Hsp22

id: P02515
gene_symbol: Hsp22
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:7227
  label: Drosophila melanogaster
description: Hsp22 is a small heat shock protein (sHSP) of Drosophila melanogaster
  that localizes to the mitochondrial matrix (PMID:10896659). 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). Hsp22
  is unique among the four in its mitochondrial localization and forms oligomeric
  complexes (PMID:10896659). In vitro, Hsp22 is the most efficient of the four sHSPs
  at preventing citrate synthase aggregation at equimolar ratios and at maintaining
  luciferase in a refoldable state (PMID:16572729). Overexpression of Hsp22 extends
  adult lifespan and confers resistance to oxidative stress and heat stress (PMID:14734639,
  PMID:15331597). Its expression increases dramatically during aging. GO:0051082 (unfolded
  protein binding) is proposed for obsoletion; as a classic holdase, the closest replacement
  is GO:0140309 (unfolded protein carrier activity), though a caveat exists regarding
  carrier semantics (the GO:0140309 definition requires escorting between cellular
  components, which does 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. Most sHSP family members across
      species localize to the cytoplasm. However, Hsp22 is specifically a mitochondrial
      matrix protein (PMID:10896659). The IBA propagation from cytoplasmic orthologs
      is phylogenetically reasonable at the family level but does not accurately reflect
      the specific localization of Hsp22. Since mitochondrial matrix is part of the
      cytoplasm in the broadest sense, this is not wrong but is misleadingly non-specific
      for this particular protein.
    action: ACCEPT
    reason: Although Hsp22 is specifically mitochondrial, cytoplasm is a broad parent
      term and the IBA annotation is not incorrect. The more specific mitochondrial
      matrix annotation (IDA) exists separately. Accepting this as consistent with
      IBA methodology.
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: IBA annotation for nuclear localization. Many sHSP orthologs including
      mammalian HSPB1/HSP27 localize to the nucleus. However, Hsp22 is specifically
      a mitochondrial matrix protein (PMID:10896659) and there is no direct evidence
      for nuclear localization.
    action: MARK_AS_OVER_ANNOTATED
    reason: Hsp22 has been experimentally shown to localize to the mitochondrial matrix
      (PMID:10896659). While many sHSP orthologs are nuclear, the IBA propagation
      does not account for the derived mitochondrial targeting of Hsp22. No direct
      experimental evidence supports nuclear localization.
- 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.
      Hsp22 is strongly heat-inducible and its overexpression confers thermotolerance.
    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 IBA
      is propagated from orthologs with similar activity. While sHSPs participate
      in the refolding pathway, the term protein refolding could be considered an
      over-annotation for a holdase that merely prevents aggregation rather than actively
      refolding.
    action: ACCEPT
    reason: Although Hsp22 is primarily a holdase rather than a foldase, it plays
      a documented role in the protein refolding pathway by maintaining substrates
      in a refoldable state. In the 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 (PMID:16572729). The IBA annotation captures
      the involvement of sHSPs in the refolding process, even though the mechanism
      is holdase-mediated.
    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
- 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), though the carrier semantics (escorting between cellular components)
      do not perfectly describe in-situ holdase function.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. As a holdase, Hsp22 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)
    supported_by:
    - 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
- 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 the IBA
      and IDA annotations for the same term. Redundant with better-evidenced annotations
      but not incorrect.
    action: ACCEPT
    reason: This IEA annotation is broader but consistent with the IDA and IBA annotations
      for the same term. Acceptable as redundant support.
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IDA
  original_reference_id: PMID:16572729
  review:
    summary: IDA annotation for protein folding based on the Morrow et al. 2006 study
      demonstrating chaperone-like activity. All four Drosophila sHSPs prevent heat-induced
      protein aggregation and maintain proteins in a refoldable state (PMID:16572729).
      This is the parent process term for the more specific protein refolding.
    action: ACCEPT
    reason: Hsp22 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 Hsp22 has chaperone-like activity, preventing heat-induced aggregation
      of citrate synthase and maintaining luciferase in a refoldable state. 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.
      sHSPs maintain substrates in a refoldable state but require the HSP70 machine
      for actual refolding.
    action: MODIFY
    reason: GO:0044183 (protein folding chaperone) is meant for foldases (ATP-dependent
      folding machines like HSP70, GroEL). Hsp22 is an ATP-independent holdase that
      prevents aggregation and maintains substrates for subsequent HSP70-dependent
      refolding (PMID:16572729, PMID:26705243). The correct replacement is GO:0140309
      (unfolded protein carrier activity), with the caveat about carrier semantics
      noted above.
    proposed_replacement_terms:
    - id: GO:0051082
      label: unfolded protein binding (retain until holdase NTR is created)
    supported_by:
    - 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
    - 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:0005759
    label: mitochondrial matrix
  evidence_type: IDA
  original_reference_id: PMID:10896659
  review:
    summary: IDA annotation for mitochondrial matrix localization. Morrow et al. (2000)
      showed that Hsp22 localizes to the mitochondrial matrix in both S2 cells and
      after heterologous expression in mammalian cells. The N-terminal WRMAEE motif
      (positions 8-13) is necessary for mitochondrial import (PMID:10896659).
    action: ACCEPT
    reason: Strong experimental evidence from subcellular fractionation, immunofluorescence,
      and deletion/mutagenesis analysis demonstrates mitochondrial matrix localization
      (PMID:10896659). This is a distinguishing feature of Hsp22 among the four Drosophila
      sHSPs.
    supported_by:
    - reference_id: PMID:10896659
      supporting_text: DmHsp22 is shown to localize in mitochondria both in D. melanogaster
        S2 cells and after heterologous expression in mammalian 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 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. The
      conserved alpha-crystallin domain is the hallmark of sHSP chaperone function.
    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:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:10896659
  review:
    summary: IPI annotation for identical protein binding. Morrow et al. (2000) showed
      through sedimentation, gel filtration, and cross-linking experiments that Hsp22
      forms oligomeric complexes in the mitochondrial matrix. Immunoprecipitation
      confirmed self-interaction (PMID:10896659). Oligomerization is a fundamental
      property of sHSPs required for chaperone function.
    action: ACCEPT
    reason: Oligomerization is experimentally demonstrated and is a core structural
      feature of sHSPs required for their holdase activity (PMID:10896659). The with/from
      column confirms interaction with itself (FB:FBgn0001223).
    supported_by:
    - reference_id: PMID:10896659
      supporting_text: DmHsp22 resides in the...mitochondrial matrix, where it is
        found in oligomeric complexes, as shown by...sedimentation and gel filtration
        analysis and by cross-linking experiments
- 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 Hsp22 binds denatured luciferase
      at 42C, as revealed by sedimentation analysis on sucrose gradients. This is
      a direct experimental demonstration of holdase activity.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. The experimental evidence (PMID:16572729)
      clearly demonstrates holdase activity. Replace with GO:0140309 (unfolded protein
      carrier activity) with the caveat about carrier semantics.
    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 analysis
      from comparative sHSP genomics. GO:0051082 is proposed for obsoletion.
    action: MODIFY
    reason: GO:0051082 is proposed for obsoletion. The ISM evidence supports chaperone/holdase
      function. 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 and compared
      the entire Drosophila sHSP family for chaperone activities.
    action: ACCEPT
    reason: Strongly supported by experimental evidence. Hsp22 is one of the most
      heat-inducible genes in Drosophila (PMID:26705243, PMID:16572729).
    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:0006979
    label: response to oxidative stress
  evidence_type: IMP
  original_reference_id: PMID:14734639
  review:
    summary: IMP annotation for response to oxidative stress. The mitochondrial localization
      of Hsp22 suggests a role in protection against oxidative damage (PMID:10896659).
      Overexpression studies demonstrated increased resistance to oxidative stress.
    action: ACCEPT
    reason: Hsp22 overexpression confers resistance to oxidative stress, consistent
      with its mitochondrial localization where it likely protects mitochondrial proteins
      from oxidative damage.
    additional_reference_ids:
    - PMID:10896659
    supported_by:
    - reference_id: PMID:10896659
      supporting_text: The mitochondrial localization of this small Hsp22 of Drosophila
        and its high level of expression in aging suggests a role for this small heat
        shock protein in protection against oxidative stress
- term:
    id: GO:0008340
    label: determination of adult lifespan
  evidence_type: IMP
  original_reference_id: PMID:14734639
  review:
    summary: IMP annotation for determination of adult lifespan. Overexpression of
      Hsp22 extends lifespan in Drosophila, as demonstrated by multiple studies (PMID:14734639,
      PMID:15331597).
    action: KEEP_AS_NON_CORE
    reason: Lifespan extension upon overexpression is well-documented but represents
      a pleiotropic phenotypic outcome of enhanced proteostasis rather than a core
      molecular function. This is a non-core biological process annotation.
    supported_by:
    - reference_id: PMID:14734639
      supporting_text: a ubiquitous or a targeted expression of Hsp22 within motorneurons
        increases the mean life span by more than 30%
- term:
    id: GO:0009408
    label: response to heat
  evidence_type: IMP
  original_reference_id: PMID:14734639
  review:
    summary: IMP annotation for response to heat based on mutant phenotype evidence.
      Overexpression of Hsp22 confers thermotolerance.
    action: ACCEPT
    reason: Consistent with the core function of Hsp22 as a heat-inducible sHSP. Redundant
      with IDA and IBA annotations for the same term but represents independent evidence.
    supported_by:
    - reference_id: PMID:14734639
      supporting_text: The motorneurons-targeted expression of Hsp22 also significantly
        increases flies' resistance to oxidative injuries induced by paraquat (up to
        35%) and thermal stress (39% at 30 degrees C and 23% at 37 degrees C)
- term:
    id: GO:0008340
    label: determination of adult lifespan
  evidence_type: IMP
  original_reference_id: PMID:15331597
  review:
    summary: IMP annotation for determination of adult lifespan from a second study
      (PMID:15331597). Independent confirmation of lifespan effects strengthens the
      annotation.
    action: KEEP_AS_NON_CORE
    reason: Duplicate evidence for lifespan effects. Retaining as non-core since lifespan
      extension is a pleiotropic outcome of enhanced proteostasis rather than a core
      evolved function.
    supported_by:
    - reference_id: PMID:15331597
      supporting_text: flies that are not expressing this mitochondrial small Hsp22
        have a 40% decrease in lifespan
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:10896659
  title: The small heat shock protein Hsp22 of Drosophila melanogaster is a mitochondrial
    protein displaying oligomeric organization.
  findings: []
- id: PMID:14734639
  title: Overexpression of the small mitochondrial Hsp22 extends Drosophila life span
    and increases resistance to oxidative stress.
  findings: []
- id: PMID:15331597
  title: Decreased lifespan in the absence of expression of the mitochondrial small
    heat shock protein Hsp22 in Drosophila.
  findings: []
- id: PMID:16572729
  title: Differences in the chaperone-like activities of the four main small heat
    shock proteins of Drosophila melanogaster.
  findings: []
- 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: []
- id: PMID:26705243
  title: Specific protein homeostatic functions of small heat-shock proteins increase
    lifespan.
  findings: []
core_functions:
- molecular_function:
    id: GO:0051082
    label: unfolded protein binding
  description: Hsp22 functions as an ATP-independent holdase chaperone that prevents
    heat-induced protein aggregation and maintains substrates in a refoldable state.
    Demonstrated by in vitro aggregation prevention and luciferase refolding assays
    (PMID:16572729). Note - GO:0140309 is the closest available term for holdases,
    though the carrier semantics (escorting between compartments) do not perfectly
    describe in-situ holdase activity. Hsp22 is uniquely localized to the mitochondrial
    matrix among the four classical Drosophila sHSPs (PMID:10896659).
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  directly_involved_in:
  - id: GO:0006457
    label: protein folding