Hsp26

id: P02517
gene_symbol: Hsp26
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:7227
  label: Drosophila melanogaster
description: >-
  Drosophila melanogaster Heat shock protein 26 (Hsp26) is a small heat shock protein (sHSP) of the
  HSP20/alpha-crystallin family. It functions primarily as a holdase chaperone, binding denaturing
  proteins to prevent their aggregation under stress conditions. Unlike ATP-dependent foldases
  (e.g., HSP70, GroEL), Hsp26 does not actively refold substrates but maintains them in a
  refoldable state for subsequent processing by HSP70 machinery (PMID:16572729). Hsp26 is one of
  four classical Drosophila sHSPs (Hsp22, Hsp23, Hsp26, Hsp27) and is highly heat-inducible
  (PMID:26705243). It forms oligomeric complexes and co-immunoprecipitates with Hsp23
  (PMID:32437379). Overexpression extends adult lifespan by approximately 30% (PMID:15308776).
  Hsp26 also plays a developmental role in synaptogenesis, where it cooperates with Hsp23 to
  modulate synapse number at the neuromuscular junction (PMID:32437379).
existing_annotations:
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:32437379
        supporting_text: >-
          sHSP23 and sHSP26 colocalize in CNS.
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    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.
    action: ACCEPT
    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. While no direct experimental evidence exists for Drosophila Hsp26 nuclear
      localization, sHSPs are known to translocate to the nucleus under stress conditions
      across species.
- term:
    id: GO:0009408
    label: response to heat
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    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).
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:26705243
        supporting_text: >-
          The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly
          induced after a heat shock
      - reference_id: PMID:16572729
        supporting_text: >-
          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
- term:
    id: GO:0042026
    label: protein refolding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    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).
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: >-
          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.
      - reference_id: PMID:26705243
        supporting_text: >-
          Consistent with in vitro data (Morrow et al., 2006), overexpression of the classical
          small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      IBA annotation for unfolded protein binding. GO:0051082 is proposed for obsoletion. 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).
      Retain GO:0051082 until the holdase NTR is created.
    action: MODIFY
    reason: >-
      GO:0051082 is being obsoleted. 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.
      Retain GO:0051082 until the holdase NTR is created.
    proposed_replacement_terms:
      - id: GO:0051082
        label: unfolded protein binding (retain until holdase NTR is created)
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: >-
          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
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: >-
      IEA (ARBA) annotation for cytoplasmic localization. Consistent with IBA and experimental
      evidence (HDA PMID:24292889, IDA PMID:32437379).
    action: ACCEPT
    reason: >-
      Correct and well-supported by multiple experimental sources. Broader than cytosol (IDA)
      but acceptable as an IEA annotation.
- term:
    id: GO:0009408
    label: response to heat
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: >-
      IEA (ARBA) annotation for response to heat. Consistent with IBA and IDA evidence
      (PMID:26705243).
    action: ACCEPT
    reason: >-
      Correct. Hsp26 is a classical heat shock protein that is highly induced upon heat stress.
      This is its namesake function.
- term:
    id: GO:0042026
    label: protein refolding
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    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.
    action: ACCEPT
    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.
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: >-
      IEA (ARBA) annotation for unfolded protein binding. GO:0051082 is being obsoleted. Same
      considerations as for the IBA and IDA annotations of this term.
    action: MODIFY
    reason: >-
      GO:0051082 is being obsoleted. Hsp26 is a holdase chaperone. Retain until holdase NTR
      is created. See review of IBA annotation for GO:0051082 above for full rationale.
    proposed_replacement_terms:
      - id: GO:0051082
        label: unfolded protein binding (retain until holdase NTR is created)
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:38944040
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
      - reference_id: PMID:32437379
        supporting_text: >-
          Both sHSPs immunoprecipitate together and the equilibrium between both chaperones
          is required for neuronal development and activity.
      - reference_id: PMID:38944040
        supporting_text: >-
          Next-generation Drosophila protein interactome map [IntAct records Hsp26-Hsp23
          interaction]
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IDA
  original_reference_id: PMID:16572729
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: >-
          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
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: IDA
  original_reference_id: PMID:16572729
  review:
    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.
    action: MODIFY
    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 (pending creation). Retain GO:0051082 as interim.
    proposed_replacement_terms:
      - id: GO:0051082
        label: unfolded protein binding (retain until holdase NTR is created)
    additional_reference_ids:
      - PMID:26705243
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: >-
          A 5 M excess of Hsp23 and Hsp26 was required to obtain the same efficiency with
          either citrate synthase or luciferase as substrate
      - reference_id: PMID:26705243
        supporting_text: >-
          our results strongly suggest that the refolding capacity of D. melanogaster HSP27 and
          CG14207 is partially dependent on an intact HSP70 machine
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: PMID:32437379
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:32437379
        supporting_text: >-
          The confocal images show an accumulation and colocalization of sHSP23 and sHSP26
          throughout the NMJ but particularly intense in the synaptic buttons
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: ISM
  original_reference_id: PMID:19715580
  review:
    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.
    action: ACCEPT
    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).
    supported_by:
      - reference_id: PMID:19715580
        supporting_text: >-
          sHSPs primarily have chaperone activity and reflect the response machine of organisms
          to some extreme stresses existing in environment
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IDA
  original_reference_id: PMID:16572729
  review:
    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 being obsoleted; Hsp26 should be annotated
      to a holdase NTR when available.
    action: MODIFY
    reason: >-
      GO:0051082 is being obsoleted. 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 (pending). Retain GO:0051082 until the holdase NTR is created.
    proposed_replacement_terms:
      - id: GO:0051082
        label: unfolded protein binding (retain until holdase NTR is created)
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: >-
          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
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: ISM
  original_reference_id: PMID:19715580
  review:
    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 being obsoleted.
    action: MODIFY
    reason: >-
      GO:0051082 is being obsoleted. The ISM inference is sound (alpha-crystallin domain
      consistently associated with chaperone/holdase function), but the term needs replacement
      with holdase NTR when available. Retain GO:0051082 as interim.
    proposed_replacement_terms:
      - id: GO:0051082
        label: unfolded protein binding (retain until holdase NTR is created)
    supported_by:
      - reference_id: PMID:19715580
        supporting_text: >-
          This stable multimeric structure formed by sHSPs has the function of molecular
          chaperone, which binds to the proteins and prevents them from thermal denaturation
- term:
    id: GO:0009408
    label: response to heat
  evidence_type: IDA
  original_reference_id: PMID:26705243
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:26705243
        supporting_text: >-
          The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly
          induced after a heat shock
- term:
    id: GO:0042026
    label: protein refolding
  evidence_type: IDA
  original_reference_id: PMID:26705243
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
      - reference_id: PMID:26705243
        supporting_text: >-
          overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased
          luciferase refolding
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: HDA
  original_reference_id: PMID:24292889
  review:
    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.
    action: ACCEPT
    reason: >-
      Consistent with all other localization data. Hsp26 is a cytoplasmic sHSP. The HDA
      evidence provides independent proteomics confirmation.
    supported_by:
      - reference_id: PMID:24292889
        supporting_text: >-
          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.
- term:
    id: GO:0017022
    label: myosin binding
  evidence_type: IPI
  original_reference_id: PMID:18045836
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
    supported_by:
      - reference_id: PMID:18045836
        supporting_text: >-
          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
- term:
    id: GO:0009631
    label: cold acclimation
  evidence_type: IEP
  original_reference_id: PMID:16313561
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
    supported_by:
      - reference_id: PMID:16313561
        supporting_text: >-
          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
- term:
    id: GO:0008340
    label: determination of adult lifespan
  evidence_type: IMP
  original_reference_id: PMID:15308776
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
    supported_by:
      - reference_id: PMID:15308776
        supporting_text: >-
          Overexpression of either hsp26 or hsp27 extended the mean lifespan by 30%, and the
          flies also displayed increased stress resistance
      - reference_id: PMID:26705243
        supporting_text: >-
          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
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings:
    - statement: IBA annotations for cytoplasm, nucleus, response to heat, protein refolding, and
        unfolded protein binding are phylogenetically well-supported across sHSP family members.
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings:
    - statement: IEA annotations for cytoplasm, response to heat, protein refolding, and unfolded
        protein binding are consistent with the IBA and experimental annotations.
- id: PMID:15308776
  title: Multiple-stress analysis for isolation of Drosophila longevity genes.
  findings:
    - statement: Overexpression of Hsp26 extends mean lifespan by 30% and increases stress resistance.
      supporting_text: Overexpression of either hsp26 or hsp27 extended the mean lifespan by 30%,
        and the flies also displayed increased stress resistance
    - statement: Hsp26 identified as a longevity gene through multiple-stress screening.
      supporting_text: This screen indeed identified 13 genes, some already known to be involved
        in longevity, plus candidate genes. Two of these, hsp26 and hsp27, were chosen to test
        for their effects on lifespan
- id: PMID:16313561
  title: Cold hardening and transcriptional change in Drosophila melanogaster.
  findings:
    - statement: Hsp26 transcript abundance increases during cold hardening treatment (0 degrees C
        for 2 h).
      supporting_text: 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
    - statement: Evidence is expression-based only (microarray and qPCR).
      supporting_text: Microarray analysis to examine the changes in transcript abundance
        associated with cold hardening treatment...Quantitative RT-PCR was used to independently
        determine transcript abundance
- id: PMID:16572729
  title: Differences in the chaperone-like activities of the four main small heat
    shock proteins of Drosophila melanogaster.
  findings:
    - statement: Hsp26 prevents heat-induced aggregation of citrate synthase but requires 5-fold
        molar excess compared to Hsp22 and Hsp27 at 1:1 ratio.
      supporting_text: A 5 M excess of Hsp23 and Hsp26 was required to obtain the same efficiency
        with either citrate synthase or luciferase as substrate
    - statement: In in vitro refolding with reticulocyte lysate, 30% luciferase recovery with Hsp26
        (vs 50% for Hsp22, 40% for Hsp27).
      supporting_text: 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.
    - statement: Sedimentation analysis shows Hsp26 binds denatured luciferase at 42 degrees C.
      supporting_text: 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
    - statement: Hsp26 is a holdase chaperone that maintains substrates in a refoldable state.
      supporting_text: 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
- id: PMID:18045836
  title: Sisyphus, the Drosophila myosin XV homolog, traffics within filopodia transporting
    key sensory and adhesion cargos.
  findings:
    - statement: Hsp26 identified as a putative cargo of Sisyphus myosin XV.
      supporting_text: 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
- id: PMID:19715580
  title: The small heat shock protein (sHSP) genes in the silkworm, Bombyx mori, and
    comparative analysis with other insect sHSP genes.
  findings:
    - statement: Comparative genomic analysis of sHSP genes across five insect species.
      supporting_text: we performed comparative analyses with the sHSPs from other four insects
        whose complete genome sequences are available
    - statement: Confirms conserved alpha-crystallin domain and chaperone function across insect sHSPs.
      supporting_text: sHSPs primarily have chaperone activity and reflect the response machine of
        organisms to some extreme stresses existing in environment
    - statement: Drosophila has 11 sHSP genes, most located on chromosome 3L.
      supporting_text: D. melanogaster has 11 sHSP genes...chromosome 3 of D. melanogaster (8
        sHSP genes)
- id: PMID:24292889
  title: Ube3a, the E3 ubiquitin ligase causing Angelman syndrome and linked to autism,
    regulates protein homeostasis through the proteasomal shuttle Rpn10.
  findings:
    - statement: Hsp26 detected in cytoplasmic fraction by high-throughput proteomics in Drosophila.
      supporting_text: "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."
  full_text_unavailable: true
- id: PMID:26705243
  title: Specific protein homeostatic functions of small heat-shock proteins increase
    lifespan.
  findings:
    - statement: Hsp26 is one of four classical Drosophila sHSPs highly induced after heat shock.
      supporting_text: The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all
        highly induced after a heat shock
    - statement: Overexpression of Hsp26 increases luciferase refolding in S2 cells.
      supporting_text: overexpression of the classical small HSPs (HSP23, HSP26, and HSP27)
        increased luciferase refolding
    - statement: sHSP-mediated refolding requires functional HSP70 machinery.
      supporting_text: our results strongly suggest that the refolding capacity of D. melanogaster
        HSP27 and CG14207 is partially dependent on an intact HSP70 machine
    - statement: Hsp26 has intermediate activity in both refolding and anti-aggregation compared to
        specialized members CG14207 (strong refolder) and HSP67BC (strong anti-aggregation).
      supporting_text: We identified CG14207 as a novel and potent small HSP member that
        exclusively assisted in HSP70-dependent refolding of stress-denatured proteins.
        Furthermore, we report that HSP67BC, which has no role in protein refolding, was the most
        effective small HSP preventing toxic protein aggregation in an HSP70-independent manner.
        ...[HSP26 showed] overexpression of the classical small HSPs (HSP23, HSP26, and HSP27)
        increased luciferase refolding
- id: PMID:32437379
  title: Small heat shock proteins determine synapse number and neuronal activity
    during development.
  findings:
    - statement: sHSP23 and sHSP26 co-localize in neurons and interact physically
        (co-immunoprecipitation).
      supporting_text: Both sHSPs immunoprecipitate together and the equilibrium between both
        chaperones is required for neuronal development and activity.
    - statement: Both sHSPs accumulate in synaptic buttons at the NMJ.
      supporting_text: The confocal images show an accumulation and colocalization of sHSP23 and
        sHSP26 throughout the NMJ but particularly intense in the synaptic buttons
    - statement: The equilibrium between Hsp23 and Hsp26 modulates synapse number and neuronal
        activity.
      supporting_text: we suggest that sHSP23 and sHSP26 together form a complex that promotes
        synapse formation in presynaptic neurons
    - statement: Hsp26-GFP-V5 fusion construct shows cytosolic localization in larval brain and NMJ.
      supporting_text: We use an Hsp26-GFP-V5 fusion construct. sHSP23 was visualized using an
        anti-Hsp23...and sHSP26 was visualized using anti-V5
    - statement: Novel gene Pinkman (pkm) regulates Hsp23/Hsp26 protein stability.
      supporting_text: Pkm regulates expression and protein stability and participates in the
        establishment of synapse number during development.
- id: PMID:38944040
  title: Next-generation Drosophila protein interactome map and its functional implications.
  findings:
    - statement: Large-scale Drosophila interactome study documenting Hsp26-Hsp23 physical
        interaction.
      supporting_text: "The network contains 32,668 interactions among 3,644 proteins, organized into 632 clusters representing putative functional modules."
core_functions:
  - molecular_function:
      id: GO:0051082
      label: unfolded protein binding
    description: >-
      Holdase chaperone activity: binds denaturing/unfolded proteins to prevent aggregation
      under stress conditions. Does not actively refold proteins but maintains them in a
      refoldable state for subsequent HSP70-dependent refolding. Requires higher
      stoichiometric excess than Hsp22 or Hsp27 for equivalent efficiency. GO:0051082 is
      used as interim until a holdase chaperone activity NTR is created. GO:0044183 (protein
      folding chaperone) is not appropriate because Hsp26 is not a foldase. GO:0140309
      (unfolded protein carrier activity) does not fit because Hsp26 acts in situ, not as
      an inter-compartment carrier.
    directly_involved_in:
      - id: GO:0006457
        label: protein folding
      - id: GO:0009408
        label: response to heat
    locations:
      - id: GO:0005829
        label: cytosol
    supported_by:
      - reference_id: PMID:16572729
        supporting_text: 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
      - reference_id: PMID:26705243
        supporting_text: The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all
          highly induced after a heat shock