HSP-4 is the major endoplasmic reticulum (ER)-resident HSP70 family chaperone in C. elegans, orthologous to mammalian BiP/GRP78. It functions as an ATP-dependent molecular chaperone that assists protein folding and quality control in the ER lumen. HSP-4 is a classical transcriptional target of the unfolded protein response (UPR), primarily induced via the IRE-1/XBP-1 pathway, with partial redundancy from the ATF-6 branch. The hsp-4::GFP reporter is the standard marker for ER stress in C. elegans. HSP-4 plays specialized roles in interorganellar signaling, ER-phagy regulation via Sec-62, and stress tolerance, representing functionally diversified activities compared to its paralog HSP-3.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA annotation from phylogenetic inference. While mammalian BiP/GRP78 has been reported in nuclear compartments under certain stress conditions, HSP-4 in C. elegans is primarily characterized as an ER lumen chaperone. The UniProt record and deep research consistently describe HSP-4 as ER-localized. Nuclear localization may reflect conservation of potential regulatory functions but is not a primary localization in C. elegans. Reason: While phylogenetically conserved from mammalian BiP, nuclear localization is not a well-documented aspect of HSP-4 function in C. elegans. The protein contains an ER retention signal (HDEL) and is primarily ER-localized according to UniProt and functional studies. This annotation may represent an ancestral or minor localization pattern. Supporting Evidence: UniProt:P20163 SUBCELLULAR LOCATION: Endoplasmic reticulum lumen |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA annotation from phylogenetic inference. HSP-4 is primarily ER-localized with an ER retention signal. Some cytoplasmic localization may occur during synthesis or under stress conditions, but the primary functional location is the ER lumen. Reason: This annotation likely reflects the biosynthetic pathway or minor localization patterns conserved in the HSP70 family. The primary functional location of HSP-4 is the ER lumen, as supported by the HDEL retention signal and functional studies. Supporting Evidence: UniProt:P20163 SUBCELLULAR LOCATION: Endoplasmic reticulum lumen |
| GO:0016887 ATP hydrolysis activity | IBA GO_REF:0000033 | ACCEPT | Summary: HSP-4/BiP is an ATPase that uses ATP hydrolysis to drive its chaperone cycle. UniProt assigns EC 3.6.4.10 (ATP-dependent chaperone activity). The deep research confirms that HSP-4 operates through an ATPase-driven chaperone cycle that binds and releases unfolded client polypeptides in the ER lumen. Reason: ATP hydrolysis is a core molecular function of HSP70 family members including BiP/HSP-4. This activity is essential for the chaperone cycle. UniProt provides catalytic activity annotation with the reaction: ATP + H2O = ADP + phosphate + H(+). Supporting Evidence: UniProt:P20163 Reaction=ATP + H2O = ADP + phosphate + H(+); Xref=Rhea:RHEA:13065, file:worm/hsp-4/hsp-4-deep-research-falcon.md HSP-4, as a BiP ortholog, operates through an ATPase-driven chaperone cycle that binds and releases unfolded client polypeptides in the ER lumen |
| GO:0031072 heat shock protein binding | IBA GO_REF:0000033 | ACCEPT | Summary: HSP70 family members interact with co-chaperones including J domain-containing proteins (DnaJ/Hsp40) and nucleotide exchange factors. HSP-4/BiP requires co-chaperones for efficient substrate recognition and ATPase regulation. Reason: This is a well-established function for HSP70/BiP family members. UniProt notes that J domain-containing co-chaperones stimulate the ATPase activity and are required for efficient substrate recognition. This interaction is central to the chaperone mechanism. Supporting Evidence: UniProt:P20163 J domain-containing co-chaperones stimulate the ATPase activity and are required for efficient substrate recognition |
| GO:0044183 protein folding chaperone | IBA GO_REF:0000033 | ACCEPT | Summary: HSP-4/BiP is an ER-resident molecular chaperone central to ER proteostasis. It assists protein folding and quality control in the ER lumen. The deep research confirms HSP-4 participates in ATP-dependent binding/release of unfolded or nascent client polypeptides. Reason: This is the core molecular function of HSP-4/BiP. It plays a key role in protein folding and quality control in the ER lumen. Overexpression of HSP-4 rescues dendrite morphogenesis defects in ire-1 mutants by restoring protein folding capacity (PMID:26052671). Supporting Evidence: UniProt:P20163 Endoplasmic reticulum chaperone that plays a key role in protein folding and quality control in the endoplasmic reticulum lumen PMID:26052671 overexpression of hsp-4 in PVD restored normal dendritic branches in ire-1 mutants |
| GO:0036503 ERAD pathway | IBA GO_REF:0000033 | ACCEPT | Summary: BiP/HSP-4 is involved in ER-associated degradation (ERAD) by recognizing misfolded proteins in the ER. The deep research notes HSP-4 involvement in ERAD-related processes and interaction with ERAD components. Reason: BiP/HSP-4 functions in ERAD by recognizing and retaining misfolded proteins for degradation. PMID:12186849 demonstrates genetic interaction between hsp-4 and ERAD component sel-1, and shows that RNAi of sel-1 activates the hsp-4::gfp ER stress reporter in an xbp-1-dependent manner. Supporting Evidence: PMID:12186849 RNAi of sel-1, the C. elegans homologue of the yeast ERAD gene HRD3 |
| GO:0016020 membrane | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: HSP-4 is a soluble ER lumen protein, not a membrane protein. It may associate with ER membrane peripherally through interactions with membrane-bound clients or ERAD components, but is not an integral membrane protein. Reason: This term is overly broad and imprecise. HSP-4/BiP is a soluble protein in the ER lumen with a HDEL retention signal. While it may interact with membrane-associated substrates, it is not itself a membrane protein. The more specific terms GO:0005788 (ER lumen) and GO:0034663 (ER chaperone complex) are more appropriate. Proposed replacements: endoplasmic reticulum lumen |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | ACCEPT | Summary: HSP70/BiP chaperones assist in protein refolding as part of their quality control function. This is a well-established activity for the HSP70 family. Reason: Protein refolding is a core function of HSP70 family chaperones including BiP/HSP-4. The ATP-driven chaperone cycle allows iterative binding and release of substrates to promote proper folding. Supporting Evidence: UniProt:P20163 The chaperone activity is regulated by ATP-induced allosteric coupling of the nucleotide-binding (NBD) and substrate- binding (SBD) domains |
| GO:0034663 endoplasmic reticulum chaperone complex | IBA GO_REF:0000033 | ACCEPT | Summary: HSP-4/BiP functions within ER chaperone complexes with co-chaperones and other quality control machinery. This is supported by both phylogenetic inference and ISS evidence from PMID:11779465. Reason: HSP-4 is a core component of the ER chaperone machinery, functioning with co-chaperones and other quality control proteins. This localization is well-supported by the protein's known function. Supporting Evidence: UniProt:P20163 J domain-containing co-chaperones stimulate the ATPase activity and are required for efficient substrate recognition |
| GO:0005788 endoplasmic reticulum lumen | IBA GO_REF:0000033 | ACCEPT | Summary: HSP-4 is localized to the ER lumen. This is the primary functional location of BiP/GRP78 homologs. The protein contains a signal peptide and HDEL ER retention motif. Reason: This is the primary and well-established localization of HSP-4/BiP. UniProt clearly states ER lumen localization. The protein has a signal peptide (aa 1-17) and HDEL ER retention motif (aa 654-657). Supporting Evidence: UniProt:P20163 SUBCELLULAR LOCATION: Endoplasmic reticulum lumen PMID:26052671 we first examined the subcellular localization of HSP-4 and found that HSP-4::GFP was exclusively localized in the PVD soma |
| GO:0030968 endoplasmic reticulum unfolded protein response | IBA GO_REF:0000033 | ACCEPT | Summary: HSP-4/BiP is a classical UPR target gene and effector. Its expression is strongly induced during ER stress, and the hsp-4::GFP reporter is the standard UPR marker in C. elegans. HSP-4 also regulates UPR signaling through interaction with IRE-1. Reason: HSP-4 is both a target and effector of the ER UPR. It is transcriptionally induced by the IRE-1/XBP-1 pathway during ER stress. Multiple publications establish hsp-4 as a core UPR marker and functional component. Supporting Evidence: PMID:11779465 C. elegans requires ire-1-mediated splicing of xbp-1 mRNA for UPR gene transcription and survival upon ER stress PMID:12186849 the signaling pathway initiated by IRE1 exerts nearly complete control over the induction of well-characterized components of ER client protein processing machinery, such as BiP (hsp-4) |
| GO:0000166 nucleotide binding | IEA GO_REF:0000043 | ACCEPT | Summary: IEA annotation from UniProt keyword mapping. HSP-4 binds ATP as part of its chaperone cycle. This is a parent term of ATP binding. Reason: This is a correct but general annotation. HSP-4 has well-characterized ATP binding and hydrolysis activities. UniProt provides detailed ATP binding site information (residues 38-41, 99, 229-231, 295-302, 366-369). Supporting Evidence: UniProt:P20163 GO; GO:0005524; F:ATP binding; IEA:UniProtKB-KW |
| GO:0005524 ATP binding | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation from automated methods. HSP-4 contains a well-characterized ATP binding domain. ATP binding is essential for the chaperone cycle. Reason: ATP binding is a core molecular function of HSP-4/BiP. UniProt provides detailed annotation of ATP binding sites and the protein has crystal structure data for the peptide-binding domain (PDB:2OP6). Supporting Evidence: UniProt:P20163 GO; GO:0005524; F:ATP binding; IEA:UniProtKB-KW |
| GO:0005788 endoplasmic reticulum lumen | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from UniProt subcellular location mapping. This duplicates the IBA annotation but from a different source. Both are correct. Reason: ER lumen localization is well-established for HSP-4/BiP. Multiple evidence types supporting the same annotation is appropriate. Supporting Evidence: UniProt:P20163 SUBCELLULAR LOCATION: Endoplasmic reticulum lumen |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | ACCEPT | Summary: IEA annotation from keyword mapping. HSP-4 has ATPase activity (hydrolyzes ATP). This is a parent term of ATP hydrolysis activity. Reason: This is a correct but general annotation. HSP-4 is an ATPase with hydrolase activity. The more specific term GO:0016887 (ATP hydrolysis activity) is also annotated. Supporting Evidence: UniProt:P20163 EC=3.6.4.10 {ECO:0000250|UniProtKB:P11021} |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation from automated methods. This duplicates the IBA annotation for ATP hydrolysis activity. Both are correct. Reason: ATP hydrolysis is a core molecular function of HSP-4/BiP, essential for the chaperone cycle. Multiple evidence types are appropriate. Supporting Evidence: UniProt:P20163 Reaction=ATP + H2O = ADP + phosphate + H(+); Xref=Rhea:RHEA:13065, |
| GO:0030968 endoplasmic reticulum unfolded protein response | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation from ARBA machine learning. This duplicates other annotations for UPR involvement. HSP-4 is a well-established UPR component. Reason: HSP-4 involvement in the ER UPR is well-established. Multiple evidence sources supporting this annotation is appropriate. Supporting Evidence: PMID:12186849 BiP (hsp-4), protein disulfide isomerases, and 4-prolyl hydroxylases |
| GO:0030968 endoplasmic reticulum unfolded protein response | IEP PMID:11779465 Complementary signaling pathways regulate the unfolded prote... | ACCEPT | Summary: IEP annotation based on expression pattern. Shen et al. (2001) established that hsp-4 is a UPR target gene regulated by ire-1/xbp-1 signaling in C. elegans. Reason: PMID:11779465 is a foundational paper establishing the UPR pathway in C. elegans. It demonstrates that ire-1-mediated splicing of xbp-1 is required for UPR gene transcription including hsp-4. Supporting Evidence: PMID:11779465 C. elegans requires ire-1-mediated splicing of xbp-1 mRNA for UPR gene transcription and survival upon ER stress |
| GO:0030968 endoplasmic reticulum unfolded protein response | IEP PMID:11780124 IRE1 couples endoplasmic reticulum load to secretory capacit... | ACCEPT | Summary: IEP annotation based on expression pattern. Calfon et al. (2002) demonstrated that xbp-1 processing by IRE1 is conserved in C. elegans and activates UPR target genes. Reason: PMID:11780124 demonstrates that IRE1-mediated XBP-1 mRNA splicing activates UPR target genes in C. elegans, establishing the conserved IRE1/XBP-1 pathway. Supporting Evidence: PMID:11780124 mutations in either ire-1 or the transcription-factor-encoding xbp-1 gene abolished the UPR in Caenorhabditis elegans |
| GO:0030968 endoplasmic reticulum unfolded protein response | IEP PMID:18216284 APY-1, a novel Caenorhabditis elegans apyrase involved in un... | ACCEPT | Summary: IEP annotation based on expression pattern. Uccelletti et al. (2008) showed that ER stress induced by tunicamycin or high temperature results in increased hsp-4 transcription, dependent on both ire-1 and atf-6. Reason: PMID:18216284 demonstrates that hsp-4 induction during ER stress requires both IRE-1 and ATF-6 sensors, showing branch redundancy in UPR regulation. Supporting Evidence: PMID:18216284 ER stress induced by tunicamycin or high temperature resulted in increased transcription of apy-1. This increase was not observed in C. elegans mutants defective in ire-1 or atf-6 |
| GO:0030968 endoplasmic reticulum unfolded protein response | HEP PMID:12186849 A survival pathway for Caenorhabditis elegans with a blocked... | ACCEPT | Summary: HEP annotation based on expression pattern from high-throughput experiment. Urano et al. (2002) used microarray analysis to characterize UPR target genes and showed hsp-4 induction is xbp-1-dependent. Reason: PMID:12186849 provides genome-wide evidence that hsp-4 is a robustly induced UPR target gene dependent on xbp-1. The study used cDNA microarrays and showed hsp-4 induction in wild-type but not xbp-1 mutants. Supporting Evidence: PMID:12186849 the signaling pathway initiated by IRE1 exerts nearly complete control over the induction of well-characterized components of ER client protein processing machinery, such as BiP (hsp-4) |
| GO:0005791 rough endoplasmic reticulum | IDA PMID:26052671 The unfolded protein response is required for dendrite morph... | ACCEPT | Summary: IDA annotation based on direct assay. Wei et al. (2015) showed that HSP-4::GFP co-localized with the rough ER marker TRAM in PVD neurons, demonstrating localization to the rough ER. Reason: PMID:26052671 provides direct imaging evidence that HSP-4 co-localizes with the rough ER marker TRAM, consistent with BiP's known function in nascent protein folding. Supporting Evidence: PMID:26052671 we first examined the subcellular localization of HSP-4 and found that HSP-4::GFP was exclusively localized in the PVD soma |
| GO:0030968 endoplasmic reticulum unfolded protein response | IEP PMID:15280428 Compartment-specific perturbation of protein handling activa... | ACCEPT | Summary: IEP annotation based on expression pattern. Yoneda et al. (2004) demonstrated compartment-specific stress responses and showed hsp-4 induction is specific to ER stress, not mitochondrial stress. Reason: PMID:15280428 establishes the specificity of hsp-4 induction to ER stress, distinguishing it from mitochondrial UPR. This supports HSP-4's specific role in ER proteostasis. Supporting Evidence: PMID:15280428 hsp-6 and hsp-60 induction was specific to perturbed mitochondrial protein handling, as neither heat-shock nor endoplasmic reticulum stress nor manipulations that impair mitochondrial steps in intermediary metabolism or ATP synthesis activated the mitochondrial chaperone genes |
| GO:0034663 endoplasmic reticulum chaperone complex | ISS PMID:11779465 Complementary signaling pathways regulate the unfolded prote... | ACCEPT | Summary: ISS annotation based on sequence similarity. HSP-4 functions in ER chaperone complexes similar to mammalian BiP. Reason: HSP-4 is orthologous to mammalian BiP, which functions in ER chaperone complexes. The functional conservation is supported by rescue experiments and pathway conservation. Supporting Evidence: UniProt:P20163 Belongs to the heat shock protein 70 family PMID:11779465 Complementary signaling pathways regulate the unfolded protein response and are required for C. |
| GO:0061629 RNA polymerase II-specific DNA-binding transcription factor binding | IPI PMID:24068940 Integration of the unfolded protein and oxidative stress res... | UNDECIDED | Summary: IPI annotation based on physical interaction. Glover-Cutter et al. (2013) showed that SKN-1/Nrf transcription factor regulates hsp-4 during ER stress. This annotation may reflect interaction data from ChIP or co-IP experiments. Reason: This annotation suggests HSP-4 physically interacts with transcription factors. While SKN-1 regulates hsp-4 transcription, HSP-4 is an ER lumen protein and direct binding to transcription factors would be unexpected. The annotation may reflect a database error or indirect interaction. Further review of the original paper is needed to verify the specific interaction claimed. Supporting Evidence: PMID:24068940 Sep 12. Integration of the unfolded protein and oxidative stress responses through SKN-1/Nrf. |
| GO:0006457 protein folding | NAS | NEW | Summary: Added to align core_functions with existing annotations. Reason: Core function term not present in existing_annotations. |
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Download this section (compressed HTML)Q: What are the specific client proteins of HSP-4 in C. elegans? While the chaperone function is well-established, specific substrates beyond DMA-1 are not well characterized.
Q: How does HSP-4 differ functionally from HSP-3 in terms of client specificity and tissue-specific roles? Recent work suggests functional diversification but the mechanistic basis is unclear.
Q: What is the basis for the GO:0061629 annotation (transcription factor binding)? As an ER lumen protein, direct interaction with transcription factors would be unusual.
Experiment: Proximity labeling (BioID/TurboID) to identify HSP-4 client proteins and interaction partners in vivo under basal and stress conditions.
Hypothesis: HSP-4 has specific client proteins distinct from HSP-3
Type: proteomics
Experiment: Comparative proteomics of hsp-4 vs hsp-3 mutants to identify specific clients and pathway dependencies of each paralog.
Hypothesis: HSP-3 and HSP-4 have distinct client protein profiles
Type: comparative proteomics
Experiment: Live imaging of endogenous HSP-4 reporters under various stress conditions to understand tissue-specific and temporal dynamics of expression.
Hypothesis: HSP-4 expression shows tissue-specific patterns during stress
Type: live imaging
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