA annotation likely propagated from ancestral thioredoxin-family proteins
that do have oxidoreductase activity.
action: MODIFY
reason: While CnoX has a thioredoxin fold and forms disulfide bonds with substrates,
it is not a classical oxidoreductase that maintains redox homeostasis. UniProt
explicitly states "Lacks oxidoreductase activity" based on PMID:21498507 and
PMID:29754824. The IBA likely reflects ancestral thioredoxin function that has
been repurposed in CnoX. A more accurate biological process annotation would
be 'cellular response to oxidative stress' (GO:0034599) which is already annotated,
or 'maintenance of unfolded protein' (GO:0036506).
proposed_replacement_terms:
- id: GO:0034599
label: cellular response to oxidative stress
supported_by:
- reference_id: PMID:21498507
supporting_text: The Trx domain lacks a canonical CXXC active site architecture
and is not a functional oxidoreductase.
- reference_id: PMID:29754824
supporting_text: CnoX uniquely combines this function with the ability to prevent
the irreversible oxidation of its substrates.
- term:
id: GO:0006950
label: response to stress
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: IEA annotation from ARBA machine learning model. CnoX is indeed involved
in the stress response -- it is activated by HOCl (bleach) stress and protects
proteins from aggregation and irreversible oxidation during oxidative stress
(PMID:29754824). The ybbN-deficient strain also shows increased sensitivity
to thermal stress (PMID:18657513). However, GO:0006950 'response to stress'
is very broad.
action: ACCEPT
reason: While GO:0006950 is very general, it is acceptable as an IEA annotation.
The more specific term GO:0034599 'cellular response to oxidative stress' is
already annotated with IMP evidence from PMID:29754824. Keeping this broader
IEA annotation is fine alongside the more specific experimental one.
supported_by:
- reference_id: PMID:29754824
supporting_text: Bleach (HOCl) is a powerful oxidant that kills bacteria in
part by causing protein aggregation. It inactivates ATP-dependent chaperones,
rendering cellular proteins mostly dependent on holdases.
- reference_id: PMID:18657513
supporting_text: an ybbN-deficient strain displays an increased sensitivity
to thermal stress
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:15690043
review:
summary: IPI annotation from high-throughput affinity purification-mass spectrometry
study (Butland et al. 2005). CnoX (YbbN) was identified as interacting with
DnaN (P0A988) in this large-scale E. coli protein complex network study. While
the interaction is likely real, 'protein binding' is uninformative. CnoX functions
as a holdase chaperone and co-chaperone, and its protein interactions reflect
this function.
action: MODIFY
reason: GO:0005515 'protein binding' is uninformative per GO curation guidelines.
CnoX's interactions with client proteins and chaperone partners (DnaK, GroEL)
reflect its holdase/co-chaperone function. A more informative MF term such as
GO:0051087 'protein-folding chaperone binding' would better capture the nature
of CnoX's protein interactions, particularly given that CnoX cooperates with
DnaK and GroEL foldase systems.
proposed_replacement_terms:
- id: GO:0051087
label: protein-folding chaperone binding
supported_by:
- reference_id: PMID:15690043
supporting_text: An interaction network of protein complexes involved in diverse
biological processes was uncovered and validated by sequential rounds of tagging
and purification.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:18657513
review:
summary: IPI annotation based on reverse purification experiments showing CnoX
(YbbN) specifically interacts with DnaK (P0A6F5), GroEL (P0A6Y8), trigger factor
(P0C8J6), and other proteins. Kthiri et al. showed that YbbN cooperates with
DnaK for protein renaturation and interacts with multiple chaperone system components
(PMID:18657513).
action: MODIFY
reason: '''Protein binding'' is uninformative. The specific interactions with
DnaK and GroEL documented in PMID:18657513 are functionally significant -- CnoX
enhances DnaK-mediated refolding 4-fold and specifically interacts with the
major foldases. GO:0051087 ''protein-folding chaperone binding'' is a more informative
term for these interactions.'
proposed_replacement_terms:
- id: GO:0051087
label: protein-folding chaperone binding
supported_by:
- reference_id: PMID:18657513
supporting_text: YbbN specifically interacts with DnaK and GroEL, as shown by
reverse purification. It increases 4-fold the rate of protein renaturation
in vitro by the DnaK chaperone machine
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:21498507
review:
summary: IPI annotation from Lin and Wilson (2011), who resolved the crystal structure
of YbbN/CnoX and identified its interacting partners including multiple ribosomal
protein subunits and a strong interaction with GroEL. They showed CnoX acts
as a mild inhibitor of GroEL/GroES chaperonin function and ATPase activity (PMID:21498507).
action: MODIFY
reason: '''Protein binding'' is uninformative. The interactions documented in
PMID:21498507 include a strong, functionally characterized interaction with
GroEL -- CnoX negatively regulates GroEL. GO:0051087 ''protein-folding chaperone
binding'' better captures this interaction.'
proposed_replacement_terms:
- id: GO:0051087
label: protein-folding chaperone binding
supported_by:
- reference_id: PMID:21498507
supporting_text: A variety of proteins in E. coli interact with YbbN, including
multiple ribosomal protein subunits and a strong interaction with GroEL. YbbN
acts as a mild inhibitor of GroESL chaperonin function and ATPase activity,
suggesting that it is a negative regulator of the GroESL system.
- term:
id: GO:0034599
label: cellular response to oxidative stress
evidence_type: IMP
original_reference_id: PMID:29754824
review:
summary: IMP annotation based on Goemans et al. (2018), the landmark study establishing
CnoX as a chaperedoxin. The cnoX mutant is highly sensitive to HOCl (hypochlorous
acid/bleach), which is a powerful oxidant. CnoX is activated by HOCl via chlorination
and then protects substrates from both aggregation and irreversible oxidation.
This is a core function of CnoX.
action: ACCEPT
reason: Well-supported by mutant phenotype data. The cnoX deletion mutant is highly
sensitive to HOCl stress. CnoX is specifically activated by oxidative stress
(HOCl-mediated chlorination of its TPR domain) and protects cellular proteins
from oxidative damage. This is a core biological process for this chaperedoxin.
supported_by:
- reference_id: PMID:29754824
supporting_text: Bleach (HOCl) is a powerful oxidant that kills bacteria in
part by causing protein aggregation. It inactivates ATP-dependent chaperones,
rendering cellular proteins mostly dependent on holdases. Here we identified
Escherichia coli CnoX (YbbN) as a folding factor that, when activated by bleach
via chlorination, functions as an efficient holdase
- term:
id: GO:0036506
label: maintenance of unfolded protein
evidence_type: IDA
original_reference_id: PMID:29754824
review:
summary: IDA annotation from Goemans et al. (2018). CnoX functions as an efficient
holdase that maintains client proteins in an unfolded but non-aggregated state
during HOCl stress, preventing their irreversible aggregation and oxidation.
After stress resolution, CnoX transfers these clients to the GroEL/GroES and
DnaK/DnaJ/GrpE foldase systems for refolding (PMID:29754824). This is a core
function of CnoX.
action: ACCEPT
reason: Directly supported by experimental data showing CnoX acts as an ATP-independent
holdase that maintains client proteins in an unfolded, refolding-competent state.
This is one of the two core molecular activities of the chaperedoxin -- the
holdase/chaperone function that prevents protein aggregation during oxidative
stress.
supported_by:
- reference_id: PMID:29754824
supporting_text: 'CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrates
from Irreversible Oxidation ... functions as an efficient holdase, protecting
the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE'
- term:
id: GO:0042026
label: protein refolding
evidence_type: IDA
original_reference_id: PMID:29754824
review:
summary: IDA annotation from Goemans et al. (2018). CnoX itself does not directly
refold proteins -- it is an ATP-independent holdase that maintains substrates
and then transfers them to GroEL/GroES and DnaK/DnaJ/GrpE for refolding (PMID:29754824).
Earlier work by Kthiri et al. showed that YbbN increases 4-fold the rate of
protein renaturation by the DnaK chaperone machine (PMID:18657513), but this
enhancement is indirect -- CnoX facilitates refolding by delivering substrates
to the foldases.
action: KEEP_AS_NON_CORE
reason: CnoX participates in protein refolding indirectly by acting as a holdase
that maintains substrates in a refolding-competent state and then hands them
off to the ATP-dependent foldases GroEL/GroES and DnaK/DnaJ/GrpE. The annotation
is not wrong -- CnoX does contribute to protein refolding as part of its overall
pathway -- but it is not the core molecular function. The core function is holdase
activity and redox protection. Protein refolding is a downstream consequence
of CnoX's holdase activity.
supported_by:
- reference_id: PMID:29754824
supporting_text: functions as an efficient holdase, protecting the substrates
of the major folding systems GroEL/ES and DnaK/J/GrpE
- reference_id: PMID:18657513
supporting_text: It increases 4-fold the rate of protein renaturation in vitro
by the DnaK chaperone machine, suggesting that it cooperates with DnaK for
the optimal expression of several cytoplasmic proteins.
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IDA
original_reference_id: PMID:29754824
review:
summary: IDA annotation from Goemans et al. (2018). CnoX does bind unfolded proteins,
but its function goes beyond simple binding. CnoX is an ATP-independent holdase
that actively prevents aggregation of client proteins and escorts them between
cellular states -- from unfolded/stressed to the GroEL/GroES and DnaK/DnaJ/GrpE
foldase systems (PMID:29754824). The term GO:0051082 'unfolded protein binding'
is a child of GO:0005515 'protein binding' and only describes the binding aspect,
not the functional chaperone/carrier activity.
action: MODIFY
reason: While CnoX does bind unfolded proteins, 'unfolded protein binding' is
too simplistic for the actual molecular function. CnoX is an ATP-independent
holdase chaperone that binds unfolded clients and escorts/carries them to the
major foldase systems. The more appropriate term is GO:0140597 'protein carrier
chaperone' (defined as "Binding to and carrying a protein between two different
cellular components by moving along with the target protein") or its child GO:0140309
'unfolded protein carrier activity'. CnoX fits the holdase/carrier paradigm
-- it binds unfolded clients, prevents their aggregation, and delivers them
to GroEL/DnaK for refolding.
proposed_replacement_terms:
- id: GO:0051082
label: unfolded protein binding (retain until holdase NTR is created)
additional_reference_ids:
- PMID:18657513
- PMID:21498507
supported_by:
- reference_id: PMID:29754824
supporting_text: CnoX (YbbN) as a folding factor that, when activated by bleach
via chlorination, functions as an efficient holdase, protecting the substrates
of the major folding systems GroEL/ES and DnaK/J/GrpE
- reference_id: PMID:21498507
supporting_text: we propose that YbbN coordinately regulates the activities
of these two prokaryotic chaperones, thereby helping to direct client protein
traffic initially to DnaK
- term:
id: GO:0005829
label: cytosol
evidence_type: IDA
original_reference_id: PMID:15911532
review:
summary: IDA annotation based on Lopez-Campistrous et al. (2005) proteomics study.
CnoX (YbbN) was identified in the cytosolic fraction of E. coli K-12 by biochemical
fractionation followed by 2D gel electrophoresis and tandem mass spectrometry.
action: ACCEPT
reason: Cytosolic localization is well supported by multiple independent proteomics
studies and is consistent with CnoX's function as a cytosolic holdase chaperone
that protects intracellular proteins. The reduced form is monomeric in the cytosol.
supported_by:
- reference_id: PMID:15911532
supporting_text: 2,160 were annotated and assigned to the cytosol, periplasm,
inner membrane, and outer membrane by biochemical fractionation followed by
two-dimensional gel electrophoresis and tandem mass spectrometry
- term:
id: GO:0005829
label: cytosol
evidence_type: IDA
original_reference_id: PMID:18304323
review:
summary: IDA annotation based on Ishihama et al. (2008) protein abundance profiling
study. CnoX (YbbN) was identified among 1103 proteins from the cytosolic fraction
of E. coli strain MC4100 by LC-MS/MS with protein and peptide fractionation.
action: ACCEPT
reason: Independent confirmation of cytosolic localization by a comprehensive
proteomics study. Consistent with all other localization data for CnoX.
supported_by:
- reference_id: PMID:18304323
supporting_text: we identified 1103 proteins from the cytosolic fraction of
the Escherichia coli strain MC4100
- term:
id: GO:0005829
label: cytosol
evidence_type: IDA
original_reference_id: PMID:16858726
review:
summary: IDA annotation based on Lasserre et al. (2006) complexomics study using
2D blue native/SDS-PAGE to separate E. coli protein complexes, identifying CnoX
(YbbN) in the cytosolic fraction.
action: ACCEPT
reason: Third independent confirmation of cytosolic localization from a complexomics
study. All three cytosol annotations are consistent and well-supported.
supported_by:
- reference_id: PMID:16858726
supporting_text: the cytosolic and membrane protein complexes of Escherichia
coli were separated. Then, the different partners of each protein complex
were identified by LC-MS/MS.
references:
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000117
title: Electronic Gene Ontology annotations created by ARBA machine learning models
findings: []
- id: PMID:15690043
title: Interaction network containing conserved and essential protein complexes
in Escherichia coli.
findings:
- statement: High-throughput affinity purification-MS identified CnoX (YbbN) interacting
with DnaN as part of a large-scale E. coli protein interaction network.
supporting_text: An interaction network of protein complexes involved in diverse
biological processes was uncovered and validated by sequential rounds of tagging
and purification.
- id: PMID:15911532
title: Localization, annotation, and comparison of the Escherichia coli K-12 proteome
under two states of growth.
findings:
- statement: CnoX (YbbN) was identified in the cytosolic fraction of E. coli K-12
by biochemical fractionation and 2D-gel/MS proteomics.
supporting_text: 2,160 were annotated and assigned to the cytosol, periplasm,
inner membrane, and outer membrane by biochemical fractionation followed by
two-dimensional gel electrophoresis and tandem mass spectrometry
- id: PMID:16563353
title: The Escherichia coli thioredoxin homolog YbbN/Trxsc is a chaperone and a
weak protein oxidoreductase.
findings:
- statement: Early characterization of CnoX (then called Trxsc/YbbN) reported both
chaperone and weak protein oxidoreductase activities. Upon oxidation, its oligomeric
state changes to tetramers and higher oligomers. [No publication text available
for direct quote]
- id: PMID:16858726
title: A complexomic study of Escherichia coli using two-dimensional blue native/SDS
polyacrylamide gel electrophoresis.
findings:
- statement: CnoX (YbbN) identified in the cytosolic fraction by 2D BN/SDS-PAGE
complexomics study.
supporting_text: the cytosolic and membrane protein complexes of Escherichia coli
were separated. Then, the different partners of each protein complex were identified
by LC-MS/MS.
- id: PMID:18304323
title: Protein abundance profiling of the Escherichia coli cytosol.
findings:
- statement: CnoX (YbbN) identified among 1103 cytosolic proteins by comprehensive
LC-MS/MS profiling.
supporting_text: we identified 1103 proteins from the cytosolic fraction of the
Escherichia coli strain MC4100
- id: PMID:18657513
title: The thioredoxin homolog YbbN functions as a chaperone rather than as an oxidoreductase.
findings:
- statement: Demonstrated that YbbN/CnoX functions primarily as a chaperone in vivo,
not an oxidoreductase. ybbN-null mutant is sensitive to thermal stress but not
oxidative stress. CnoX specifically interacts with DnaK and GroEL and increases
DnaK-mediated refolding rates 4-fold.
supporting_text: an ybbN-deficient strain displays an increased sensitivity to
thermal stress but not to oxidative stress ... YbbN specifically interacts with
DnaK and GroEL, as shown by reverse purification. It increases 4-fold the rate
of protein renaturation in vitro by the DnaK chaperone machine
- id: PMID:21498507
title: Escherichia coli thioredoxin-like protein YbbN contains an atypical tetratricopeptide
repeat motif and is a negative regulator of GroEL.
findings:
- statement: Crystal structure of CnoX revealed a mobile Trx domain and four atypical
TPR motifs. The Trx domain lacks a canonical CXXC active site and is not a functional
oxidoreductase. CnoX is a negative regulator of GroEL and coordinates the activities
of GroEL and DnaK chaperone pathways.
supporting_text: The Trx domain lacks a canonical CXXC active site architecture
and is not a functional oxidoreductase ... YbbN acts as a mild inhibitor of
GroESL chaperonin function and ATPase activity, suggesting that it is a negative
regulator of the GroESL system
- id: PMID:29754824
title: 'CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrates from Irreversible
Oxidation.'
findings:
- statement: Landmark study establishing CnoX as the founding member of the chaperedoxin
family. CnoX is activated by HOCl (bleach) via chlorination of its TPR domain
and functions as an efficient ATP-independent holdase. It protects substrates
of GroEL/ES and DnaK/J/GrpE from aggregation and prevents their irreversible
oxidation through mixed disulfide bond formation via Cys-63. After stress resolution,
CnoX transfers substrates to the major foldases for refolding.
supporting_text: CnoX (YbbN) as a folding factor that, when activated by bleach
via chlorination, functions as an efficient holdase, protecting the substrates
of the major folding systems GroEL/ES and DnaK/J/GrpE. Remarkably, CnoX uniquely
combines this function with the ability to prevent the irreversible oxidation
of its substrates.
core_functions:
- description: ATP-independent holdase chaperone (chaperedoxin). When activated by
HOCl via chlorination of its TPR domain, CnoX binds unfolded client proteins to
prevent their aggregation, then delivers them to the major ATP-dependent foldase
systems GroEL/GroES and DnaK/DnaJ/GrpE for refolding. Also protects substrates
from irreversible oxidation via mixed disulfide bond formation through Cys-63
(PMID:29754824).
molecular_function:
id: GO:0140597
label: protein carrier chaperone
directly_involved_in:
- id: GO:0034599
label: cellular response to oxidative stress
- id: GO:0036506
label: maintenance of unfolded protein
locations:
- id: GO:0005829
label: cytosol
supported_by:
- reference_id: PMID:29754824
supporting_text: CnoX (YbbN) as a folding factor that, when activated by bleach
via chlorination, functions as an efficient holdase, protecting the substrates
of the major folding systems GroEL/ES and DnaK/J/GrpE
- reference_id: PMID:18657513
supporting_text: YbbN specifically interacts with DnaK and GroEL ... It increases
4-fold the rate of protein renaturation in vitro by the DnaK chaperone machine
- reference_id: PMID:21498507
supporting_text: we propose that YbbN coordinately regulates the activities of
these two prokaryotic chaperones, thereby helping to direct client protein traffic
initially to DnaK