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). Beyond acute
  heat shock, Hsp26 is developmentally and stress-independently regulated: it is highly
  expressed in early embryos (4-6 h after egg laying) and enriched in ovaries and testes,
  with germline expression in nurse cells/oocyte and in male primary spermatocytes,
  spermatogonia and spermatids (PMID:30400176). Ubiquitous RNAi knockdown of several
  Drosophila sHsps including Hsp26 causes lethality, indicating an essential developmental
  role for the sHsp family (PMID:30400176). Hsp26 is predominantly cytosolic with a
  minor nuclear-matrix pool, and shows distinctive basal regulation (it is an exception
  to Med15-dependent basal Hsp expression in ovaries; PMID:39353569).
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. Falcon deep research corroborates this: Hsp26
      is reported to be predominantly cytosolic/cytoplasmic with a granular cytosolic
      staining pattern distinct from Hsp23.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    supported_by:
    - reference_id: PMID:32437379
      supporting_text: >-
        sHSP23 and sHSP26 colocalize in CNS.
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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. 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.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    supported_by:
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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.
      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.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
    - 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. 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_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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]
      full_text_unavailable: true
- 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.
      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_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
    additional_reference_ids:
    - PMID:26705243
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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. Falcon deep research concurs that Hsp26 is
      predominantly cytosolic/cytoplasmic with a granular staining pattern distinct
      from Hsp23.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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. 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.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
- 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. 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.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        Hsp26 is also reported as **cold-inducible** in review-level summaries.
- 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. 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.
    additional_reference_ids:
    - file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    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
    - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
      supporting_text: |-
        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.
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."
- id: PMID:30400176
  title: Developmental Expression and Functions of the Small Heat Shock Proteins in
    Drosophila.
  findings:
  - statement: |-
      sHSPs prevent nonspecific aggregation of substrate proteins in an ATP-independent
      manner, confirming Hsp26 acts as an ATP-independent holdase rather than an
      ATP-dependent foldase.
    supporting_text: |-
      They play a crucial role in the maintenance of protein homeostasis, preventing nonspecific aggregation of the substrate protein in an ATP-independent manner
    reference_section_type: INTRODUCTION
  - statement: |-
      Hsp26 is highly expressed in early stage embryos (4-6 h after egg laying) and
      enriched in testis and ovaries, indicating developmentally regulated,
      stress-independent expression.
    supporting_text: |-
      Hsp26, Hsp67Ba, Hsp23, and Hsp27 are highly expressed in early stage embryos (4–6 h after egg laying, AEL)
    reference_section_type: RESULTS
  - statement: |-
      Hsp26 mRNA is present in nurse cells and later in the oocyte during oogenesis.
    supporting_text: |-
      Hsp26 mRNA is present in nurse cells and later in the oocyte
    reference_section_type: RESULTS
  - statement: |-
      Hsp26 is expressed in male germline cells, including primary spermatocytes and
      some spermatogonia and spermatids, via dedicated spermatocyte-specific regulatory
      elements.
    supporting_text: |-
      Hsp26 is expressed in germline cell types including primary spermatocytes, and in some spermatogonia and spermatids
    reference_section_type: RESULTS
  - statement: |-
      Ubiquitous RNAi knockdown of six Drosophila sHsps including Hsp26 caused
      lethality, indicating an essential developmental role for the sHsp family.
    supporting_text: |-
      ubiquitous RNAi knockdown of six Drosophila sHsps (Hsp23, Hsp26, Hsp27, CG4461, l(2)efl, and CG14207) resulted in lethality
    reference_section_type: INTRODUCTION
- id: PMID:39353569
  title: Moesin contributes to heat shock gene response through direct binding to
    the Med15 subunit of the Mediator complex in the nucleus.
  findings:
  - statement: |-
      In Med15-silenced ovaries, Hsp26 (together with Hsp70Ba) was an exception:
      unlike most surveyed Hsp genes, its basal expression was not significantly
      reduced, suggesting Hsp26 has a distinct/compensatory basal regulatory
      architecture.
    supporting_text: |-
      We then measured the activity of Hsp genes in Med15-silenced ovaries and found that, with the exception of the Hsp70Ba and Hsp26 genes, the expression of the examined Hsp genes was significantly reduced, however to varying degrees compared to that of the WT
    reference_section_type: RESULTS
- id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
  title: Falcon deep research report on Drosophila melanogaster Hsp26 (CG4183, P02517)
  findings:
  - statement: |
      Hsp26 is a small heat shock protein of the alpha-crystallin/HSP20 family that
      functions primarily as an ATP-independent holdase chaperone, binding
      misfolded/unfolding proteins to prevent aggregation and keeping them
      folding-competent for downstream ATP-dependent chaperone systems.
    supporting_text: |-
      They typically assemble as **dimers** (ACD-mediated) that serve as building blocks for **dynamic oligomers**, and 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.
    reference_section_type: OTHER
  - statement: |
      In comparative in vitro assays, Hsp26 inhibits heat-induced aggregation of
      citrate synthase and luciferase but is less efficient than Hsp22/Hsp27,
      requiring approximately 5-fold molar excess to match their suppression.
    supporting_text: |-
      Hsp26 required approximately a **5-fold molar excess** to reach suppression levels achieved by Hsp22/Hsp27 near 1:1 ratios in citrate synthase aggregation assays.
    reference_section_type: OTHER
  - statement: |
      In a luciferase refolding paradigm, recovery in the presence of Hsp26 was
      35.7% +/- 3.5%, versus 54.9% +/- 2.8% (Hsp22) and 42.8% +/- 3.3% (Hsp27),
      consistent with the holdase model feeding ATP-dependent refolding.
    supporting_text: |-
      luciferase activity recovery after denaturation in the presence of Hsp26 was **35.7% ± 3.5%**, compared with **54.9% ± 2.8%** (Hsp22) and **42.8% ± 3.3%** (Hsp27).
    reference_section_type: OTHER
  - statement: |
      Hsp26 is predominantly cytosolic/cytoplasmic with a granular staining pattern
      distinct from Hsp23, with a minor fraction also detected in the nuclear matrix
      of embryos and S2 cells.
    supporting_text: |-
      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.
    reference_section_type: OTHER
  - statement: |
      Hsp26 exists as multiple molecular forms (reported as five isoforms), with
      three phosphorylatable serines and observed ubiquitination in fly neurons.
    supporting_text: |-
      Hsp26 exists as multiple molecular forms: reports summarize the presence of **five isoforms**, and note that **three serines can be phosphorylated** and that **ubiquitination** of Hsp26 has been observed in fly neurons.
    reference_section_type: OTHER
  - statement: |
      Hsp26 is developmentally regulated, highly expressed in early embryos (4-6 h
      after egg laying) and enriched in ovaries and testes.
    supporting_text: |-
      It is described as highly expressed in **early embryos (4–6 h after egg laying)**, and enriched in **ovaries** and **testes**.
    reference_section_type: OTHER
  - statement: |
      Ubiquitous RNAi knockdown of multiple sHsps including Hsp26 caused lethality,
      supporting an indispensable contribution to proteostasis and tissue robustness
      during development.
    supporting_text: |-
      ubiquitous RNAi knockdown of multiple sHsps including Hsp26 caused **lethality**, supporting that sHsps (including Hsp26) contribute indispensably to proteostasis and tissue robustness during development.
    reference_section_type: OTHER
  - statement: |
      In Med15-silenced ovaries, Hsp26 (and Hsp70Ba) was an explicit exception that
      did not show the basal expression reduction seen for other Hsp genes,
      indicating distinct basal regulatory control.
    supporting_text: |-
      **Hsp26 (and Hsp70Ba)** were explicit **exceptions** and did not show the basal reduction observed for other Hsp genes under those conditions.
    reference_section_type: OTHER
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
  - reference_id: file:DROME/Hsp26/Hsp26-deep-research-falcon.md
    supporting_text: |-
      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.