Hsp26

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005737 cytoplasm	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for cytoplasmic localization, supported by phylogenetic inference across multiple sHSP orthologs. Consistent with HDA data (PMID:24292889) and IDA cytosol annotation (PMID:32437379). Drosophila Hsp26 is a cytoplasmic sHSP. Reason: Cytoplasmic localization is well-supported by multiple lines of evidence including proteomics (HDA, PMID:24292889), direct assay showing cytosol localization (IDA, PMID:32437379), and phylogenetic inference. This is consistent with the known biology of sHSPs as cytoplasmic chaperones. Falcon deep research corroborates this: Hsp26 is reported to be predominantly cytosolic/cytoplasmic with a granular cytosolic staining pattern distinct from Hsp23. Supporting Evidence: PMID:32437379 sHSP23 and sHSP26 colocalize in CNS. file:DROME/Hsp26/Hsp26-deep-research-falcon.md Hsp26 is reported to be predominantly cytosolic/cytoplasmic and displays a granular cytosolic staining pattern that differs from the distribution of Hsp23, suggesting sHsp specialization within shared compartments.
GO:0005634 nucleus	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for nuclear localization based on phylogenetic inference from mammalian orthologs (HSPB1, CRYAB, CRYAA, HSPB8) that have documented nuclear localization. No direct experimental evidence for nuclear localization of Drosophila Hsp26 specifically, but mammalian sHSPs do shuttle to the nucleus under stress, making this plausible by homology. Reason: Nuclear localization is well established for mammalian sHSP orthologs included in the IBA with/from set (HSPB1, CRYAB, CRYAA, HSPB8). The IBA inference is phylogenetically sound. Falcon deep research adds Drosophila-specific support: a minor nuclear fraction of Hsp26 has been detected in the nuclear matrix of embryos and S2 cells (alongside Hsp27), while whole-cell immunostaining indicates it is mainly cytoplasmic. This indicates nuclear association is limited/conditional rather than a primary localization, but is consistent with the IBA inference. Supporting Evidence: 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 (alongside Hsp27), while whole-cell immunostaining indicates it is mainly cytoplasmic, consistent with a model in which nuclear association is limited/conditional.
GO:0009408 response to heat	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for response to heat, well-supported by phylogenetic conservation of heat shock response across sHSPs. Hsp26 is one of four classical Drosophila sHSPs that are highly heat-inducible (PMID:26705243, PMID:16572729). Reason: Core function. Hsp26 is a classical heat shock protein, highly induced after heat shock at 38 degrees C (PMID:26705243). The name itself reflects this core function. Falcon deep research confirms Hsp26 is a canonical heat shock gene regulated by HSF/HSE logic in the 67B cluster and among the most abundant heat-responsive transcripts in S2 cells. Supporting Evidence: PMID:26705243 The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly induced after a heat shock file:DROME/Hsp26/Hsp26-deep-research-falcon.md Hsp26 is a canonical heat shock gene regulated by HSF/HSE logic in the 67B cluster; it is described as among the most abundant stress-responsive transcripts in Drosophila S2 cells after heat stress, and HSF binding at its promoter is reported in review-level summaries. 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. A 5 M excess of Hsp23 and Hsp26 was required to obtain the same efficiency
GO:0042026 protein refolding	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for protein refolding. While sHSPs do facilitate eventual protein refolding, this is misleading because Hsp26 itself does not refold proteins. Rather, it maintains denatured proteins in a refoldable state for subsequent processing by HSP70 (PMID:16572729, PMID:26705243). The refolding step is performed by the HSP70 machine. However, in the context of cellular assays, overexpression of Hsp26 does increase luciferase refolding because the endogenous HSP70 machinery completes the refolding (PMID:26705243). Reason: Although Hsp26 does not directly catalyze protein refolding, its holdase activity maintains substrates in a refoldable state, which is a prerequisite for refolding by the HSP70 machine. In cellular assays, Hsp26 overexpression enhances refolding of heat-denatured luciferase (PMID:16572729, PMID:26705243). The GO annotation captures the biological outcome (refolding is achieved) even though the molecular mechanism is indirect (holdase feeding into HSP70-dependent refolding). IBA inference is phylogenetically sound across sHSP family members. Supporting Evidence: PMID:16572729 In an in vitro refolding assay with reticulocyte lysate, more than 50% of luciferase activity was recovered when heat denaturation was performed in the presence of Hsp22, 40% with Hsp27, and 30% with Hsp23 or Hsp26. PMID:26705243 Consistent with in vitro data (Morrow et al., 2006), overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding
GO:0051082 unfolded protein binding	IBA GO_REF:0000033	MODIFY	Summary: IBA annotation for unfolded protein binding. GO:0051082 is now formally obsolete. For sHSPs/holdases, the correct replacement is a holdase chaperone activity NTR (not GO:0140309 which is carrier-specific, and not GO:0044183 which implies active folding). Replace with the proposed holdase chaperone activity NTR. Reason: GO:0051082 is now formally obsolete. Hsp26 functions as a holdase chaperone, binding unfolded proteins to prevent aggregation in situ without actively refolding them. Per UPB project decision rules, sHSPs should be annotated to a holdase chaperone activity NTR (pending creation). GO:0140309 (unfolded protein carrier activity) is not appropriate because it was created for TIM carrier-holdases that transport substrates between compartments. GO:0044183 (protein folding chaperone) is not appropriate because Hsp26 is not a foldase. The replacement is therefore the proposed holdase chaperone activity NTR (see proposed_new_terms). Falcon deep research independently reaches the same conclusion, describing Drosophila sHSPs including Hsp26 as ATP-independent holdase chaperones that bind misfolded/unfolding proteins to prevent aggregation and keep clients folding-competent for downstream ATP-dependent systems. Proposed replacements: holdase chaperone activity 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/Hsp26/Hsp26-deep-research-falcon.md they function primarily as ATP-independent “holdase” chaperones: they bind misfolded/unfolding proteins to prevent nonspecific aggregation and keep clients in a folding-competent state for subsequent refolding or processing by ATP-dependent chaperone systems.
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: IEA (ARBA) annotation for unfolded protein binding. GO:0051082 is now formally obsolete. Same considerations as for the IBA and IDA annotations of this term. Reason: GO:0051082 is now formally obsolete. Hsp26 is a holdase chaperone. The replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). See review of IBA annotation for GO:0051082 above for full rationale. Proposed replacements: holdase chaperone activity
GO:0005515 protein binding	IPI PMID:38944040 Next-generation Drosophila protein interactome map and its f...	MARK AS OVER ANNOTATED	Summary: IPI annotation for protein binding based on large-scale Drosophila interactome study. The interacting partner is Hsp23 (P02516), as documented in UniProt and IntAct. This interaction is biologically meaningful (sHSPs form homo- and hetero-oligomeric complexes, and Hsp23/Hsp26 co-immunoprecipitate in PMID:32437379), but GO:0005515 (protein binding) is uninformative and does not convey the nature of the interaction. Reason: GO:0005515 (protein binding) is uninformative as per GO curation guidelines. While the Hsp23-Hsp26 interaction is real and biologically meaningful (co-immunoprecipitation confirmed in PMID:32437379; large-scale interactome in PMID:38944040), the term 'protein binding' does not capture any useful functional information. A more specific term describing sHSP oligomerization or chaperone substrate binding would be preferable. Supporting Evidence: PMID:32437379 Both sHSPs immunoprecipitate together and the equilibrium between both chaperones is required for neuronal development and activity. PMID:38944040 Next-generation Drosophila protein interactome map [IntAct records Hsp26-Hsp23 interaction]
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: IDA annotation for protein folding chaperone based on Morrow et al. (2006). This MF term implies active protein folding chaperone activity (foldase). However, Hsp26 is a holdase that prevents aggregation and maintains proteins in a refoldable state for HSP70-dependent refolding. It does not actively refold proteins through iterative ATP-dependent binding/release cycles. Per UPB project rules, GO:0044183 is not appropriate for pure holdases. Reason: GO:0044183 (protein folding chaperone) implies active foldase activity, which is not the mechanism of Hsp26. Morrow et al. (2006) showed that a 5-fold molar excess of Hsp26 was required for aggregation prevention, and refolding depended on reticulocyte lysate (containing HSP70 machinery). Vos et al. (2016) confirmed that sHSP-mediated refolding requires HSP70. Hsp26 is a holdase, not a foldase. The correct annotation should be to a holdase chaperone activity NTR (see proposed_new_terms). Falcon deep research reinforces the holdase (not foldase) characterization: Hsp26 is best described as an ATP-independent chaperone that buffers proteostasis by binding destabilized proteins during stress and facilitating their downstream handling by ATP-driven chaperone/refolding pathways, and required ~5-fold molar excess to match Hsp22/Hsp27 in citrate synthase aggregation assays. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:16572729 A 5 M excess of Hsp23 and Hsp26 was required to obtain the same efficiency with either citrate synthase or luciferase as substrate PMID:26705243 our results strongly suggest that the refolding capacity of D. melanogaster HSP27 and CG14207 is partially dependent on an intact HSP70 machine file:DROME/Hsp26/Hsp26-deep-research-falcon.md In functional annotation terms, Hsp26 is best described as an ATP-independent chaperone that buffers proteostasis by binding destabilized proteins during stress and facilitating their downstream handling by ATP-driven chaperone/refolding pathways.
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: IDA annotation for unfolded protein binding based on Morrow et al. (2006). The study directly demonstrated that Hsp26 binds heat-denatured luciferase by sedimentation analysis on sucrose gradients. GO:0051082 is now formally obsolete; Hsp26 should be annotated to a holdase NTR when available. Reason: GO:0051082 is now formally obsolete. The experimental evidence directly demonstrates unfolded protein binding: sedimentation analysis showed Hsp26 co-sediments with denatured luciferase at 42 degrees C (PMID:16572729). This binding constitutes holdase chaperone activity. Per UPB project decision rules, the correct replacement for sHSPs is a holdase chaperone activity NTR (see proposed_new_terms). 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
GO:0051082 unfolded protein binding	ISM PMID:19715580 The small heat shock protein (sHSP) genes in the silkworm, B...	MODIFY	Summary: ISM annotation for unfolded protein binding based on sequence model inference from the conserved alpha-crystallin domain in Li et al. (2009). GO:0051082 is now formally obsolete. Reason: GO:0051082 is now formally obsolete. The ISM inference is sound (alpha-crystallin domain consistently associated with chaperone/holdase function), but the term needs replacement with the proposed holdase chaperone activity NTR (see proposed_new_terms). 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 dREGγ; 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.

Gene Description

Proposed New Ontology Terms

holdase chaperone activity

Existing Annotations Review

Core Functions

References

Deep Research

Falcon

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