CnoX (formerly YbbN) is a cytosolic chaperedoxin that combines ATP-independent holdase activity with protection of client cysteines from irreversible oxidation. Hypochlorous acid activates its tetratricopeptide-repeat region by N-chlorination, enabling CnoX to bind non-native proteins and suppress their aggregation while its thioredoxin-like domain forms reversible mixed disulfides with oxidized clients. The thioredoxin-like domain lacks the canonical CXXC active-site architecture, and CnoX is not a functional oxidoreductase. CnoX subsequently cooperates with the GroEL/GroES and DnaK/DnaJ/GrpE folding systems. It also binds stably to the apical domain of GroEL outside the substrate-binding site; GroES binding releases CnoX, coupling redox protection to chaperonin-assisted folding.
Definition: Binding to an unfolded or misfolded protein to prevent its aggregation without actively catalyzing refolding. The holdase maintains the client in a soluble, folding-competent state.
Justification: CnoX directly prevents aggregation of non-native cytosolic clients and maintains them for later refolding. Obsolete GO:0051082 represented binding only, whereas GO:0140309 additionally requires escort to an acceptor or specific location.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0045454 cell redox homeostasis | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Current PAINT places cell redox homeostasis at PTN001625774 from two experimentally grounded descendant sources. CnoX is not a classical thiol oxidoreductase, and its deletion does not alter bulk cytoplasmic, membrane, or periplasmic protein redox state. It nevertheless directly maintains client redox integrity by trapping oxidized cysteines in reversible mixed disulfides during HOCl stress. Reason: GO:0045454 does not require canonical thioredoxin oxidoreductase activity. CnoX's demonstrated client-level redox-protective activity makes the IBA defensible. However, the normal bulk protein redox state of the knockout and absence of oxidoreductase activity argue against treating general cellular redox homeostasis as a defining core process. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN001625774 Β· PANTHER:PTN001625774 SUPPORTS TRANSFER Current PTHR45663 PAINT retains GO:0045454 at this ancestral node. AGI_LocusCode:AT2G15570 Β· AGI_LocusCode:AT2G15570 SUPPORTS TRANSFER Listed descendant evidence in the exact current GOA and PAINT assertion. UniProtKB:O53161 Β· UniProtKB:O53161 SUPPORTS TRANSFER Listed descendant evidence in the exact current GOA and PAINT assertion. Supporting Evidence: PMID:29754824 CnoX uniquely combines this function with the ability to prevent the irreversible oxidation of its substrates. PMID:18657513 not to oxidative stress, a normal redox state of its cellular proteins but a decreased expression of several cytoplasmic proteins |
| GO:0005515 protein binding | IPI PMID:15690043 Interaction network containing conserved and essential prote... | UNDECIDED | Summary: This high-throughput interaction-network row reports CnoX with DnaN. IntAct records the pair across two experiments, and later work connects the interaction to a bona fide CnoX chaperone role in refolding the urea-unfolded beta clamp. Reason: The interaction is experimentally and physiologically credible: a cnoX mutant has DNA-replication defects and CnoX refolds unfolded DnaN in vitro. Protein binding remains uninformative, while GO:0051087 would be wrong because DnaN is the client rather than a folding chaperone. No current GO molecular-function term captures the demonstrated client-directed holdase activity, so the row remains undecided pending the proposed general holdase term. Supporting Evidence: PMID:21195694 Moreover, YbbN functions as a bona fide chaperone in the refolding of the urea-unfolded Ξ²-clamp. |
| GO:0005515 protein binding | IPI PMID:15690043 Interaction network containing conserved and essential prote... | UNDECIDED | Summary: This second high-throughput row reports CnoX with GroEL and FbaB. The cached abstract does not expose the pair-level assays, and the two partners do not support one common informative molecular-function replacement. Reason: The source is a high-throughput interaction atlas whose abstract does not expose the pair-level assays. Later work directly establishes CnoX-GroEL binding, but FbaB is not a folding chaperone, so this combined row cannot safely be recast as chaperone binding without pair-level splitting. |
| GO:0005515 protein binding | IPI PMID:18657513 The thioredoxin homolog YbbN functions as a chaperone rather... | MODIFY | Summary: Reverse purification and functional assays establish interactions with GroEL and DnaK, while this source row also includes GatY, ribosomal protein uL5, and CnoX itself. Reason: GO:0051087 captures the demonstrated GroEL and DnaK interactions. Because the WITH/FROM list also contains non-chaperone partners, the replacement should apply only to the GroEL and DnaK pair assertions rather than the combined row. Proposed replacements: protein-folding chaperone binding Supporting Evidence: PMID:18657513 YbbN specifically interacts with DnaK and GroEL, as shown by reverse purification. |
| GO:0005515 protein binding | IPI PMID:21498507 Escherichia coli thioredoxin-like protein YbbN contains an a... | MODIFY | Summary: Structural and biochemical work reports a strong CnoX interaction with GroEL and mild inhibition of GroEL/GroES chaperonin function and ATPase activity, together with non-chaperone partners represented in the same source row. Reason: GO:0051087 is informative for the demonstrated GroEL interaction. The mild in-vitro inhibition is compatible with the later stable-plugin model, in which GroES binding releases CnoX, but it does not by itself establish a directional in-vivo regulatory biological process. No regulatory BP term is proposed. The source row must be split so the MF replacement is not propagated to its ribosomal and metabolic partners. 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. PMID:21498507 YbbN acts as a mild inhibitor of GroESL chaperonin function and ATPase activity, suggesting that it is a negative regulator of the GroESL system. PMID:36764293 Here, we report that the bacterial Hsp60 chaperonin GroEL forms a stable, functionally relevant complex with the chaperedoxin CnoX, a protein combining a chaperone and a redox function. |
| GO:0034599 cellular response to oxidative stress | IMP PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... | ACCEPT | Summary: Loss-of-function evidence and biochemical characterization establish CnoX as an HOCl-activated factor that protects proteins from aggregation and irreversible oxidation during oxidative stress. Reason: This is a core physiological role of CnoX and is directly supported by its experimentally characterized response to hypochlorous acid. Supporting Evidence: PMID:29754824 Here we identified Escherichia coli CnoX (YbbN) as a folding factor that, when activated by bleach via chlorination, functions as an efficient holdase |
| GO:0042026 protein refolding | IDA PMID:29754824 CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrat... | ACCEPT | Summary: CnoX does not catalyze ATP-dependent folding. It maintains clients in a folding-competent state and transfers them to GroEL/GroES and DnaK/DnaJ/GrpE, acting directly within the protein-refolding pathway. Reason: The biological-process term includes helper proteins and does not imply that CnoX is a foldase. Client capture, transfer, and productive cooperation with the major foldases make this a core process. Supporting Evidence: PMID:29754824 functions as an efficient holdase, protecting the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE. PMID:36764293 Binding of GroES (Hsp10 cofactor) to GroEL induces CnoX release. |
| GO:0005829 cytosol | IDA PMID:15911532 Localization, annotation, and comparison of the Escherichia ... | ACCEPT | Summary: Biochemical fractionation and proteomics place CnoX in the E. coli cytosol. Reason: The annotation is consistent with two independent cytosolic-proteome studies and with CnoX's work on cytosolic foldase substrates. 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: Quantitative LC-MS/MS independently identifies CnoX in the cytosolic proteome. Reason: This localization agrees with the other cytosol annotations and the known intracellular clients and chaperone partners of 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: Blue-native/SDS-PAGE complexomics provides a third experimental cytosol annotation for CnoX. Reason: Although the cached paper is abstract-only, this curator's row agrees with two independent localization studies and the established biology. Supporting Evidence: PMID:16858726 the cytosolic and membrane protein complexes of Escherichia coli were separated. |
| GO:0051087 protein-folding chaperone binding | IPI PMID:36764293 A molecular device for the redox quality control of GroEL/ES... | NEW | Summary: Biochemistry and cryo-EM directly establish a stable, functionally relevant CnoX-GroEL complex, with CnoX bound outside the GroEL substrate-binding site. Reason: This direct pair-specific evidence supports protein-folding chaperone binding independently of the older mixed-partner GOA rows. Supporting Evidence: PMID:36764293 Here, we report that the bacterial Hsp60 chaperonin GroEL forms a stable, functionally relevant complex with the chaperedoxin CnoX, a protein combining a chaperone and a redox function. |
| GO:0009408 response to heat | IMP PMID:18657513 The thioredoxin homolog YbbN functions as a chaperone rather... | NEW | Summary: A cnoX-deficient strain has increased sensitivity to thermal stress, consistent with CnoX's chaperone activity in maintaining cytosolic protein folding. Reason: The mutant phenotype directly supports a response-to-heat process annotation that is absent from the current GOA snapshot. Supporting Evidence: PMID:18657513 an ybbN-deficient strain displays an increased sensitivity to thermal stress |
Loading supporting contentβ¦
Download this section (compressed HTML)Q: Which endogenous proteins are the principal CnoX clients during HOCl stress, and what features determine capture by its holdase and thiol-protection surfaces?
Q: Does transfer of held clients to DnaK satisfy carrier-specific GO:0140309, or is CnoX released before a direct client-acceptor handoff?
Q: How broadly is the stable GroEL-plugin mechanism conserved among bacterial and eukaryotic thioredoxin-TPR proteins?
Q: Does CnoX inhibit GroEL/GroES under physiological conditions, or does the mild in-vitro inhibition reflect occupancy of the transient redox-plugin complex?
Experiment: Compare time-resolved CnoX client capture and release in wild type, C63A, and TPR-surface mutants during and after defined HOCl pulses, with parallel aggregation and cysteine-oxidation proteomics.
Type: client-trapping proteomics
Experiment: Reconstitute fluorescent client transfer from CnoX to GroEL/GroES and DnaK/DnaJ/GrpE at single-turnover resolution to distinguish direct carrier-like handoff from release-and-recapture.
Type: chaperone handoff assay
Experiment: Test separation-of-function CnoX variants defective in N-chlorination activation, client mixed-disulfide formation, or GroEL binding for HOCl survival and recovery of client solubility in vivo.
Type: genetic complementation
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)