HSPB2 (heat shock protein beta-2; also MKBP, "DMPK-binding protein") is an ATP-independent small heat-shock protein (sHSP) of the alpha-crystallin/HSP20 family, expressed preferentially in skeletal and cardiac muscle. Like other sHSPs it acts as a holdase chaperone, binding non-native/destabilized client proteins through its alpha-crystallin domain to prevent their aggregation under stress, holding them for downstream ATP-dependent refolding. HSPB2 forms its own oligomeric complex in muscle cytosol, distinct from the alphaB-crystallin (CRYAB)/HSP27 complex, and is notable for binding and activating the myotonic dystrophy protein kinase (DMPK), enhancing its kinase activity and protecting it from heat-induced inactivation. It localizes to the cytoplasm (Z-membrane of myofibrils and the neuromuscular junction) and to nuclear foci. HSPB2 is not strongly heat-inducible but participates in the muscle stress response and is cardioprotective during ischemia, helping maintain ATP levels.
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: HSPB2: a small heat-shock protein whose holdase activity rests mainly on family membership; direct evidence is limited to GAPDH being validated as a potential client through chaperone assays (PMID:26465331, assay not detailed in the cached abstract), and the aggregation-protection assays listed in suggested_experiments would confirm it. 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: Phylogenetic (IBA) nuclear localization, corroborated by direct experimental evidence that HSPB2 localizes to nuclear foci. Reason: Nuclear localization is directly supported (PMID:19464326, HPA nucleoplasm IDA) in addition to the IBA transfer. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Note=Localizes to nuclear foci. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) cytoplasmic localization, corroborated by direct evidence; HSPB2 is a cytosolic muscle sHSP localizing to the myofibrillar Z-membrane. Reason: Cytoplasmic localization is directly supported (PMID:19464326 IDA; TAS cytosol) and is a principal site of HSPB2 chaperone action. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0043066 negative regulation of apoptotic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic transfer of the cytoprotective/anti-apoptotic role common to small HSPs. Consistent with HSPB2's cardioprotective (anti-ischemic) phenotype, but a downstream consequence of its chaperone activity rather than a direct molecular function. Reason: A plausible downstream cytoprotective effect supported by the cardioprotection phenotype, but secondary to HSPB2's core holdase/kinase-activator functions. Supporting Evidence: PMID:26465331 transgenic overexpressing mice including reduced infarct size and maintenance of |
| GO:0009408 response to heat | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic transfer of "response to heat" from the heat-shock protein family. Notably HSPB2/MKBP is NOT induced by heat shock, though it participates in the muscle stress response and protects clients from heat-induced damage. Reason: HSPB2 expression is not heat-inducible, so the term applies only loosely via its stress-protective chaperone activity; retained as non-core. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt The expression of MKBP is not induced by heat shock |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Protein refolding IBA propagated from the small heat shock protein node PTN000897708 (Drosophila sHSP donors); small heat shock proteins like HSPB2 are ATP-independent holdases that prevent aggregation rather than catalyzing active protein refolding. Reason: This IBA is propagated from the Eumetazoa (taxon:6072) small heat shock protein (sHSP) node PANTHER:PTN000897708 (seeded by Drosophila sHSPs FB:FBgn0001224, FB:FBgn0001225, FB:FBgn0001226, FB:FBgn0011296, FB:FBgn0031037, and FB:FBgn0035817). Small heat shock proteins, including human HSPB2, are ATP-independent holdase chaperones that bind and sequester unfolding intermediates to prevent irreversible aggregation (and for HSPB2, suppress the heat-induced inactivation of DMPK; PMID:9490724); restoring native structure and activity to an unfolded client is performed by ATP-dependent HSP70/HSP100 foldase systems rather than the sHSP itself. Although GO:0042026 (protein refolding) is a biological process term, sHSP holdase sequestration occurs during stress to prevent irreversible aggregation independently of whether the client's downstream fate is HSP70-mediated refolding or proteasomal/autophagic degradation, and sHSPs are neither subunits nor cofactors of the ATP-dependent HSP70/J-domain/NEF refolding reaction once substrate is released, so attributing the downstream protein refolding process (GO:0042026) to the upstream stress holdase conflates distinct proteostasis steps (go-ontology#30552). Across non-donor vertebrate and nematode PTN000897708 descendants in this repository, DANRE/cryaa, DANRE/cryaba, DANRE/cryabb, human/HSPB1, human/CRYAB, human/HSPB6, and C. elegans hsp-16.1/16.2/16.41/16.48 are all MARK_AS_OVER_ANNOTATED on GO:0042026 (while human/CRYAA is REMOVE due to a negated ISS GO:0042026 row and C. elegans hsp-12.3 and hsp-12.6 are REMOVE on direct in vitro refutation of chaperone activity in PMID:9744800 and PMID:9139746 plus negated GO:0051082 rows; the four Drosophila reviews DROME/Hsp22, DROME/Hsp23, DROME/Hsp26, and DROME/Hsp27 read the GO:0042026 participation test the other way based on PMID:16572729 and PMID:26705243 and are deferred to follow-up pending upstream resolution of go-ontology#30552). Because HSPB2's holdase role is already captured in core_functions (proposed_molecular_function: holdase chaperone activity) and GO:0006986 (response to unfolded protein) and there is no direct evidence that HSPB2 catalyzes active protein refolding, GO:0042026 is MARK_AS_OVER_ANNOTATED. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: ROLE CONFLATION GRANULARITY MISMATCH Sources checked: PANTHER:PTN000897708 · PTN000897708 SUPPORTS SOURCE BUT NOT TARGET Ancestral sHSP node placement (Eumetazoa, taxon:6072) is sound for chaperone-like holdase function, but GO:0042026 (protein refolding) ascribes to the sHSP an active foldase step performed by ATP-dependent chaperone machinery rather than HSPB2's ATP-independent holdase suppression of heat-induced client inactivation/aggregation. FB:FBgn0001224 · Hsp23 SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. FB:FBgn0001225 · Hsp26 SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. FB:FBgn0001226 · Hsp27 SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. FB:FBgn0011296 · l(2)efl SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. FB:FBgn0031037 · CG14207 SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. FB:FBgn0035817 · CG7409 SUPPORTS SOURCE BUT NOT TARGET Drosophila sHSP donor seeding PTN000897708; the holdase biology is shared, but the holdase-vs-foldase distinction is not captured by GO:0042026 so the GO:0042026 term does not transfer. Supporting Evidence: PMID:9490724 MKBP suppresses the heat-induced inactivation of DMPK by binding to its kinase domain PMID:26465331 validated as a potential client protein of HspB2 through chaperone assays |
| GO:0005212 structural constituent of eye lens | IEA GO_REF:0000002 | REMOVE | Summary: InterPro2GO transfer of the lens structural role from the alpha-crystallin domain signature. HSPB2 is explicitly not expressed in the lens, so this is an erroneous family-level over-annotation. Reason: HSPB2 is not a lens crystallin and is expressly absent from the lens; the term is incorrectly transferred from the broader alpha-crystallin/sHSP family. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Expressed preferentially in skeletal muscle and heart but not in the lens. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic (UniProt SubCell) nuclear localization, redundant with the experimental IDA nuclear annotation. Reason: Correct, supported by direct experimental evidence for nuclear foci. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Note=Localizes to nuclear foci. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic (UniProt SubCell) cytoplasmic localization, redundant with the experimental IDA cytoplasm annotation. Reason: Correct, supported by direct experimental evidence. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0005515 protein binding | IPI PMID:14594798 Interaction of human HSP22 (HSPB8) with other small heat sho... | MODIFY | Summary: Interaction with the small heat-shock protein HSPB8/HSP22 (Q9UJY1). This sHSP-sHSP interaction is more informatively captured as heat shock protein binding than bare protein binding. Reason: The partner is another heat-shock protein (HSPB8), so heat shock protein binding is the appropriate specific molecular function. Proposed replacements: heat shock protein binding Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; Q9UJY1: HSPB8; NbExp=3; IntAct=EBI-739395, EBI-739074 |
| GO:0005515 protein binding | IPI PMID:23188086 Binding determinants of the small heat shock protein, αB-cry... | MODIFY | Summary: Interaction with the small heat-shock protein alphaB-crystallin/CRYAB (P02511), reflecting sHSP hetero-oligomer recognition via the IxI motif. Reason: The partner is another heat-shock protein (CRYAB), so heat shock protein binding is the appropriate specific molecular function. Proposed replacements: heat shock protein binding Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; P02511: CRYAB; NbExp=3; IntAct=EBI-739395, EBI-739060 |
| GO:0005515 protein binding | IPI PMID:26465331 Characterization of the Cardiac Overexpression of HSPB2 Reve... | MODIFY | Summary: Cardiac yeast two-hybrid interactome again capturing the HSPB2-CRYAB interaction (and broader myofibrillar/mitochondrial clients). The sHSP-sHSP interaction is better captured as heat shock protein binding. Reason: Partner is the heat-shock protein CRYAB; heat shock protein binding is the specific molecular function. The wider client-handling role is captured by the proposed holdase chaperone activity in core functions. Proposed replacements: heat shock protein binding Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; P02511: CRYAB; NbExp=3; IntAct=EBI-739395, EBI-739060 |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Large binary HuRI interactome screen capturing many heterogeneous partners (BAG3, A1CF, CEP19, POGZ, VEZF1 and others). Bare protein binding is uninformative. Reason: Bare GO:0005515 protein binding carries no functional information, so under the generic protein-binding policy this row is REMOVE. Removal does not mean the reported interaction is false; the summary and any supporting text keep it on record. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; O95817: BAG3; NbExp=5; IntAct=EBI-739395, EBI-747185 |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | REMOVE | Summary: Neurodegeneration interactome screen capturing an HSPB2-APP (amyloid precursor protein) interaction. Bare protein binding from a single high-throughput screen. Reason: Bare GO:0005515 protein binding carries no functional information, so under the generic protein-binding policy this row is REMOVE. Removal does not mean the reported interaction is false; the summary and any supporting text keep it on record. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; P05067: APP; NbExp=3; IntAct=EBI-739395, EBI-77613 |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | REMOVE | Summary: BioPlex affinity-purification interactome capturing an HSPB2-BAG3 interaction. BAG3 is a co-chaperone that cooperates with small HSPs; nonetheless this is a bare protein binding annotation. Reason: Bare GO:0005515 protein binding carries no functional information, so under the generic protein-binding policy this row is REMOVE. Removal does not mean the reported interaction is false; the summary and any supporting text keep it on record. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Q16082; O95817: BAG3; NbExp=5; IntAct=EBI-739395, EBI-747185 |
| GO:0005654 nucleoplasm | IDA GO_REF:0000052 | ACCEPT | Summary: HPA immunofluorescence localization to the nucleoplasm, consistent with the reported nuclear foci. Reason: Direct antibody-based localization consistent with the documented nuclear pool. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Note=Localizes to nuclear foci. |
| GO:0005515 protein binding | IPI PMID:25556234 New host factors important for respiratory syncytial virus (... | REMOVE | Summary: AgBase-curated interaction (with P0DOE7) reported in a respiratory syncytial virus host-factor screen. Bare protein binding, unrelated to HSPB2's core function. Reason: Bare GO:0005515 protein binding carries no functional information, so under the generic protein-binding policy this row is REMOVE. Removal does not mean the reported interaction is false; the summary and any supporting text keep it on record. |
| GO:0005634 nucleus | IDA PMID:19464326 HSPB7 is a SC35 speckle resident small heat shock protein. | ACCEPT | Summary: Direct experimental demonstration that HSPB2 localizes to the nucleus (nuclear foci). Reason: Strongest-evidence nuclear localization. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Note=Localizes to nuclear foci. |
| GO:0005737 cytoplasm | IDA PMID:19464326 HSPB7 is a SC35 speckle resident small heat shock protein. | ACCEPT | Summary: Direct experimental demonstration that HSPB2 localizes to the cytoplasm. Reason: Strongest-evidence cytoplasmic localization; principal site of HSPB2 action. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0006986 response to unfolded protein | NAS PMID:9344664 Identification and characterization of the gene encoding a n... | KEEP AS NON CORE | Summary: HSPB2 is an alpha-crystallin/small-HSP family member implicated in the stress response to unfolded proteins. Consistent with its holdase chaperone activity. Reason: A reasonable stress-response process annotation; the underlying molecular activity (binding non-native proteins) is the core feature captured by the proposed holdase chaperone activity. Supporting Evidence: file:human/HSPB2/HSPB2-uniprot.txt Belongs to the small heat shock protein (HSP20) family. |
| GO:0005829 cytosol | TAS PMID:9490724 MKBP, a novel member of the small heat shock protein family,... | ACCEPT | Summary: Author-stated cytosolic localization; in muscle cytosol HSPB2/MKBP exists as an oligomeric complex distinct from the CRYAB/HSP27 complex. Reason: Supported by the original MKBP characterization; cytosol is a principal site of HSPB2 oligomers. Supporting Evidence: PMID:9490724 MKBP exists as an oligomeric complex separate from the complex |
| GO:0008047 enzyme activator activity | TAS PMID:9490724 MKBP, a novel member of the small heat shock protein family,... | ACCEPT | Summary: HSPB2/MKBP binds the myotonic dystrophy protein kinase (DMPK), enhances its kinase activity and protects it from heat-induced inactivation, acting as a kinase activator. This is a distinctive, well-documented HSPB2 function. Reason: Directly demonstrated in vitro; HSPB2 enhances DMPK kinase activity, a genuine and specific enzyme-activator (kinase-activator) function. Supporting Evidence: PMID:9490724 enhances the kinase activity of DMPK and protects it from heat-induced |
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Download this section (compressed HTML)Q: Does HSPB2 form a defined hetero-oligomer with HSPB3 in muscle, and how does this complex differ functionally from HSPB2 homo-oligomers and the CRYAB/HSP27 complex?
Q: Is DMPK activation a direct allosteric effect of HSPB2 binding, and is it disrupted in myotonic dystrophy where MKBP is selectively upregulated?
Experiment: In vitro aggregation-protection (holdase) assays with recombinant HSPB2 against model and physiological clients (e.g. GAPDH) to quantify chaperone activity and ATP-independence.
Experiment: Reconstitute DMPK with HSPB2 to map the binding interface and measure kinase-activation kinetics, testing whether disease-associated changes alter activation.
Experiment: Define the muscle HSPB2/HSPB3 complex stoichiometry by size-exclusion/native MS and test client specificity relative to HSPB2 alone.
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