Hsp27 is a small heat shock protein (sHSP) of Drosophila melanogaster belonging to the HSP20/alpha-crystallin family. It is one of four classical Drosophila sHSPs (Hsp22, Hsp23, Hsp26, Hsp27) that share a conserved alpha-crystallin domain and possess ATP-independent chaperone-like (holdase) activity. Hsp27 prevents heat-induced protein aggregation and maintains substrates in a refoldable state, with high efficiency at a 1:1 molar ratio to substrate (PMID:16572729). Approximately 40% of luciferase activity is recovered in in vitro refolding assays with Hsp27 (PMID:16572729), and its refolding capacity is partially dependent on the HSP70 machine (PMID:26705243). Hsp27 localizes primarily to the cytoplasm and is strongly heat-inducible. Overexpression of Hsp27 extends mean lifespan by 30% and increases stress resistance (PMID:15308776). A characterized knockout allele is homozygous viable, without obvious defects, and fertile, indicating that Hsp27 is not essential for development; its loss is instead associated with reduced starvation resistance (PMID:18229455). A developmental review (Jagla et al. 2018, cited in the falcon report) reports that ubiquitous RNAi knockdown of Hsp27 is lethal; this apparent discrepancy with the viable knockout allele likely reflects RNAi-specific effects (e.g. off-target activity or co-knockdown of related sHSPs) or genetic-background differences rather than a true essential developmental requirement, with the definitive loss-of-function allele study (PMID:18229455) taking precedence. Hsp27 interacts with the SUMO-conjugating enzyme DmUbc9 (PMID:9514881) and with the ER chaperone XPORT in the secretory pathway (PMID:22099462).
Definition: Binding to an unfolded or misfolded protein to prevent its aggregation without actively catalyzing refolding. The holdase maintains the client protein in a soluble, folding-competent state. This is mechanistically distinct from foldase activity (GO:0044183) and from carrier-holdase activity (GO:0140309).
Justification: Hsp27: Hsp27 functions as an ATP-independent holdase chaperone that prevents heat-induced protein aggregation and maintains substrates in a refoldable state. Obsolete GO:0051082 captured binding only; GO:0044183 requires assisting folding, and GO:0140309 (relabelled 'unfolded protein holdase activity') keeps a carrier-specific definition requiring escort to an acceptor molecule or location, which is not demonstrated here. See go-ontology#30552.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Manual review: cytoplasm is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Manual review: nucleus may be context-dependent or peripheral for Hsp27. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. |
| GO:0009408 response to heat | IBA GO_REF:0000033 | ACCEPT | Summary: Manual review: response to heat is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Protein refolding (GO:0042026) is the restoration of biological activity of an unfolded/misfolded protein. Hsp27 is a strict ATP-independent holdase; the actual ATP-dependent refolding (restoration of activity) is performed by the downstream Hsp70 machine, with Hsp27 holding substrates in a refoldable state upstream (PMID:26705243). Kept as non-core: Hsp27's contribution to refolding is indirect and upstream of the ATP-dependent refolding step, not its core holdase activity. Reason: Hsp27 is a holdase, not an ATP-dependent foldase; its refolding contribution is indirect (it maintains substrates in a refoldable state for handoff to the Hsp70 system). Kept as non-core rather than core, consistent with the ATP-independent holdase characterization applied throughout this review. |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MODIFY | Summary: GO:0051082 is now formally obsolete. Hsp27 is an sHSP holdase that binds unfolded proteins to prevent aggregation, consistent with experimental evidence for holdase activity (PMID:16572729). Reason: The biology is supported by direct experimental evidence, but GO:0051082 is obsolete. GO:0140309 (relabelled unfolded protein holdase activity) is not appropriate because its definition remains carrier-specific (escort to an acceptor; go-ontology#30552), so the replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). Falcon deep research confirms Hsp27 acts as an ATP-independent anti-aggregation holdase rather than an enzyme or ATP-dependent chaperone. Propagation Review Root cause: TERM SCOPING PROBLEM Proposed replacements: holdase chaperone activity Supporting Evidence: file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 is **not an enzyme** and does not catalyze a chemical reaction. file:DROME/Hsp27/Hsp27-deep-research-falcon.md Its primary biochemical function is **molecular chaperone activity** (ATP-independent anti-aggregation/holdase activity) |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | ACCEPT | Summary: Manual review: cytoplasm is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. |
| GO:0009408 response to heat | IEA GO_REF:0000117 | ACCEPT | Summary: Manual review: response to heat is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. |
| GO:0042026 protein refolding | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Protein refolding (GO:0042026) is the restoration of biological activity of an unfolded/misfolded protein. Hsp27 is a strict ATP-independent holdase; the actual ATP-dependent refolding is performed by the downstream Hsp70 machine, with Hsp27 holding substrates in a refoldable state upstream (PMID:26705243). Kept as non-core: Hsp27's contribution to refolding is indirect and upstream of the ATP-dependent step. Reason: Hsp27 is a holdase, not an ATP-dependent foldase; its refolding contribution is indirect (it maintains substrates in a refoldable state for handoff to the Hsp70 system). Kept as non-core rather than core, consistent with the ATP-independent holdase characterization applied throughout this review. |
| GO:0051082 unfolded protein binding | IEA GO_REF:0000117 | MODIFY | Summary: Manual review: unfolded protein binding is consistent with known biology of Hsp27, but GO:0051082 is now obsolete. Reason: Holdase binding of unfolded clients is well supported for Hsp27, but GO:0051082 is obsolete and GO:0140309 keeps a carrier-specific definition (go-ontology#30552). Replace with the proposed holdase chaperone activity NTR (see proposed_new_terms), as for the IBA row above. Proposed replacements: holdase chaperone activity |
| GO:0006457 protein folding | IDA PMID:16572729 Differences in the chaperone-like activities of the four mai... | ACCEPT | Summary: Manual review: protein folding is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. Supporting Evidence: PMID:16572729 the 4 main sHsps of Drosophila share the ability to prevent heat-induced protein aggregation and are able to maintain proteins in a refoldable state, although with different efficiencies file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 can prevent heat-induced aggregation of model substrates such as **citrate synthase and luciferase** and can maintain heat-denatured luciferase in a **refoldable** state. |
| GO:0044183 protein folding chaperone | IDA PMID:16572729 Differences in the chaperone-like activities of the four mai... | ACCEPT | Summary: Manual review: protein folding chaperone is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. Supporting Evidence: PMID:16572729 Heat-induced aggregation of citrate synthase was decreased from 100 to 17 arbitrary units in the presence of Hsp22 and Hsp27 at a 1:1 molar ratio of sHsp to citrate synthase file:DROME/Hsp27/Hsp27-deep-research-falcon.md Its primary biochemical function is **molecular chaperone activity** (ATP-independent anti-aggregation/holdase activity) |
| GO:0034663 endoplasmic reticulum chaperone complex | IPI PMID:22099462 XPORT-dependent transport of TRP and rhodopsin. | KEEP AS NON CORE | Summary: Manual review: endoplasmic reticulum chaperone complex may be context-dependent or peripheral for Hsp27. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. Supporting Evidence: PMID:22099462 XPORT is a resident ER and secretory pathway protein that interacts with TRP and Rh1, as well as with Hsp27 and Hsp90 |
| GO:0006457 protein folding | ISM PMID:19715580 The small heat shock protein (sHSP) genes in the silkworm, B... | ACCEPT | Summary: Manual review: protein folding is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. Supporting Evidence: PMID:19715580 sHSPs primarily have chaperone activity and reflect the response machine of organisms to some extreme stresses existing in environment |
| GO:0051082 unfolded protein binding | IDA PMID:16572729 Differences in the chaperone-like activities of the four mai... | MODIFY | Summary: Manual review: unfolded protein binding is consistent with known biology of Hsp27, but GO:0051082 is now obsolete. Reason: Holdase binding of unfolded clients is well supported for Hsp27, but GO:0051082 is obsolete and GO:0140309 keeps a carrier-specific definition (go-ontology#30552). Replace with the proposed holdase chaperone activity NTR (see proposed_new_terms), as for the IBA row above. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:16572729 These differences in luciferase reactivation efficiency seemed related to the ability of sHsps to bind their substrate at 42 degrees C, as revealed by sedimentation analysis of sHsp and luciferase on sucrose gradients file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 is part of the set of ATP-independent chaperones that prevent nonspecific aggregation of misfolded proteins under stress and non-stress contexts. |
| GO:0051082 unfolded protein binding | ISM PMID:19715580 The small heat shock protein (sHSP) genes in the silkworm, B... | MODIFY | Summary: Manual review: unfolded protein binding is consistent with known biology of Hsp27, but GO:0051082 is now obsolete. Reason: Holdase binding of unfolded clients is well supported for Hsp27, but GO:0051082 is obsolete and GO:0140309 keeps a carrier-specific definition (go-ontology#30552). Replace with the proposed holdase chaperone activity NTR (see proposed_new_terms), as for the IBA row above. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:19715580 This stable multimeric structure formed by sHSPs has the function of molecular chaperone, which binds to the proteins and prevents them from thermal denaturation |
| GO:0009408 response to heat | IDA PMID:26705243 Specific protein homeostatic functions of small heat-shock p... | ACCEPT | Summary: Manual review: response to heat is consistent with known biology of Hsp27. Reason: Retained as supported or plausible for this gene and evidence context. Supporting Evidence: PMID:26705243 The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly induced after a heat shock |
| GO:0042026 protein refolding | IDA PMID:26705243 Specific protein homeostatic functions of small heat-shock p... | KEEP AS NON CORE | Summary: Direct evidence: Hsp27 overexpression increases luciferase refolding (PMID:26705243). However, the refolding depends on the Hsp70 machinery (falcon report), and Hsp27 is a strict ATP-independent holdase that maintains substrates in a refoldable state for handoff to the ATP-dependent Hsp70 foldase. Kept as non-core: Hsp27's refolding contribution is indirect and upstream of the ATP-dependent refolding step rather than its core holdase activity (GO:0051082). Reason: Hsp27 is a holdase, not an ATP-dependent foldase; the observed increase in refolding reflects an upstream holding role that hands substrates to the Hsp70 system. Kept as non-core rather than core, consistent with the ATP-independent holdase characterization applied throughout this review. Supporting Evidence: PMID:26705243 overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 can assist refolding of **nuclear luciferase** in *Drosophila* S2 cells, and the refolding depends on **Hsp70 machinery**, consistent with a holdase role upstream of ATP-dependent refolding. |
| GO:0005634 nucleus | HDA PMID:24292889 Ube3a, the E3 ubiquitin ligase causing Angelman syndrome and... | KEEP AS NON CORE | Summary: Nuclear localization is reported by falcon deep research as a distinctive feature of Hsp27 among the Drosophila sHSPs (the other classical sHSPs being mitochondrial, cytosolic, or otherwise distributed). However, localization is stage- and context-dependent: during oogenesis Hsp27 is nuclear in nurse cells through germarium stage ~6 and then shifts to perinuclear/cytoplasmic from stage ~8. Retained as a valid but context-specific (non-core) location rather than a constitutive one. Provenance note: the HDA evidence derives from a high-throughput proteomics screen in the full text of PMID:24292889; the cached abstract concerns Ube3a/Rpn10 and does not mention Hsp27, so the screen-specific localization data could not be independently verified from the abstract cache and rests on the FlyBase 2014 curation. Reason: Kept as non-core: nuclear localization is well-supported as a hallmark of Hsp27 relative to other Drosophila sHSPs but is developmentally/stress regulated rather than constitutive, so it is not elevated to the core (cytoplasmic holdase) function. Supporting Evidence: file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 is specifically described as **nuclear** (in contrast to other sHsps with mitochondrial, cytosolic, or other localizations). file:DROME/Hsp27/Hsp27-deep-research-falcon.md In nurse cells: **nuclear** up to germarium stage ~6, then **perinuclear/cytoplasmic** from stage ~8. |
| GO:0005737 cytoplasm | HDA PMID:24292889 Ube3a, the E3 ubiquitin ligase causing Angelman syndrome and... | ACCEPT | Summary: Cytoplasmic localization is consistent with the core role of Hsp27 as an ATP-independent holdase. Falcon deep research documents perinuclear/cytoplasmic localization in nurse cells from oogenesis stage ~8 and predominant detection in somatic follicle cells after heat shock, consistent with a cytoplasmic compartment for chaperone action. Provenance note: the HDA evidence derives from a high-throughput proteomics screen in the full text of PMID:24292889; the cached abstract concerns Ube3a/Rpn10 and does not mention Hsp27, so the screen-specific localization data could not be independently verified from the abstract cache and rests on the FlyBase 2014 curation. Reason: Retained as supported by the cytoplasmic holdase role; cytoplasm is the principal compartment for Hsp27 anti-aggregation activity. Supporting Evidence: file:DROME/Hsp27/Hsp27-deep-research-falcon.md In nurse cells: **nuclear** up to germarium stage ~6, then **perinuclear/cytoplasmic** from stage ~8. file:DROME/Hsp27/Hsp27-deep-research-falcon.md After heat shock: Hsp27 predominantly detected in somatic follicle cells surrounding germline cysts. |
| GO:0042595 behavioral response to starvation | IMP PMID:18229455 The Hsp27 gene is not required for Drosophila development bu... | KEEP AS NON CORE | Summary: Manual review: behavioral response to starvation may be context-dependent or peripheral for Hsp27. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. Supporting Evidence: PMID:18229455 a significant reduction in starvation resistance was associated with the genotype without a functional Hsp27 gene file:DROME/Hsp27/Hsp27-deep-research-falcon.md silencing *D. melanogaster* hsp27 reduces the ability to endure starvation |
| GO:0005515 protein binding | IPI PMID:9514881 Cloning and developmental expression of a nuclear ubiquitin-... | MARK AS OVER ANNOTATED | Summary: Manual review: protein binding is too generic or over-extended for Hsp27. Reason: Marked over-annotated because more specific terms capture the biology more accurately. Supporting Evidence: PMID:9514881 two-hybrid system analysis reveals DmUbc9 interaction with Drosophila and mammalian Hsp27 file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 can bind the ubiquitin-conjugating enzyme **DmUbc9** |
| GO:0042742 defense response to bacterium | IMP PMID:21076039 Participation of the p38 pathway in Drosophila host defense ... | KEEP AS NON CORE | Summary: Defense response to bacterium may be context-dependent or peripheral for Hsp27. Provenance note: the cached abstract of PMID:21076039 describes the p38 pathway and host defense but does not name Hsp27 explicitly, so the IMP evidence derives from full-paper data not present in the abstract cache; the falcon supporting text (from the Morrow & Tanguay 2015 review) corroborates the p38-Hsp27 link. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. Supporting Evidence: PMID:21076039 p38-activated heat-shock factor and suppressed JNK collectively contributed to host defense against infection file:DROME/Hsp27/Hsp27-deep-research-falcon.md Hsp27 is required for proper **p38 MAPK–dependent host defense** |
| GO:0050832 defense response to fungus | IMP PMID:21076039 Participation of the p38 pathway in Drosophila host defense ... | KEEP AS NON CORE | Summary: Defense response to fungus may be context-dependent or peripheral for Hsp27. Provenance note: the cached abstract of PMID:21076039 describes the p38 pathway and host defense but does not name Hsp27 explicitly, so the IMP evidence derives from full-paper data not present in the abstract cache; the falcon supporting text (from the Morrow & Tanguay 2015 review) corroborates the p38-Hsp27 link. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. Supporting Evidence: PMID:21076039 the p38 pathway-mediated stress response contribute to Drosophila host defense against microbial infection file:DROME/Hsp27/Hsp27-deep-research-falcon.md hsp27 mutants are described as more susceptible to infection |
| GO:0008340 determination of adult lifespan | IMP PMID:15308776 Multiple-stress analysis for isolation of Drosophila longevi... | KEEP AS NON CORE | Summary: Manual review: determination of adult lifespan may be context-dependent or peripheral for Hsp27. Reason: Kept as non-core to preserve potentially valid context-specific annotation without elevating it to core function. Supporting Evidence: PMID:15308776 Overexpression of either hsp26 or hsp27 extended the mean lifespan by 30%, and the flies also displayed increased stress resistance |
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