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.
Definition: Binding to an unfolded or misfolded protein to prevent its aggregation without actively catalyzing refolding. The holdase maintains the client protein in a soluble, folding-competent state. This is mechanistically distinct from foldase activity (GO:0044183) and from carrier-holdase activity (GO:0140309).
Justification: HSPB8: ATP-independent small heat shock protein (holdase) that binds aggregation-prone and stress-destabilized client proteins, preventing their aggregation. Obsolete GO:0051082 captured binding only; GO:0044183 requires assisting folding, and GO:0140309 (relabelled 'unfolded protein holdase activity') keeps a carrier-specific definition requiring escort to an acceptor molecule or location, which is not demonstrated here. See go-ontology#30552. GO:0051787 misfolded protein binding is live but, like the obsolete GO:0051082, records client binding only and not the suppression of aggregation, so it does not capture the activity. The proposed parent is the molecular_function root because GO has no general chaperone-activity grouping term: GO:0044183 protein folding chaperone is itself a direct child of GO:0003674.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Nuclear localization inferred phylogenetically; HSPB8 is directly documented in the nucleus and translocates to nuclear foci during heat shock. Reason: Corroborated by direct experimental (IDA) evidence (PMID:19464326) and the UniProt subcellular-location record. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Nucleus {ECO:0000269|PubMed:19464326} |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasmic localization inferred phylogenetically; the cytoplasm is the principal site where HSPB8 acts in CASA and as a holdase. Reason: Corroborated by direct experimental (IDA) evidence (PMID:19464326, PMID:28144995) and the UniProt subcellular-location record. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0034620 cellular response to unfolded protein | IBA GO_REF:0000033 | ACCEPT | 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. Reason: Phylogenetic inference is consistent with experimental evidence that HSPB8 prevents aggregation of unfolded/aggregation-prone clients and is induced by stress. Supporting Evidence: PMID:14985082 Hsp22 effectively prevents heat-induced aggregation of yeast alcohol dehydrogenase and bovine liver rhodanese |
| GO:0101031 protein folding chaperone complex | IBA GO_REF:0000033 | ACCEPT | 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. Reason: Direct experimental evidence places HSPB8 in the CASA complex and in ternary complexes with BAG3 and Hsp70, consistent with this phylogenetic annotation. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Component of the chaperone-assisted selective autophagy (CASA) complex consisting of BAG3, HSPA8/HSC70, HSPB8 and STUB1/CHIP |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation of nuclear localization, consistent with the IBA and IDA nucleus annotations. Reason: Agrees with stronger experimental evidence (PMID:19464326) placing HSPB8 in the nucleus. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Nucleus {ECO:0000269|PubMed:19464326} |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation of cytoplasmic localization, redundant with the IBA and IDA cytoplasm annotations. Reason: Correct compartment, agreeing with stronger experimental evidence. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0034620 cellular response to unfolded protein | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic annotation to the cellular response to unfolded protein, consistent with HSPB8's sHSP holdase function. Reason: Redundant with the IBA and IMP annotations of the same process, all supported by HSPB8's documented anti-aggregation chaperone activity. Supporting Evidence: PMID:14985082 Hsp22 effectively prevents heat-induced aggregation of yeast alcohol dehydrogenase and bovine liver rhodanese |
| GO:0042803 protein homodimerization activity | IEA GO_REF:0000002 | ACCEPT | Summary: HSPB8 forms stable homodimers, the basic oligomeric unit of this small HSP. Supported by direct biochemical evidence. Reason: Recombinant HSPB8 forms stable dimers by size-exclusion and crosslinking, corroborating this electronic annotation. Supporting Evidence: PMID:14985082 Hsp22 forms stable dimers |
| GO:0005515 protein binding | IPI PMID:14594798 Interaction of human HSP22 (HSPB8) with other small heat sho... | MODIFY | 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. 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 replacements: heat shock protein binding Supporting Evidence: PMID:14594798 HSP22 interacts with itself, cvHSP (HSPB7), MKBP (HSPB2) and HSP27 |
| GO:0005515 protein binding | IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in... | KEEP AS NON CORE | Summary: Proteome-scale interactome screen capturing HSPB8 interactions. The bare protein binding term is uninformative. 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. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:16189514 |
| GO:0005515 protein binding | IPI PMID:21516116 Next-generation sequencing to generate interactome datasets. | KEEP AS NON CORE | Summary: Next-generation-sequencing interactome dataset capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; records genuine interactions but uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:21516116 |
| GO:0005515 protein binding | IPI PMID:23414517 A human skeletal muscle interactome centered on proteins inv... | KEEP AS NON CORE | Summary: Human skeletal-muscle interactome study capturing HSPB8 interactions in a muscle context. The bare protein binding term is uninformative. Reason: Muscle-focused interactome data is contextually relevant but the bare term is uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:23414517 |
| GO:0005515 protein binding | IPI PMID:25036637 A quantitative chaperone interaction network reveals the arc... | KEEP AS NON CORE | Summary: Quantitative chaperone interaction network capturing HSPB8 within chaperone modules. The bare protein binding term is uninformative. Reason: Places HSPB8 in the chaperone interactome; genuine but uninformative as a bare molecular-function term. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:25036637 |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | KEEP AS NON CORE | Summary: Proteome-scale yeast two-hybrid interactome map capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:25416956 |
| GO:0005515 protein binding | IPI PMID:26496610 A human interactome in three quantitative dimensions organiz... | KEEP AS NON CORE | Summary: Quantitative stoichiometry-resolved interactome capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:26496610 |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | KEEP AS NON CORE | Summary: Human interactome (protein communities) study capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:28514442 |
| GO:0005515 protein binding | IPI PMID:31273097 The heme-regulated inhibitor is a cytosolic sensor of protei... | KEEP AS NON CORE | Summary: Study of the heme-regulated inhibitor (HRI) misfolding sensor capturing an HSPB8 interaction. The bare protein binding term is uninformative. Reason: Records a genuine interaction relevant to a misfolding-stress pathway but the bare term is uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:31273097 |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | KEEP AS NON CORE | Summary: Reference binary interactome map capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:32296183 |
| GO:0005515 protein binding | IPI PMID:32707033 Kinase Interaction Network Expands Functional and Disease Ro... | KEEP AS NON CORE | Summary: Kinase interaction network study capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:32707033 |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | KEEP AS NON CORE | Summary: BioPlex affinity-purification interactome capturing HSPB8 interactions. The bare protein binding term is uninformative. Reason: High-throughput interactome data; uninformative as a molecular function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005515 protein binding molecular_function ECO:0000353 IPI PMID:33961781 |
| GO:0042802 identical protein binding | IPI PMID:14594798 Interaction of human HSP22 (HSPB8) with other small heat sho... | ACCEPT | Summary: HSPB8 self-associates (HSP22 interacts with itself), the basis for its homodimer/homo-oligomer. Identical protein binding is supported. Reason: Direct evidence that HSP22 interacts with itself supports the self-association molecular function. Supporting Evidence: PMID:14594798 HSP22 interacts with itself, cvHSP (HSPB7), MKBP (HSPB2) and HSP27 |
| GO:0005654 nucleoplasm | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Direct immunofluorescence (HPA) localization to the nucleoplasm, consistent with HSPB8's documented nuclear pool. Reason: Supported by HPA IDA evidence; consistent with nuclear localization but peripheral to the core cytoplasmic CASA/holdase function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005654 nucleoplasm cellular_component ECO:0000314 IDA |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: Direct immunofluorescence (HPA) localization to the cytosol, the principal compartment for HSPB8's chaperone activity. Reason: IDA-supported cytosolic localization, consistent with the cytoplasmic site of CASA and holdase activity. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005829 cytosol cellular_component ECO:0000314 IDA |
| GO:0042803 protein homodimerization activity | IPI PMID:18006506 HspB8 chaperone activity toward poly(Q)-containing proteins ... | ACCEPT | Summary: HSPB8 homodimerization activity, the basic oligomeric unit of this small HSP, supported by direct evidence. Reason: HSPB8 forms stable dimers/self-associates; this IPI-supported homodimerization is a genuine molecular function. Supporting Evidence: PMID:14985082 Hsp22 forms stable dimers |
| GO:0005515 protein binding | IPI PMID:18006506 HspB8 chaperone activity toward poly(Q)-containing proteins ... | MODIFY | 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. 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 replacements: protein-folding chaperone binding Supporting Evidence: PMID:18006506 HspB8 forms a stable complex with Bag3 in cells and that the formation of this complex is essential for the activity of HspB8 |
| GO:0034620 cellular response to unfolded protein | IMP PMID:18006506 HspB8 chaperone activity toward poly(Q)-containing proteins ... | ACCEPT | 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. Reason: Direct experimental (IMP) evidence that HSPB8 handles aggregation-prone misfolded clients supports this process annotation as a core function. Supporting Evidence: PMID:18006506 overexpression of HspB8 in cultured cells prevents the accumulation of aggregation-prone proteins such as the polyglutamine protein Htt43Q |
| GO:0101031 protein folding chaperone complex | IDA PMID:18006506 HspB8 chaperone activity toward poly(Q)-containing proteins ... | ACCEPT | Summary: HSPB8 forms a stable complex with BAG3 (and within the larger CASA/Hsp70 machinery); direct evidence places it in a chaperone complex. Reason: Direct (IDA) evidence for a stable HSPB8-BAG3 complex supports membership in a protein-folding chaperone complex. Supporting Evidence: PMID:18006506 HspB8 forms a stable complex with Bag3 in cells |
| GO:1905337 positive regulation of aggrephagy | IMP PMID:18006506 HspB8 chaperone activity toward poly(Q)-containing proteins ... | ACCEPT | Summary: HSPB8, in complex with BAG3, promotes the macroautophagic degradation of aggregation-prone clients (Htt43Q). This is the core CASA/aggrephagy role of HSPB8. 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. Supporting Evidence: PMID:18006506 the HspB8-Bag3 complex might stimulate the degradation of Htt43Q by macroautophagy |
| GO:0005515 protein binding | IPI PMID:28144995 Axonal Neuropathies due to Mutations in Small Heat Shock Pro... | MODIFY | 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. 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 replacements: protein-folding chaperone binding Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Interacts with BAG3 (PubMed:28144995) |
| GO:0005737 cytoplasm | IDA PMID:28144995 Axonal Neuropathies due to Mutations in Small Heat Shock Pro... | ACCEPT | Summary: Direct experimental evidence for cytoplasmic localization of HSPB8 from the disease-variant characterization study. Reason: IDA-supported cytoplasmic localization, the principal compartment for HSPB8's CASA/holdase function. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:19464326, ECO:0000269|PubMed:28144995} |
| GO:0005654 nucleoplasm | TAS Reactome:R-HSA-5082356 | KEEP AS NON CORE | Summary: Reactome-curated nucleoplasm localization, consistent with the HPA IDA nucleoplasm annotation and HSPB8's nuclear pool. Reason: Supported by curated pathway annotation and HPA evidence; consistent with nuclear localization but peripheral to the core cytoplasmic function. Supporting Evidence: file:human/HSPB8/HSPB8-goa.tsv GO:0005654 nucleoplasm cellular_component ECO:0000304 TAS Reactome:R-HSA-5082356 |
| GO:0005515 protein binding | IPI PMID:22366786 Mutations affecting the cytoplasmic functions of the co-chap... | MODIFY | 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. 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 replacements: heat shock protein binding Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Interacts with DNAJB6 (PubMed:22366786) |
| GO:0004672 protein kinase activity | IDA NOT PMID:14985082 Some properties of human small heat shock protein Hsp22 (H11... | ACCEPT | 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. Reason: Direct experimental evidence refutes protein kinase activity for HSPB8, supporting this correctly-negated annotation. Supporting Evidence: PMID:14985082 Hsp22 possesses a negligibly low autophosphorylation activity and under the conditions used is unable to phosphorylate casein or |
| GO:0005634 nucleus | IDA PMID:19464326 HSPB7 is a SC35 speckle resident small heat shock protein. | ACCEPT | Summary: Direct experimental (confocal microscopy) evidence for nuclear localization of HSPB8 from the HSPB-family subcellular-distribution survey. Reason: IDA-supported nuclear localization, consistent with UniProt and the heat-shock-induced nuclear foci. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt Nucleus {ECO:0000269|PubMed:19464326} |
| GO:0005737 cytoplasm | IDA PMID:19464326 HSPB7 is a SC35 speckle resident small heat shock protein. | ACCEPT | Summary: Direct experimental (confocal microscopy) evidence for cytoplasmic localization of HSPB8. Reason: IDA-supported cytoplasmic localization, the principal site of HSPB8 action. Supporting Evidence: file:human/HSPB8/HSPB8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
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Download this section (compressed HTML)Q: How do the K141 hot-spot variants mechanistically impair HSPB8 chaperone function and CASA to cause the spectrum of neuropathy and myopathy phenotypes?
Q: What determines the client selectivity of HSPB8 within the BAG3/Hsp70 CASA machinery versus its standalone holdase activity?
Q: Does the nuclear pool of HSPB8 have a function distinct from its cytoplasmic CASA role?
Experiment: 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.
Experiment: Quantitative autophagy-flux and aggregate-clearance assays in HSPB8-null versus reconstituted muscle cells with poly-Q or aggregation-prone reporters.
Experiment: Knock-in mouse or iPSC-derived motor neuron/myocyte models of HSPB8 K141N/E to assess Z-disk integrity, aggrephagy, and progressive degeneration.
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