Hsp23

id: P02516
gene_symbol: Hsp23
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:7227
  label: Drosophila melanogaster
description: Hsp23 is a small heat shock protein (sHSP) of Drosophila melanogaster
  that localizes to the cytoplasm/cytosol (PMID:32437379). It is one of four classical
  Drosophila sHSPs (Hsp22, Hsp23, Hsp26, Hsp27) encoded in a gene cluster at chromosomal
  locus 67B. All four sHSPs share a conserved alpha-crystallin domain and possess
  ATP-independent chaperone-like (holdase) activity, preventing heat-induced protein
  aggregation and maintaining substrates in a refoldable state (PMID:16572729). Hsp23
  requires a 5-fold molar excess over substrate (citrate synthase or luciferase) for
  equivalent anti-aggregation activity compared to Hsp22 and Hsp27 (PMID:16572729).
  Approximately 30% of luciferase activity is recovered in in vitro refolding assays
  with Hsp23 (PMID:16572729). Hsp23 interacts physically with Hsp26, and both proteins
  colocalize in CNS neurons, playing a role in synaptogenesis during development (PMID:32437379).
  Hsp23 also interacts with the SUMO-conjugating enzyme DmUbc9 (PMID:9514881). Hsp23
  is upregulated during cold hardening (PMID:16313561) and constant hypoxia, where
  it plays an important role in hypoxia tolerance (PMID:19401761). GO:0051082 (unfolded
  protein binding) is proposed for obsoletion; as a classic holdase, the closest replacement
  is GO:0140309 (unfolded protein carrier activity), though the carrier semantics
  (escorting between cellular components) do not perfectly describe in-situ holdase
  activity.
existing_annotations:
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: IBA annotation for cytoplasm localization. sHSP family members across
      species localize to the cytoplasm. Hsp23 has been experimentally shown to localize
      in the cytoplasm/cytosol of CNS cells in Drosophila (PMID:32437379). The IBA
      annotation is consistent with direct experimental evidence.
    action: ACCEPT
    reason: Cytoplasmic localization is well supported by IBA phylogenetic inference
      and confirmed experimentally by immunofluorescence in CNS cells (PMID:32437379).
    supported_by:
    - reference_id: PMID:32437379
      supporting_text: The data show that sHSP23 and sHSP26 localize in the cytoplasm
        of CNS cells, in particular in the optic lobes and the central nerve cord
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: IBA annotation for nuclear localization. Many mammalian sHSP orthologs
      (e.g. HSPB1/HSP27, alphaB-crystallin) localize to the nucleus. For Hsp23, direct
      experimental evidence shows cytoplasmic localization in CNS cells (PMID:32437379)
      but no specific nuclear localization has been reported.
    action: MARK_AS_OVER_ANNOTATED
    reason: While the IBA inference is phylogenetically reasonable for the broader
      sHSP family, the available direct evidence for Drosophila Hsp23 shows cytoplasmic
      localization (PMID:32437379). No direct experimental evidence supports nuclear
      localization of Hsp23 specifically.
- 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. All four classical Drosophila sHSPs
      are highly heat-inducible (PMID:26705243, PMID:16572729). This is a core conserved
      function across the sHSP family.
    action: ACCEPT
    reason: Response to heat is a fundamental, conserved function of the sHSP family.
      Hsp23 is strongly heat-inducible as confirmed by qPCR (PMID:26705243).
    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: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: IBA annotation for protein refolding. sHSPs do not themselves refold
      proteins; they are holdases that maintain substrates in a refoldable state for
      subsequent HSP70-dependent refolding (PMID:16572729, PMID:26705243). The refolding
      capacity of sHSPs is partially dependent on an intact HSP70 machine (PMID:26705243).
      The IBA annotation captures the involvement of sHSPs in the refolding pathway.
    action: ACCEPT
    reason: Although Hsp23 is primarily a holdase, it participates in the protein
      refolding pathway by maintaining substrates in a refoldable state. In the in
      vitro refolding assay, 30% of luciferase activity was recovered in the presence
      of Hsp23 (PMID:16572729).
    supported_by:
    - reference_id: PMID:16572729
      supporting_text: 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
- 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. sHSPs are classic holdases that bind unfolded/denatured proteins
      and prevent their aggregation in an ATP-independent manner (PMID:16572729).
      As holdases, the closest replacement term is GO:0140309 (unfolded protein carrier
      activity).
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. As a holdase, Hsp23 prevents aggregation
      of unfolded proteins (PMID:16572729). GO:0140309 (unfolded protein carrier activity)
      is not appropriate because it is carrier-specific (per go-ontology#30552). Retain
      until a holdase chaperone activity NTR is created.
    proposed_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: IEA annotation from ARBA for cytoplasm localization. Consistent with
      IBA and IDA annotations for cytoplasm/cytosol. Redundant with better-evidenced
      annotations.
    action: ACCEPT
    reason: This IEA annotation is consistent with the IBA and experimental (IDA cytosol)
      annotations. Acceptable as redundant support.
- term:
    id: GO:0009408
    label: response to heat
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: IEA annotation from ARBA for response to heat. Consistent with IBA and
      IDA annotations for the same term.
    action: ACCEPT
    reason: This IEA annotation is consistent with the IDA and IBA annotations for
      the same term. Acceptable as redundant support.
- term:
    id: GO:0042026
    label: protein refolding
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: IEA annotation from ARBA for protein refolding. Consistent with IBA and
      IDA annotations.
    action: ACCEPT
    reason: This IEA annotation is consistent with the IDA and IBA annotations for
      the same term. Acceptable as redundant support.
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: IEA annotation from ARBA for unfolded protein binding. GO:0051082 is
      proposed for obsoletion. Same recommendation as for the IBA annotation.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. Replace with GO:0140309 (unfolded
      protein carrier activity) as for the IBA annotation.
    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 from a large-scale Drosophila interactome
      study. The with/from column indicates interaction with P02517 (Hsp26). This
      interaction is independently confirmed by co-immunoprecipitation (PMID:32437379).
      However, protein binding is uninformative; the interaction with Hsp26 should
      be captured more specifically.
    action: ACCEPT
    reason: The interaction between Hsp23 and Hsp26 is well-documented by co-immunoprecipitation
      (PMID:32437379) and large-scale interactomics (PMID:38944040). While protein
      binding is uninformative as a GO term, the IPI evidence with a specific interactor
      is acceptable and the interaction is biologically meaningful for sHSP oligomerization.
    supported_by:
    - reference_id: PMID:32437379
      supporting_text: These results confirm the physical interaction between sHSP23
        and sHSP26
- 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 demonstrating
      chaperone-like activity. All four Drosophila sHSPs prevent heat-induced protein
      aggregation and maintain proteins in a refoldable state (PMID:16572729).
    action: ACCEPT
    reason: Hsp23 has demonstrated chaperone-like activity in preventing heat-induced
      protein aggregation and maintaining substrates in a refoldable state (PMID:16572729).
      Protein folding is an appropriate broad process annotation for a chaperone.
    supported_by:
    - reference_id: PMID:16572729
      supporting_text: Therefore, 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. Morrow et al. (2006) demonstrated
      that Hsp23 has chaperone-like activity, though less efficient than Hsp22 or
      Hsp27, requiring a 5-fold molar excess for equivalent anti-aggregation. However,
      GO:0044183 is defined as an ATP-dependent protein folding chaperone (foldase),
      which does not accurately describe sHSPs that function as ATP-independent holdases.
    action: MODIFY
    reason: GO:0044183 (protein folding chaperone) is for foldases (ATP-dependent).
      Hsp23 is an ATP-independent holdase requiring HSP70 for actual refolding (PMID:26705243).
      The correct replacement is GO:0140309 (unfolded protein carrier activity).
    proposed_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
    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: 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).
      Immunofluorescence in third instar larval brain shows that sHSP23 and sHSP26
      localize in the cytoplasm of CNS cells (PMID:32437379). The more specific cytosol
      annotation is appropriate.
    action: ACCEPT
    reason: Direct experimental evidence from immunofluorescence shows cytoplasmic
      localization of Hsp23 in CNS cells (PMID:32437379). Cytosol is an appropriate
      specific term.
    supported_by:
    - reference_id: PMID:32437379
      supporting_text: The data show that sHSP23 and sHSP26 localize in the cytoplasm
        of CNS cells, in particular in the optic lobes and the central nerve cord
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: ISM
  original_reference_id: PMID:19715580
  review:
    summary: ISM annotation for protein folding based on sequence model analysis.
      Li et al. (2009) performed comparative analysis of sHSP genes across insects,
      identifying conserved alpha-crystallin domains characteristic of chaperone function.
    action: ACCEPT
    reason: The ISM evidence from comparative genomic analysis is consistent with
      experimental evidence (PMID:16572729) demonstrating chaperone activity.
    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. GO:0051082 is proposed for
      obsoletion. Morrow et al. (2006) demonstrated that all four sHSPs bind denatured
      substrates, with Hsp23 requiring a 5-fold molar excess for efficient binding.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. The experimental evidence clearly
      demonstrates holdase activity. Replace with GO:0140309 (unfolded protein carrier
      activity).
    proposed_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
    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
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: ISM
  original_reference_id: PMID:19715580
  review:
    summary: ISM annotation for unfolded protein binding from comparative sHSP genomics.
      GO:0051082 is proposed for obsoletion.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. Replace with GO:0140309 (unfolded
      protein carrier activity).
    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 from Vos et al. (2016). This study
      confirmed that the four classical sHSPs are all highly heat-inducible by qPCR
      in S2 cells.
    action: ACCEPT
    reason: Strongly supported by experimental evidence. Hsp23 is one of the most
      abundantly constitutively expressed sHSPs and is highly heat-inducible (PMID:26705243).
    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 from Vos et al. (2016). In cell-based
      luciferase refolding assays, overexpression of the classical sHSPs (HSP23, HSP26,
      HSP27) increased luciferase refolding in S2 cells (PMID:26705243).
    action: ACCEPT
    reason: Cellular refolding assay confirms that Hsp23 overexpression enhances luciferase
      refolding. The refolding capacity requires HSP70 (PMID:26705243).
    supported_by:
    - reference_id: PMID:26705243
      supporting_text: overexpression of the classical small HSPs (HSP23, HSP26, and
        HSP27) increased luciferase refolding
- term:
    id: GO:0009631
    label: cold acclimation
  evidence_type: IEP
  original_reference_id: PMID:16313561
  review:
    summary: IEP annotation for cold acclimation. Qin et al. (2005) used microarray
      analysis to examine transcript changes during cold hardening in Drosophila.
      Hsp23 was identified among the stress proteins differentially expressed during
      cold hardening treatment. IEP (inferred from expression pattern) is appropriate
      for transcript upregulation data.
    action: KEEP_AS_NON_CORE
    reason: Cold acclimation is a stress response that may involve sHSP chaperone
      activity but represents a pleiotropic environmental response rather than a core
      molecular function. The IEP evidence (transcript upregulation) is relatively
      weak.
    supported_by:
    - reference_id: PMID:16313561
      supporting_text: stress proteins, including Hsp23, Hsp26, Hsp83 and Frost as
        well as membrane-associated proteins may contribute to the cold hardening
        response
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:9514881
  review:
    summary: IPI annotation for protein binding based on Joanisse et al. (1998). The
      with/from column indicates interaction with FB:FBgn0010602 (DmUbc9, the SUMO-conjugating
      enzyme). This was demonstrated by yeast two-hybrid and confirmed by co-immunoprecipitation.
    action: ACCEPT
    reason: The interaction between Hsp23 and DmUbc9 is experimentally validated by
      both yeast two-hybrid and co-immunoprecipitation (PMID:9514881). While protein
      binding is uninformative, the IPI evidence documents a specific interactor.
    supported_by:
    - reference_id: PMID:9514881
      supporting_text: a Drosophila melanogaster homologue of yeast and human ubc9
        (Dmubc9) was found to interact with Drosophila Hsp23
- term:
    id: GO:0001666
    label: response to hypoxia
  evidence_type: IEP
  original_reference_id: PMID:19401761
  review:
    summary: IEP annotation for response to hypoxia. Azad et al. (2009) showed by
      microarray and qPCR that Hsp23 is significantly upregulated during constant
      hypoxia in Drosophila.
    action: KEEP_AS_NON_CORE
    reason: Hypoxia response is a stress response that involves sHSP chaperone activity
      but is a pleiotropic response rather than a core molecular function. The IEP
      evidence (transcript upregulation) supports involvement but not direct participation.
    supported_by:
    - reference_id: PMID:19401761
      supporting_text: Heat shock proteins up-regulation (specifically Hsp23 and Hsp70)
        led to a significant increase in adult survival (as compared to controls)
        of P-element lines during CH
- term:
    id: GO:0001666
    label: response to hypoxia
  evidence_type: IMP
  original_reference_id: PMID:19401761
  review:
    summary: IMP annotation for response to hypoxia. Azad et al. (2009) showed that
      Hsp23 P-element lines with increased expression had significantly higher survival
      under constant hypoxia (55% survival vs 31% for controls). This functional evidence
      goes beyond expression data.
    action: KEEP_AS_NON_CORE
    reason: The IMP evidence demonstrates a functional role for Hsp23 in hypoxia tolerance
      via increased survival of P-element lines. However, this represents a pleiotropic
      stress response phenotype rather than a core evolved function of Hsp23.
    supported_by:
    - reference_id: PMID:19401761
      supporting_text: Heat shock proteins up-regulation (specifically Hsp23 and Hsp70)
        led to a significant increase in adult survival (as compared to controls)
        of P-element lines during CH
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: PMID:9514881
  title: Cloning and developmental expression of a nuclear ubiquitin-conjugating enzyme
    (DmUbc9) that interacts with small heat shock proteins in Drosophila melanogaster.
  findings:
    - statement: DmUbc9 interacts with Hsp23 in yeast two-hybrid and co-immunoprecipitation assays.
      supporting_text: "In a two hybrid screen designed to identify proteins that interact with small heat shock proteins (sHsps), a Drosophila melanogaster homologue of yeast and human ubc9 (Dmubc9) was found to interact with Drosophila Hsp23"
- id: PMID:16313561
  title: Cold hardening and transcriptional change in Drosophila melanogaster.
  findings:
    - statement: Hsp23 is among stress proteins differentially expressed during cold hardening treatment.
      supporting_text: "Taken together, 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."
- id: PMID:16572729
  title: Differences in the chaperone-like activities of the four main small heat
    shock proteins of Drosophila melanogaster.
  findings:
    - statement: All four sHSPs have chaperone-like activity; Hsp23 requires 5-fold molar excess for equivalent efficiency.
      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: Approximately 30% of luciferase activity is recovered with Hsp23 in refolding assays.
      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."
- id: PMID:19401761
  title: Distinct mechanisms underlying tolerance to intermittent and constant hypoxia
    in Drosophila melanogaster.
  findings:
    - statement: Hsp23 is upregulated during constant hypoxia; P-element lines show increased survival.
      supporting_text: "Heat shock proteins up-regulation (specifically Hsp23 and Hsp70) led to a significant increase in adult survival (as compared to controls) of P-element lines during CH."
- 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 analysis identifies conserved alpha-crystallin domains across insect sHSPs.
      supporting_text: "The sHSPs have an α-crystalling domain comprising about 100 amino acid residues, which is the conserved structure of all sHSP sequences [6-8]"
- id: PMID:26705243
  title: Specific protein homeostatic functions of small heat-shock proteins increase
    lifespan.
  findings:
    - statement: Hsp23 assists in HSP70-dependent refolding of luciferase in S2 cell-based assays.
      supporting_text: "Consistent with in vitro data (Morrow et al., 2006), overexpression of the classical small HSPs (HSP23, HSP26, and HSP27) increased luciferase refolding (Fig"
    - statement: All four classical sHSPs are highly heat-inducible.
      supporting_text: "The four classical small HSPs (HSP22, HSP23, HSP26, and HSP27) were all highly induced after a heat shock (Fig"
- id: PMID:32437379
  title: Small heat shock proteins determine synapse number and neuronal activity
    during development.
  findings:
    - statement: Hsp23 and Hsp26 colocalize in the cytoplasm of CNS cells and physically interact.
      supporting_text: "The data show that sHSP23 and sHSP26 localize in the cytoplasm of CNS cells, in particular in the optic lobes and the central nerve cord (Fig 2A and 2B`)"
    - statement: Hsp23 and Hsp26 modulate synapse number during development.
      supporting_text: "we suggest that sHSP23 and sHSP26 together form a complex that promotes synapse formation in presynaptic neurons, Pkm is an anti-synaptogenic element in neurons through, but not restricted to, the modulation of sHsp23 and sHsp26 (Fig 5C)."
- id: PMID:38944040
  title: Next-generation Drosophila protein interactome map and its functional implications.
  findings:
    - statement: Large-scale interactome confirms Hsp23-Hsp26 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: Hsp23 functions as an ATP-independent holdase chaperone that prevents
    heat-induced protein aggregation and maintains substrates in a refoldable state.
    Less efficient than Hsp22 or Hsp27, requiring a 5-fold molar excess for equivalent
    anti-aggregation activity (PMID:16572729). Refolding of substrates held by Hsp23
    requires the HSP70 machine (PMID:26705243). Hsp23 physically interacts with Hsp26,
    and both proteins colocalize in the cytoplasm of CNS cells (PMID:32437379). Note
    - GO:0140309 is the closest available term for holdases, though the carrier semantics
    do not perfectly describe in-situ holdase activity.
  locations:
  - id: GO:0005829
    label: cytosol
  directly_involved_in:
  - id: GO:0006457
    label: protein folding