id: Q9UJY1
gene_symbol: HSPB8
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: HSPB8 (heat shock protein beta-8, also called HSP22, H11 kinase, E2IG1 or alpha-crystallin C chain) is a member of the small heat shock protein (sHSP / HSP20, alpha-crystallin domain) family, predominantly expressed in skeletal muscle and heart. It is an ATP-independent molecular chaperone (holdase) that binds aggregation-prone and stress-destabilized client proteins. Its central role is in chaperone-assisted selective autophagy (CASA), where together with the co-chaperone BAG3, the Hsp70 chaperones HSPA8/HSC70 and HSPA1A, and the ubiquitin ligase STUB1/CHIP, HSPB8 routes damaged clients (e.g. filamin and polyglutamine-expanded proteins) for autophagic degradation via p62/SQSTM1, a process essential for maintenance of the muscle Z-disk under mechanical stress. HSPB8 forms homodimers and hetero-oligomers with other small HSPs (HSPB1, HSPB2, HSPB7) and DNAJB6. Despite an early report of protein kinase activity, recombinant HSPB8 lacks detectable kinase activity. It localizes to the cytoplasm and nucleus and translocates to nuclear foci during heat shock. Dominant variants (notably at the K141 hot spot) cause distal hereditary motor neuropathy, Charcot-Marie-Tooth disease type 2L, and myofibrillar/rimmed-vacuole myopathy.
existing_annotations:
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Nuclear localization inferred phylogenetically; HSPB8 is directly documented in the nucleus and translocates to nuclear foci during heat shock.
    action: ACCEPT
    reason: Corroborated by direct experimental (IDA) evidence (PMID:19464326) and the UniProt subcellular-location record.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Nucleus {ECO:0000269|PubMed:19464326}
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Cytoplasmic localization inferred phylogenetically; the cytoplasm is the principal site where HSPB8 acts in CASA and as a holdase.
    action: ACCEPT
    reason: Corroborated by direct experimental (IDA) evidence (PMID:19464326, PMID:28144995) and the UniProt subcellular-location record.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm'
- term:
    id: GO:0034620
    label: cellular response to unfolded protein
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: As a stress-inducible small HSP holdase, HSPB8 participates in the cellular response to unfolded protein. Supported by family inference and by direct functional data.
    action: ACCEPT
    reason: Phylogenetic inference is consistent with experimental evidence that HSPB8 prevents aggregation of unfolded/aggregation-prone clients and is induced by stress.
    supported_by:
    - reference_id: PMID:14985082
      supporting_text: Hsp22 effectively prevents heat-induced aggregation of yeast alcohol dehydrogenase and bovine liver rhodanese
- term:
    id: GO:0101031
    label: protein folding chaperone complex
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: part_of
  review:
    summary: HSPB8 is a component of multi-chaperone complexes (the CASA complex with BAG3/HSPA8/STUB1, and sHSP hetero-oligomers). Membership in a protein-folding chaperone complex is well supported.
    action: ACCEPT
    reason: Direct experimental evidence places HSPB8 in the CASA complex and in ternary complexes with BAG3 and Hsp70, consistent with this phylogenetic annotation.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Component of the chaperone-assisted selective autophagy (CASA) complex consisting of BAG3, HSPA8/HSC70, HSPB8 and STUB1/CHIP
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Electronic annotation of nuclear localization, consistent with the IBA and IDA nucleus annotations.
    action: ACCEPT
    reason: Agrees with stronger experimental evidence (PMID:19464326) placing HSPB8 in the nucleus.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Nucleus {ECO:0000269|PubMed:19464326}
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Electronic annotation of cytoplasmic localization, redundant with the IBA and IDA cytoplasm annotations.
    action: ACCEPT
    reason: Correct compartment, agreeing with stronger experimental evidence.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm'
- term:
    id: GO:0034620
    label: cellular response to unfolded protein
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: InterPro-based electronic annotation to the cellular response to unfolded protein, consistent with HSPB8's sHSP holdase function.
    action: ACCEPT
    reason: Redundant with the IBA and IMP annotations of the same process, all supported by HSPB8's documented anti-aggregation chaperone activity.
    supported_by:
    - reference_id: PMID:14985082
      supporting_text: Hsp22 effectively prevents heat-induced aggregation of yeast alcohol dehydrogenase and bovine liver rhodanese
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: HSPB8 forms stable homodimers, the basic oligomeric unit of this small HSP. Supported by direct biochemical evidence.
    action: ACCEPT
    reason: Recombinant HSPB8 forms stable dimers by size-exclusion and crosslinking, corroborating this electronic annotation.
    supported_by:
    - reference_id: PMID:14985082
      supporting_text: Hsp22 forms stable dimers
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:14594798
  qualifier: enables
  review:
    summary: HSPB8 (HSP22) interacts with the small heat shock proteins HSPB7 (cvHSP), HSPB2 (MKBP) and HSPB1 (HSP27). The bare protein binding term is uninformative; the partners are heat shock proteins.
    action: MODIFY
    reason: Per curation guidelines, bare protein binding (GO:0005515) is uninformative. The documented partners are small HSPs, so heat shock protein binding (GO:0031072) is the appropriate specific molecular function.
    proposed_replacement_terms:
    - id: GO:0031072
      label: heat shock protein binding
    supported_by:
    - reference_id: PMID:14594798
      supporting_text: HSP22 interacts with itself, cvHSP (HSPB7), MKBP (HSPB2) and HSP27
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16189514
  qualifier: enables
  review:
    summary: Proteome-scale interactome screen capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data records genuine interactions but the bare term is uninformative; HSPB8's biologically meaningful chaperone interactions are captured by more specific terms.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:16189514
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:21516116
  qualifier: enables
  review:
    summary: Next-generation-sequencing interactome dataset capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; records genuine interactions but uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:21516116
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:23414517
  qualifier: enables
  review:
    summary: Human skeletal-muscle interactome study capturing HSPB8 interactions in a muscle context. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Muscle-focused interactome data is contextually relevant but the bare term is uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:23414517
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25036637
  qualifier: enables
  review:
    summary: Quantitative chaperone interaction network capturing HSPB8 within chaperone modules. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Places HSPB8 in the chaperone interactome; genuine but uninformative as a bare molecular-function term.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:25036637
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  qualifier: enables
  review:
    summary: Proteome-scale yeast two-hybrid interactome map capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:25416956
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:26496610
  qualifier: enables
  review:
    summary: Quantitative stoichiometry-resolved interactome capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:26496610
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:28514442
  qualifier: enables
  review:
    summary: Human interactome (protein communities) study capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:28514442
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:31273097
  qualifier: enables
  review:
    summary: Study of the heme-regulated inhibitor (HRI) misfolding sensor capturing an HSPB8 interaction. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records a genuine interaction relevant to a misfolding-stress pathway but the bare term is uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:31273097
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: Reference binary interactome map capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:32296183
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32707033
  qualifier: enables
  review:
    summary: Kinase interaction network study capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:32707033
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: BioPlex affinity-purification interactome capturing HSPB8 interactions. The bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome data; uninformative as a molecular function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:33961781
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:14594798
  qualifier: enables
  review:
    summary: HSPB8 self-associates (HSP22 interacts with itself), the basis for its homodimer/homo-oligomer. Identical protein binding is supported.
    action: ACCEPT
    reason: Direct evidence that HSP22 interacts with itself supports the self-association molecular function.
    supported_by:
    - reference_id: PMID:14594798
      supporting_text: HSP22 interacts with itself, cvHSP (HSPB7), MKBP (HSPB2) and HSP27
- term:
    id: GO:0005654
    label: nucleoplasm
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) localization to the nucleoplasm, consistent with HSPB8's documented nuclear pool.
    action: KEEP_AS_NON_CORE
    reason: Supported by HPA IDA evidence; consistent with nuclear localization but peripheral to the core cytoplasmic CASA/holdase function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005654 nucleoplasm cellular_component ECO:0000314 IDA
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) localization to the cytosol, the principal compartment for HSPB8's chaperone activity.
    action: ACCEPT
    reason: IDA-supported cytosolic localization, consistent with the cytoplasmic site of CASA and holdase activity.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005829 cytosol cellular_component ECO:0000314 IDA
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IPI
  original_reference_id: PMID:18006506
  qualifier: enables
  review:
    summary: HSPB8 homodimerization activity, the basic oligomeric unit of this small HSP, supported by direct evidence.
    action: ACCEPT
    reason: HSPB8 forms stable dimers/self-associates; this IPI-supported homodimerization is a genuine molecular function.
    supported_by:
    - reference_id: PMID:14985082
      supporting_text: Hsp22 forms stable dimers
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:18006506
  qualifier: enables
  review:
    summary: Demonstrates that HSPB8 forms a stable, functionally essential complex with the co-chaperone BAG3. The bare protein binding term is uninformative; BAG3 is a chaperone/co-chaperone.
    action: MODIFY
    reason: Bare protein binding is uninformative. The documented partner BAG3 is a Hsp70 co-chaperone/scaffold, so protein-folding chaperone binding (GO:0051087) is the appropriate specific molecular function.
    proposed_replacement_terms:
    - id: GO:0051087
      label: protein-folding chaperone binding
    supported_by:
    - reference_id: PMID:18006506
      supporting_text: HspB8 forms a stable complex with Bag3 in cells and that the formation of this complex is essential for the activity of HspB8
- term:
    id: GO:0034620
    label: cellular response to unfolded protein
  evidence_type: IMP
  original_reference_id: PMID:18006506
  qualifier: involved_in
  review:
    summary: HSPB8 overexpression prevents accumulation of aggregation-prone clients (poly-Q Htt43Q); this anti-aggregation activity, dependent on BAG3-stimulated autophagy, is part of the cellular response to unfolded/misfolded protein.
    action: ACCEPT
    reason: Direct experimental (IMP) evidence that HSPB8 handles aggregation-prone misfolded clients supports this process annotation as a core function.
    supported_by:
    - reference_id: PMID:18006506
      supporting_text: overexpression of HspB8 in cultured cells prevents the accumulation of aggregation-prone proteins such as the polyglutamine protein Htt43Q
- term:
    id: GO:0101031
    label: protein folding chaperone complex
  evidence_type: IDA
  original_reference_id: PMID:18006506
  qualifier: part_of
  review:
    summary: HSPB8 forms a stable complex with BAG3 (and within the larger CASA/Hsp70 machinery); direct evidence places it in a chaperone complex.
    action: ACCEPT
    reason: Direct (IDA) evidence for a stable HSPB8-BAG3 complex supports membership in a protein-folding chaperone complex.
    supported_by:
    - reference_id: PMID:18006506
      supporting_text: HspB8 forms a stable complex with Bag3 in cells
- term:
    id: GO:1905337
    label: positive regulation of aggrephagy
  evidence_type: IMP
  original_reference_id: PMID:18006506
  qualifier: involved_in
  review:
    summary: HSPB8, in complex with BAG3, promotes the macroautophagic degradation of aggregation-prone clients (Htt43Q). This is the core CASA/aggrephagy role of HSPB8.
    action: ACCEPT
    reason: Direct experimental evidence that the HSPB8-BAG3 complex stimulates macroautophagic degradation of poly-Q clients strongly supports positive regulation of aggrephagy as a core biological process.
    supported_by:
    - reference_id: PMID:18006506
      supporting_text: the HspB8-Bag3 complex might stimulate the degradation of Htt43Q by macroautophagy
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:28144995
  qualifier: enables
  review:
    summary: Disease-variant study documenting the HSPB8-BAG3 interaction (with several HMND2 mutations altering it). The bare protein binding term is uninformative; the partner BAG3 is a co-chaperone.
    action: MODIFY
    reason: Bare protein binding is uninformative. The documented partner is the co-chaperone BAG3, so protein-folding chaperone binding (GO:0051087) is the appropriate specific molecular function.
    proposed_replacement_terms:
    - id: GO:0051087
      label: protein-folding chaperone binding
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Interacts with BAG3 (PubMed:28144995)
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IDA
  original_reference_id: PMID:28144995
  qualifier: located_in
  review:
    summary: Direct experimental evidence for cytoplasmic localization of HSPB8 from the disease-variant characterization study.
    action: ACCEPT
    reason: IDA-supported cytoplasmic localization, the principal compartment for HSPB8's CASA/holdase function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:19464326, ECO:0000269|PubMed:28144995}'
- term:
    id: GO:0005654
    label: nucleoplasm
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5082356
  qualifier: located_in
  review:
    summary: Reactome-curated nucleoplasm localization, consistent with the HPA IDA nucleoplasm annotation and HSPB8's nuclear pool.
    action: KEEP_AS_NON_CORE
    reason: Supported by curated pathway annotation and HPA evidence; consistent with nuclear localization but peripheral to the core cytoplasmic function.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-goa.tsv
      supporting_text: GO:0005654 nucleoplasm cellular_component ECO:0000304 TAS Reactome:R-HSA-5082356
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:22366786
  qualifier: enables
  review:
    summary: Documents the HSPB8 interaction with the co-chaperone DNAJB6 (a Hsp40/J-domain protein mutated in limb-girdle muscular dystrophy). The bare protein binding term is uninformative; the partner is a chaperone.
    action: MODIFY
    reason: Bare protein binding is uninformative. The documented partner DNAJB6 is a J-domain co-chaperone, so heat shock protein binding (GO:0031072) is the appropriate specific molecular function.
    proposed_replacement_terms:
    - id: GO:0031072
      label: heat shock protein binding
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Interacts with DNAJB6 (PubMed:22366786)
- term:
    id: GO:0004672
    label: protein kinase activity
  evidence_type: IDA
  original_reference_id: PMID:14985082
  qualifier: enables
  negated: true
  review:
    summary: Although HSPB8 was originally named protein kinase H11, recombinant HSPB8 has negligible autophosphorylation and cannot phosphorylate model substrates; the kinase activity is refuted. The NOT (negated) annotation is correct.
    action: ACCEPT
    reason: Direct experimental evidence refutes protein kinase activity for HSPB8, supporting this correctly-negated annotation.
    supported_by:
    - reference_id: PMID:14985082
      supporting_text: Hsp22 possesses a negligibly low autophosphorylation activity and under the conditions used is unable to phosphorylate casein or
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IDA
  original_reference_id: PMID:19464326
  qualifier: located_in
  review:
    summary: Direct experimental (confocal microscopy) evidence for nuclear localization of HSPB8 from the HSPB-family subcellular-distribution survey.
    action: ACCEPT
    reason: IDA-supported nuclear localization, consistent with UniProt and the heat-shock-induced nuclear foci.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: Nucleus {ECO:0000269|PubMed:19464326}
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IDA
  original_reference_id: PMID:19464326
  qualifier: located_in
  review:
    summary: Direct experimental (confocal microscopy) evidence for cytoplasmic localization of HSPB8.
    action: ACCEPT
    reason: IDA-supported cytoplasmic localization, the principal site of HSPB8 action.
    supported_by:
    - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm'
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation of UniProtKB entries based on the manual curation of subcellular locations
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: PMID:14594798
  title: Interaction of human HSP22 (HSPB8) with other small heat shock proteins.
  findings:
  - statement: HSP22 (HSPB8) self-associates and hetero-dimerizes with HSPB7 (cvHSP), HSPB2 (MKBP) and HSPB1 (HSP27), defining the small-HSP hetero-oligomer network.
    reference_section_type: RESULTS
- id: PMID:14985082
  title: Some properties of human small heat shock protein Hsp22 (H11 or HspB8).
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached publications/PMID_14985082.md title matches; the cached Results establish ATP-independent holdase activity (suppresses heat aggregation of ADH and rhodanese), supporting core_function GO:0051082. Notably the same paper reports negligible kinase activity, which contradicts the GOA IDA GO:0004672 (protein kinase activity) tied to this PMID - a likely over-annotation. Cited in core_functions supported_by."
  findings:
  - statement: Recombinant HSPB8 forms stable dimers, has negligible kinase/autophosphorylation activity and cannot phosphorylate casein or histone, but possesses ATP-independent chaperone (holdase) activity preventing heat-induced aggregation of alcohol dehydrogenase and rhodanese.
    reference_section_type: RESULTS
- id: PMID:16189514
  title: Towards a proteome-scale map of the human protein-protein interaction network.
  findings: []
- id: PMID:18006506
  title: HspB8 chaperone activity toward poly(Q)-containing proteins depends on its association with Bag3, a stimulator of macroautophagy.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached publications/PMID_18006506.md title matches; richly anchored in GOA as the source for GO:1905337 (positive regulation of aggrephagy, IMP), GO:0034620 (cellular response to unfolded protein, IMP), GO:0101031 (protein folding chaperone complex, IDA) and GO:0042803 (homodimerization). Establishes the HSPB8-BAG3 CASA/aggrephagy core function. Cited in core_functions supported_by."
  findings:
  - statement: HSPB8 forms a stable complex with BAG3 that is essential for its activity; the HSPB8-BAG3 complex promotes macroautophagic degradation of the polyglutamine protein Htt43Q, preventing accumulation of aggregation-prone proteins.
    reference_section_type: RESULTS
- id: PMID:19464326
  title: HSPB7 is a SC35 speckle resident small heat shock protein.
  findings:
  - statement: Confocal-microscopy survey of HSPB-family subcellular distribution; HSPB8 localizes to cytoplasm and nucleus.
    reference_section_type: RESULTS
- id: PMID:20060297
  title: Chaperone-assisted selective autophagy is essential for muscle maintenance.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: "Cached publications/PMID_20060297.md title matches and confirms HSPB8 as a small-HSP component of the BAG3/HSPA8/CHIP chaperone-assisted selective autophagy (CASA) machinery degrading damaged Z-disk filamin in muscle; supports the CASA/aggrephagy core function. Not directly GOA-anchored for HSPB8 but cached content is on-target."
  findings:
  - statement: BAG3 coordinates Hsc70 (HSPA8) and the small HSP HSPB8 in chaperone-assisted selective autophagy (CASA), initiated by the ubiquitin ligase CHIP/STUB1 and the autophagy adaptor p62, to degrade damaged Z-disk components such as filamin; impaired CASA causes Z-disk disintegration and progressive muscle weakness.
    reference_section_type: RESULTS
- id: PMID:21516116
  title: Next-generation sequencing to generate interactome datasets.
  findings: []
- id: PMID:22366786
  title: Mutations affecting the cytoplasmic functions of the co-chaperone DNAJB6 cause limb-girdle muscular dystrophy.
  findings:
  - statement: HSPB8 interacts with the J-domain co-chaperone DNAJB6, linking it to the chaperone network whose disruption causes limb-girdle muscular dystrophy.
    reference_section_type: RESULTS
- id: PMID:23414517
  title: 'A human skeletal muscle interactome centered on proteins involved in muscular dystrophies: LGMD interactome.'
  findings: []
- id: PMID:25036637
  title: A quantitative chaperone interaction network reveals the architecture of cellular protein homeostasis pathways.
  findings: []
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
- id: PMID:26496610
  title: A human interactome in three quantitative dimensions organized by stoichiometries and abundances.
  findings: []
- id: PMID:28144995
  title: 'Axonal Neuropathies due to Mutations in Small Heat Shock Proteins: Clinical, Genetic, and Functional Insights into Novel Mutations.'
  findings:
  - statement: Characterized HMND2-causing HSPB8 variants; documents HSPB8-BAG3 interaction and that several mutations alter it, with cytoplasmic localization of HSPB8.
    reference_section_type: RESULTS
- id: PMID:28514442
  title: Architecture of the human interactome defines protein communities and disease networks.
  findings: []
- id: PMID:31273097
  title: The heme-regulated inhibitor is a cytosolic sensor of protein misfolding that controls innate immune signaling.
  findings: []
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
- id: PMID:32707033
  title: Kinase Interaction Network Expands Functional and Disease Roles of Human Kinases.
  findings: []
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
  findings: []
- id: Reactome:R-HSA-5082356
  title: Reactome annotation (nucleoplasm localization of HSPB8)
  findings: []
- id: file:human/HSPB8/HSPB8-uniprot.txt
  title: UniProt entry Q9UJY1 (HSPB8_HUMAN), Heat shock protein beta-8 (HSP22)
  findings:
  - statement: Small heat shock protein involved in chaperone-assisted selective autophagy (CASA) with BAG3, HSPA8/HSC70 and STUB1/CHIP; temperature-dependent chaperone activity; interacts with HSPB1, DNAJB6, BAG3 and HSPA1A; cytoplasm and nucleus; muscle-enriched; dominant variants cause HMND2/CMT2L/MFM13.
    reference_section_type: OTHER
core_functions:
- description: ATP-independent small heat shock protein (holdase) that binds aggregation-prone and stress-destabilized client proteins, preventing their aggregation.
  molecular_function:
    id: GO:0051082
    label: unfolded protein binding
  locations:
  - id: GO:0005737
    label: cytoplasm
  supported_by:
  - reference_id: PMID:14985082
    supporting_text: Hsp22 effectively prevents heat-induced aggregation of yeast alcohol dehydrogenase and bovine liver rhodanese
  - reference_id: PMID:18006506
    supporting_text: overexpression of HspB8 in cultured cells prevents the accumulation of aggregation-prone proteins such as the polyglutamine protein Htt43Q
- description: Co-chaperone partner of BAG3 that, with HSPA8/Hsp70 and STUB1/CHIP, drives chaperone-assisted selective autophagy (CASA), promoting aggrephagy/macroautophagic clearance of damaged clients (e.g. filamin, polyglutamine proteins) for muscle proteostasis.
  molecular_function:
    id: GO:0051087
    label: protein-folding chaperone binding
  locations:
  - id: GO:0005737
    label: cytoplasm
  supported_by:
  - reference_id: PMID:18006506
    supporting_text: HspB8 forms a stable complex with Bag3 in cells and that the formation of this complex is essential for the activity of HspB8
  - reference_id: file:human/HSPB8/HSPB8-uniprot.txt
    supporting_text: Component of the chaperone-assisted selective autophagy (CASA) complex consisting of BAG3, HSPA8/HSC70, HSPB8 and STUB1/CHIP
- description: Positive regulator of aggrephagy / selective autophagy, routing aggregation-prone misfolded clients for autophagic degradation as part of the cellular response to proteotoxic stress.
  molecular_function:
    id: GO:0051082
    label: unfolded protein binding
  locations:
  - id: GO:0005737
    label: cytoplasm
  supported_by:
  - reference_id: PMID:18006506
    supporting_text: the HspB8-Bag3 complex might stimulate the degradation of Htt43Q by macroautophagy
  directly_involved_in:
  - id: GO:1905337
    label: positive regulation of aggrephagy
proposed_new_terms: []
suggested_questions:
- question: How do the K141 hot-spot variants mechanistically impair HSPB8 chaperone function and CASA to cause the spectrum of neuropathy and myopathy phenotypes?
- question: What determines the client selectivity of HSPB8 within the BAG3/Hsp70 CASA machinery versus its standalone holdase activity?
- question: Does the nuclear pool of HSPB8 have a function distinct from its cytoplasmic CASA role?
suggested_experiments:
- description: Reconstitute the CASA complex (HSPB8/BAG3/HSPA8/STUB1) in vitro and measure HSPB8-dependent ubiquitination and autophagic targeting of a model damaged client (e.g. filamin), comparing wild-type and K141 variants.
- description: Quantitative autophagy-flux and aggregate-clearance assays in HSPB8-null versus reconstituted muscle cells with poly-Q or aggregation-prone reporters.
- description: Knock-in mouse or iPSC-derived motor neuron/myocyte models of HSPB8 K141N/E to assess Z-disk integrity, aggrephagy, and progressive degeneration.