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. hsp-4 is also induced in intestinal cells by the bacterial pore-forming toxin Cry5B, through the IRE-1/XBP-1 branch and the p38 MAPK PMK-1, as part of the UPR-dependent defence against such toxins. In early embryos, depletion of BiP blocks the mitotic transition of the ER into sheet-like structures, a step thought to need homotypic ER membrane fusion.
| 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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Download this section (compressed HTML)hsp-4 encodes one of two C. elegans heat shock response 70 (hsp70) proteins homologous to mammalian grp78/BiP (glucose regulated protein 78/immunoglobulin heavy chain-binding protein, OMIM:138120); hsp-4 affects embryonic and larval viability; hsp-4 is expressed in several tissues including the hypodermis, intestine, and spermatheca; hsp-4 transcription is upregulated in response to endoplasmic reticulum stress induced by dithiothreitol (DTT) or tunicamycin (unfolded protein response) as well as in response to heat shock and treatment of animals with pore-forming toxins such as Cry5B.
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