Drosophila melanogaster Hsp26 is an alpha-crystallin-domain small heat shock protein with ATP-independent holdase activity. It binds non-native clients during stress, suppresses heat-induced aggregation, and preserves damaged proteins for subsequent refolding by other chaperones. Hsp26 is highly heat-inducible, predominantly cytosolic, and less efficient than Hsp22 or Hsp27 in purified citrate synthase and luciferase protection assays. It associates with Hsp23; Hsp23/Hsp26 dosage modulates neuromuscular-junction synapse number in developing motor neurons. Hsp26 is also developmentally regulated, with strong early-embryo and germline expression, and its basal expression in ovaries is maintained differently from most Hsp genes after Med15 depletion.
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: Hsp26 is a Drosophila small heat shock protein that suppresses heat-induced client aggregation in situ and maintains clients in a refoldable state for later reactivation by ATP-dependent chaperones. GO:0051082 is obsolete, GO:0044183 is a foldase term, and GO:0140309 remains carrier-specific because it requires escort to an acceptor molecule or location. Hsp26 does not show that escort mechanism, so the proposed general holdase chaperone activity term is the correct molecular-function replacement.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasm is consistent with conserved small heat-shock protein biology and with Hsp26-specific cytosolic localization. Reason: Current PAINT places the cytoplasm IBD at PTN000897708 in PTHR45640, and Hsp26 descends from this small-HSP ancestral node. Full PMID:32437379 localizes tagged Hsp26 with Hsp23 through the larval CNS cytoplasm and NMJ synaptic boutons, while full PMID:16572729 cites the classical Drosophila localization comparison that placed Hsp23 and Hsp26 in the cytosol. Retain the inherited broad cytoplasm annotation. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000897708 SUPPORTS TRANSFER Current PTHR45640 PAINT places this source IBD at the Eumetazoa small-HSP node that gives rise to Drosophila Hsp26; target experimental descendants are valid grounding, not circularity. Supporting Evidence: PMID:32437379 The confocal images show an accumulation and colocalization of sHSP23 and sHSP26 throughout the NMJ but particularly intense in the synaptic buttons PMID:16572729 Hsp23 and Hsp26 in the cytosol, and Hsp27 in the nucleus |
| GO:0005634 nucleus | IBA GO_REF:0000033 | UNDECIDED | Summary: A minor or conditional nuclear Hsp26 pool remains unresolved pending primary Hsp26-specific evidence. Reason: The current PTHR45640 nucleus IBD is genuinely placed at PTN000897708, the same ancestral node inherited by Hsp26, so this is not a simple bad-transfer call. However, unlike the cytoplasm, heat-response, and refolding IBDs, the nucleus IBD has no Drosophila descendant seeds in the current PAINT extract. The cached primary Hsp26 papers verify cytosolic larval CNS/NMJ localization, and the only Hsp26 nuclear-matrix claim in the review comes from secondary/generative summaries. Keep this IBA unresolved until the primary Hsp26 nuclear-matrix paper can be inspected for target specificity, fraction purity, and a genuine Hsp26 signal distinct from Hsp23/Hsp27 cross-reactivity. Propagation Review Root cause: UNRESOLVED Sources checked: PANTHER:PTN000897708 UNRESOLVED The nucleus IBD is at the same ancestral small-HSP node as the supported cytoplasm, heat-response and refolding IBDs, but unlike those Drosophila-rich assertions its WITH/FROM list has no Drosophila seeds; target-specific primary support for a nuclear Hsp26 pool still needs retrieval. Supporting Evidence: PMID:16572729 Hsp23 and Hsp26 in the cytosol, and Hsp27 in the nucleus file:DROME/Hsp26/Hsp26-deep-research-falcon.md A **minor nuclear fraction** has been detected: Hsp26 was observed in the **nuclear matrix** of embryos and S2 cells |
| GO:0009408 response to heat | IBA GO_REF:0000033 | ACCEPT | Summary: Response to heat is consistent with the conserved small heat-shock protein function. Reason: Heat induction and experimentally demonstrated client protection support the conserved heat-response function. Transcript induction alone is not the whole evidence: full PMID:16572729 shows purified Hsp26 protects heated citrate synthase and luciferase clients, and full PMID:26705243 directly confirms strong Hsp26 induction after 38 C heat shock. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000897708 SUPPORTS TRANSFER Current PTHR45640 PAINT places this source IBD at the Eumetazoa small-HSP node that gives rise to Drosophila Hsp26; target experimental descendants are valid grounding, not circularity. Supporting Evidence: PMID:26705243 The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly induced after a heat shock PMID:16572729 A 5-fold excess of these 2 sHsps (5:1 sHsp:luciferase) was necessary to prevent luciferase aggregation as well as the 1:1 ratio for Hsp22 |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | ACCEPT | Summary: Protein refolding is consistent with the conserved small heat-shock protein function. Reason: Hsp26 binds heat-damaged clients, prevents their aggregation and preserves a pool that the ATP-supplemented reticulocyte chaperone machinery can reactivate in full PMID:16572729. This is direct participation in the refolding process even though Hsp26 itself performs the holding step rather than the ATP-driven folding reaction. Vos et al. also showed that Hsp26 overexpression increases recovery of heat-denatured luciferase in S2 cells. Current target ancestry agrees with this inference. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000897708 SUPPORTS TRANSFER Current PTHR45640 PAINT places this source IBD at the Eumetazoa small-HSP node that gives rise to Drosophila Hsp26; target experimental descendants are valid grounding, not circularity. Supporting Evidence: PMID:16572729 Altogether, these results demonstrate that Hsp22, Hsp23, Hsp26, and Hsp27 can maintain heat-treated luciferase in a refoldable state, from which it can be refolded by other chaperones into an active enzyme. PMID:26705243 overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MODIFY | Summary: Use the proposed general holdase term for the demonstrated client-binding mechanism. Reason: GO:0051082 is obsolete, but carrier-specific GO:0140309 does not fit Hsp26's in-situ small-HSP holdase activity because no escort to a defined acceptor or location is demonstrated. Full PMID:16572729 directly demonstrates client holding and later recovery by the reticulocyte chaperone machinery, with different efficiencies for each small Hsp. Use the proposed general holdase chaperone activity term; the current PAINT extract no longer carries GO:0051082 because it is obsolete, not because Hsp26 lacks inherited holdase activity. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: PANTHER:PTN000897708 SOURCE STALE OR MISSING The frozen IBA traced to this node, but current PTHR45640 PAINT no longer carries obsolete GO:0051082; the target ancestry and client-holding biology remain consistent with the active holdase replacement. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:16572729 Altogether, these results demonstrate that Hsp22, Hsp23, Hsp26, and Hsp27 can maintain heat-treated luciferase in a refoldable state, from which it can be refolded by other chaperones into an active enzyme. |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | ACCEPT | Summary: IEA (ARBA) annotation for cytoplasmic localization. Consistent with IBA and experimental evidence (HDA PMID:24292889, IDA PMID:32437379). Reason: Correct and well-supported by multiple experimental sources. Broader than cytosol (IDA) but acceptable as an IEA annotation. |
| GO:0009408 response to heat | IEA GO_REF:0000117 | ACCEPT | Summary: IEA (ARBA) annotation for response to heat. Consistent with IBA and IDA evidence (PMID:26705243). Reason: Correct. Hsp26 is a classical heat shock protein that is highly induced upon heat stress. This is its namesake function. |
| GO:0042026 protein refolding | IEA GO_REF:0000117 | ACCEPT | Summary: IEA (ARBA) annotation for protein refolding. Consistent with IBA and IDA evidence (PMID:26705243, PMID:16572729). Hsp26 participates in the protein refolding process by holding substrates for HSP70-dependent refolding. Reason: Consistent with the IBA and IDA annotations for the same term. Hsp26 participates in protein refolding by maintaining substrates in a refoldable state, although the actual refolding step is performed by the HSP70 machine. |
| GO:0051082 unfolded protein binding | IEA GO_REF:0000117 | MODIFY | Summary: Use the proposed general holdase term for the demonstrated client-binding mechanism. Reason: GO:0051082 is obsolete. Hsp26-mediated ATP-independent client holding is an in-situ small-HSP activity, not carrier-holdase activity, so the row should point to the proposed general holdase chaperone activity term rather than GO:0140309. Proposed replacements: holdase chaperone activity |
| GO:0005515 protein binding | IPI PMID:38944040 Next-generation Drosophila protein interactome map and its f... | MODIFY | Summary: The Hsp23 association can be recorded as heat shock protein binding. Reason: The source interactome supplies a curated Hsp23-Hsp26 association, and full PMID:32437379 independently confirms Hsp23/Hsp26 co-immunoprecipitation with controls. GO:0031072 describes the binding partner more informatively than generic protein binding. This does not assert a fixed stoichiometry, obligatory complex or one-directional synapse phenotype. Proposed replacements: heat shock protein binding Supporting Evidence: PMID:32437379 These results confirm the physical interaction between sHSP23 and sHSP26 |
| GO:0006457 protein folding | IDA PMID:16572729 Differences in the chaperone-like activities of the four mai... | ACCEPT | Summary: IDA annotation for protein folding based on Morrow et al. (2006). The study showed Hsp26 prevents heat-induced aggregation of citrate synthase and maintains luciferase in a refoldable state. However, Hsp26 itself does not fold proteins; it holds them for HSP70-dependent refolding. This annotation is acceptable as participation in the broader protein folding process. Reason: The annotation captures Hsp26's role in the protein folding process at the biological process level. While Hsp26 does not directly catalyze folding (it is a holdase), it is an essential participant in the protein folding pathway by maintaining substrates in a folding-competent state. The BP term protein folding encompasses all steps of the process, not just the catalytic step. 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 |
| GO:0044183 protein folding chaperone | IDA PMID:16572729 Differences in the chaperone-like activities of the four mai... | MODIFY | Summary: Holdase activity is the more precise description of the source experiment. Reason: GO:0044183 does not define chaperones as necessarily ATP dependent, but its current comment distinguishes folding a client from holding it unfolded. Full PMID:16572729 demonstrates protection during heat treatment followed by reactivation by other chaperones in reticulocyte lysate plus ATP. Hsp26 performs the in-situ small-HSP holdase step, so a general holdase chaperone activity NTR is more precise than foldase GO:0044183 or carrier-specific GO:0140309. Full PMID:26705243 tested Hsp26 in the S2 luciferase refolding assay, but its direct HSP70-knockdown tests were for Hsp27 and CG14207, so those knockdown data should not be described as a direct Hsp26 requirement. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:16572729 Altogether, these results demonstrate that Hsp22, Hsp23, Hsp26, and Hsp27 can maintain heat-treated luciferase in a refoldable state, from which it can be refolded by other chaperones into an active enzyme. |
| GO:0005829 cytosol | IDA PMID:32437379 Small heat shock proteins determine synapse number and neuro... | ACCEPT | Summary: IDA annotation for cytosol localization based on Santana et al. (2020). The study used an Hsp26-GFP-V5 fusion construct and immunostaining in third instar larval brains and NMJs, showing cytosolic localization particularly enriched in synaptic buttons. Reason: Well-supported by direct immunofluorescence data. Cytosolic localization is consistent with the broader cytoplasm annotations (IBA, HDA, IEA) and with the known biology of sHSPs as cytoplasmic chaperones. Falcon deep research concurs that Hsp26 is predominantly cytosolic/cytoplasmic with a granular staining pattern distinct from Hsp23. Supporting Evidence: PMID:32437379 The confocal images show an accumulation and colocalization of sHSP23 and sHSP26 throughout the NMJ but particularly intense in the synaptic buttons file:DROME/Hsp26/Hsp26-deep-research-falcon.md Hsp26 is **predominantly cytosolic/cytoplasmic**, with a **granular cytosolic staining pattern** distinct from Hsp23; a **minor fraction** was also detected in the **nuclear matrix** of embryos and S2 cells. |
| GO:0006457 protein folding | ISM PMID:19715580 The small heat shock protein (sHSP) genes in the silkworm, B... | ACCEPT | Summary: ISM annotation for protein folding based on Li et al. (2009), a comparative genomic study of sHSP genes across insects. The annotation is based on sequence model inference from the conserved alpha-crystallin domain. The paper itself focuses on genomic organization and evolution of sHSP genes, not direct functional characterization of Drosophila Hsp26. Reason: The ISM inference is sound: the alpha-crystallin domain is a hallmark of sHSPs with chaperone function. Li et al. (2009) confirmed that insect sHSPs share conserved structural features associated with chaperone function. Hsp26 participation in protein folding is also confirmed by direct experimental evidence (PMID:16572729). 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: Use the proposed general holdase term for the demonstrated client-binding mechanism. Reason: GO:0051082 is obsolete. Sedimentation analysis in full PMID:16572729 directly showed Hsp26 binding to heat-denatured luciferase, and the same work showed recovery of the held client by other chaperones in reticulocyte lysate plus ATP. A general holdase chaperone activity NTR is the right replacement for this ATP-independent in-situ holdase activity; carrier-specific GO:0140309 does not fit. 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 42Β°C, as revealed by sedimentation analysis of sHsp and luciferase on sucrose gradients. |
| GO:0051082 unfolded protein binding | ISM PMID:19715580 The small heat shock protein (sHSP) genes in the silkworm, B... | MODIFY | Summary: Use the proposed general holdase term for the conserved small-HSP client-binding mechanism. Reason: GO:0051082 is obsolete. The alpha-crystallin-domain sequence inference is consistent with Hsp26 holdase activity, but the row should point to the proposed general holdase chaperone activity term, not carrier-specific GO:0140309, as the replacement for client holding. 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: IDA annotation for response to heat based on Vos et al. (2016). The study showed that Hsp26 is one of the four classical Drosophila sHSPs that are highly induced after heat shock at 38 degrees C, and its overexpression enhances refolding of heat-denatured substrates and prevents protein aggregation. Reason: Core function. Heat shock response is a defining characteristic of Hsp26. The study directly measured Hsp26 mRNA induction upon heat shock and characterized its chaperone-like activities in the context of heat stress. 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... | ACCEPT | Summary: IDA annotation for protein refolding based on Vos et al. (2016). The study used a cellular luciferase refolding assay in Drosophila S2 cells and showed that overexpression of Hsp26 increased luciferase refolding after heat shock. However, the refolding depends on the endogenous HSP70 machinery; Hsp26 functions as a holdase to maintain substrates in a refoldable state. Reason: The annotation captures the biological process outcome correctly. In cellular assays, Hsp26 overexpression does enhance protein refolding (PMID:26705243). The mechanistic nuance (holdase feeding into HSP70-dependent refolding) does not invalidate the BP annotation for protein refolding, as Hsp26 is an essential participant in this process. Supporting Evidence: PMID:26705243 overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding |
| GO:0005737 cytoplasm | HDA PMID:24292889 Ube3a, the E3 ubiquitin ligase causing Angelman syndrome and... | ACCEPT | Summary: HDA annotation for cytoplasmic localization based on Lee et al. (2014). This study focused on Ube3a and protein homeostasis, and Hsp26 was identified in the cytoplasmic proteome by high-throughput methods. While Hsp26 is not a focus of this paper, the detection of Hsp26 in cytoplasmic fractions is consistent with its known localization. Reason: Consistent with all other localization data. Hsp26 is a cytoplasmic sHSP. The HDA evidence provides independent proteomics confirmation. Supporting Evidence: PMID:24292889 We have now devised a protocol to screen for substrates of this particular ubiquitin ligase. In a neuronal cell system, we find direct ubiquitination by Ube3a of three proteasome-related proteins Rpn10, Uch-L5, and CG8209, as well as of the ribosomal protein Rps10b. |
| GO:0017022 myosin binding | IPI PMID:18045836 Sisyphus, the Drosophila myosin XV homolog, traffics within ... | KEEP AS NON CORE | Summary: IPI annotation for myosin binding based on Liu et al. (2008). The study focused on Sisyphus (Drosophila myosin XV homolog) and identified Hsp26 as a putative cargo transported within filopodia. The paper identified several putative Sisyphus cargos including DE-cadherin, Katanin-60, EB1, Milton, and aPKC, but Hsp26 is not mentioned in the abstract as a primary cargo. This appears to be an incidental interaction rather than a core function of Hsp26. Reason: The interaction with Sisyphus (myosin XV) was identified in a study focused on myosin cargo identification, not on Hsp26 function. While the physical interaction may be real, myosin binding does not represent a core function of Hsp26 as a holdase chaperone. It may reflect Hsp26 being transported as cargo or an incidental interaction in the cytoplasm. Falcon deep research likewise frames Hsp26-myosin interaction as a review-level, non-core observation suggesting a possible role in filopodial/cytoskeletal dynamics, supporting the KEEP_AS_NON_CORE action. Supporting Evidence: PMID:18045836 We have identified several putative Sisyphus cargos, including DE-cadherin (also known as Shotgun) and the microtubule-linked proteins Katanin-60, EB1, Milton and aPKC file:DROME/Hsp26/Hsp26-deep-research-falcon.md Reported interactions include **myosin 10A** (suggesting roles in **filopodial/cytoskeletal dynamics**) and a two-hybrid interaction with **lawc**, a factor linked to the **nuclear proteasome regulator dREGΞ³**; possible association with **DmUbc9** has been proposed. |
| GO:0009631 cold acclimation | IEP PMID:16313561 Cold hardening and transcriptional change in Drosophila mela... | KEEP AS NON CORE | Summary: IEP annotation for cold acclimation based on Qin et al. (2005). The study used microarray analysis to examine transcriptional changes during cold hardening (0 degrees C for 2 h) and found Hsp26 among genes with increased transcript abundance. The evidence is expression-based (IEP) and suggests Hsp26 participates in the cold hardening response, but does not demonstrate a direct functional role. Reason: The evidence is only expression-based (IEP). Cold acclimation is not a core function of Hsp26 but rather reflects the broader stress response role of sHSPs. The transcriptional induction during cold hardening is plausible given that protein misfolding can occur during cold stress, but this is a secondary/pleiotropic function. Falcon deep research independently notes that Hsp26 is reported as cold-inducible in review-level summaries, consistent with retaining this as a non-core stress-response role. Supporting Evidence: PMID:16313561 these assays suggest that stress proteins, including Hsp23, Hsp26, Hsp83 and Frost as well as membrane-associated proteins may contribute to the cold hardening response file:DROME/Hsp26/Hsp26-deep-research-falcon.md Hsp26 is also reported as **cold-inducible** in review-level summaries. |
| GO:0008340 determination of adult lifespan | IMP PMID:15308776 Multiple-stress analysis for isolation of Drosophila longevi... | KEEP AS NON CORE | Summary: IMP annotation for determination of adult lifespan based on Wang et al. (2004). The study showed that overexpression of Hsp26 using the UAS/GAL4 system extended mean lifespan by 30% in Drosophila and increased stress resistance. This is a real biological phenotype but likely reflects the downstream consequence of improved protein homeostasis rather than a core molecular function. Reason: The lifespan extension phenotype is well-supported by IMP evidence. However, determination of adult lifespan is a downstream consequence of the core holdase chaperone function maintaining protein homeostasis during aging, not a direct molecular activity. Vos et al. (2016) showed that diverse sHSP activities (both refolding-promoting and anti-aggregation) can support longevity, indicating the lifespan effect is a pleiotropic outcome of improved proteostasis. Falcon deep research corroborates this as a pleiotropic/downstream readout: it notes Hsp26 overexpression can increase lifespan and oxidative-stress resistance, but its direct thermoprotective effect in larvae appears small, supporting KEEP_AS_NON_CORE. Supporting Evidence: PMID:15308776 Overexpression of either hsp26 or hsp27 extended the mean lifespan by 30%, and the flies also displayed increased stress resistance PMID:26705243 overexpression of both CG14207 and HSP67BC in Drosophila leads to a mild increase in lifespan, demonstrating that increased levels of functionally diverse small HSPs can promote longevity in vivo file:DROME/Hsp26/Hsp26-deep-research-falcon.md Overexpression studies summarized in reviews report that Hsp26 can **increase lifespan** and **oxidative-stress resistance**; however, its direct thermoprotective effect in larvae appears **small** and it had **no effect on neural function** in one summarized assay. |
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Download this section (compressed HTML)Q: Can primary Hsp26 nuclear-matrix fractionation or imaging data distinguish a true conditional nuclear pool from Hsp23/Hsp27 antibody cross-reactivity and cytosolic contamination?
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Download this section (compressed HTML)Heat shock protein 26 (Hsp26) encodes a protein involved in protein folding. It contributes to lifespan determination and the response to cold and heat.
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