HSPB3 (heat shock protein beta-3, also called HSP17 or HSPL27) is a member of the small heat shock protein (sHSP / HSP20, alpha-crystallin domain) family. It is the most divergent of the human small HSPs, possessing a unique N-terminal domain and essentially lacking the C-terminal extension found in other family members, while retaining the conserved alpha-crystallin domain. Like other small HSPs it is an ATP-independent molecular chaperone that binds partially unfolded client proteins and holds them in a folding-competent state, and it has been reported to inhibit actin polymerization. HSPB3 forms a hetero-oligomeric complex with its partner small HSP HSPB2 (MKBP). It is expressed predominantly in striated and smooth muscle (heart, skeletal muscle, tongue), localizes to the cytoplasm and nucleus, and translocates to nuclear foci during heat shock. Dominant missense variants in HSPB3 cause autosomal dominant distal hereditary motor neuronopathy.
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: HSPB3: ATP-independent small heat shock protein (holdase) chaperone that binds partially unfolded/stress-destabilized client proteins and maintains them in a folding-competent state, contributing to cellular proteostasis under stress, predominantly in striated and smooth muscle. 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:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasmic localization inferred phylogenetically across the small HSP family; HSPB3 is directly documented in the cytoplasm and acts there as a sHSP chaperone. Reason: The cytoplasm is the primary site of action for this small HSP, corroborated by direct experimental evidence (PMID:19464326) and the UniProt subcellular-location record. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0016607 nuclear speck | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Nuclear speck (SC35/splicing speckle) residence inferred phylogenetically. Constitutive speckle localization is a documented hallmark of the paralog HSPB7, not robustly established as a constitutive feature of HSPB3, though HSPB3 does translocate to nuclear foci during heat shock and has IDA (HPA) support for nuclear speck. Reason: The speckle localization is family-transferred (the source study PMID:19464326 established constitutive SC35-speckle residence specifically for HSPB7). HSPB3 shows nucleus/nuclear-foci localization and an HPA IDA nuclear-speck call, so the term is retained but is peripheral to its core cytoplasmic/muscle chaperone role. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt Note=Translocates to nuclear foci during heat shock. PMID:19464326 HSPB7 constitutively localized to SC35 splicing speckles, driven by its |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation of nuclear localization, consistent with the experimentally documented (IDA) nuclear localization and heat-shock-induced nuclear foci. Reason: Agrees with stronger IDA evidence (PMID:19464326) and the UniProt subcellular-location record placing HSPB3 in the nucleus. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt Nucleus {ECO:0000269|PubMed:19464326} |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation of cytoplasmic localization, redundant with and consistent with the IBA and IDA cytoplasm annotations. Reason: Correct compartment for this cytoplasmic small HSP; agrees with stronger experimental (IDA) evidence. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0016607 nuclear speck | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Direct immunofluorescence (HPA) localization to nuclear speckles. Provides experimental support for a nuclear-speck pool of HSPB3, although this is not its principal site of action. Reason: IDA (HPA) evidence supports a nuclear-speck pool, but speckle residence is peripheral to HSPB3's core cytoplasmic/muscle small-HSP chaperone function. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt GO:0016607; C:nuclear speck; IDA:HPA. |
| 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 HSPB3 from the HSPB-family subcellular-distribution survey. Reason: IDA-supported nuclear localization, consistent with UniProt and the heat-shock-induced translocation to nuclear foci. Supporting Evidence: file:human/HSPB3/HSPB3-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 HSPB3, the primary site of action for this small HSP. Reason: IDA-supported cytoplasmic localization, the principal compartment for HSPB3's chaperone activity. Supporting Evidence: file:human/HSPB3/HSPB3-uniprot.txt Cytoplasm {ECO:0000269|PubMed:19464326} PMID:29969581 HspB2 partly co-localizes with HspB3 in the cytoplasm [6], as a [HspB2/B3] complex. PMID:29969581 we present the structure of human HspB2/B3, which crystallized as a hetero-tetramer in a 3:1 ratio...In the HspB2/B3 tetramer, the four Ξ±-crystallin domains (ACDs) assemble into a flattened tetrahedron |
| GO:0006986 response to unfolded protein | TAS PMID:8972725 Isolation and characterization of a human heart cDNA encodin... | ACCEPT | Summary: HSPB3 is a small heat shock protein of the stress-responsive sHSP family; participation in the response to unfolded protein is the core biological process for this chaperone. Reason: Family membership and the stress-response role of small HSPs support this process annotation; HSPB3 was identified and characterized as a member of the small heat shock protein family. Supporting Evidence: PMID:8972725 a new member of the small heat shock protein family file:human/HSPB3/HSPB3-uniprot.txt Belongs to the small heat shock protein (HSP20) family. PMID:29969581 This first structure of a full-length human sHsp heteromer reveals the heterogeneous interactions of the terminal regions and suggests a plasticity that is important for the cytoprotective functions of sHsps. |
| GO:0006986 response to unfolded protein | TAS PMID:9858786 HspB3, the most deviating of the six known human small heat ... | ACCEPT | Summary: Second curated TAS annotation for the stress-response role of HSPB3 as a small heat shock protein, from the paper defining the corrected HspB3 sequence. Reason: Consistent with HSPB3's identity as a small heat shock protein; redundant with the other response-to-unfolded-protein annotation but appropriately supported. Supporting Evidence: PMID:9858786 a novel human small heat shock protein (sHsp), called HspB3 |
| GO:0051291 protein heterooligomerization | IPI PMID:29969581 Terminal Regions Confer Plasticity to the Tetrameric Assembl... | NEW | Summary: The crystal structure of full-length human HspB2/HspB3 directly establishes that HSPB3 assembles with HSPB2 into a defined hetero-oligomer - a 3:1 HspB2/HspB3 hetero-tetramer whose four alpha-crystallin domains form a flattened tetrahedron. This hetero-oligomerization is the structure-distinctive functional content for HSPB3 and had no corresponding GO annotation in the GOA. Reason: Structure-motivated net-new annotation. PDB 6F2R (PMID:29969581) resolves the HspB2-HspB3 hetero-tetramer; HSPB3's assembly into this hetero-oligomer (which modulates its chaperone activity) was previously captured only in free-text and had no GO term in existing_annotations. Added here as the structure-derived gap-fill. The falcon deep-research synthesis independently frames this HSPB2:HSPB3 pairing as an obligate, fixed 3:1-stoichiometry hetero-oligomer, reinforcing that hetero-oligomerization (not homo-oligomerization) is the defining assembly behavior of HSPB3. Supporting Evidence: PMID:29969581 we present the structure of human HspB2/B3, which crystallized as a hetero-tetramer in a 3:1 ratio...In the HspB2/B3 tetramer, the four Ξ±-crystallin domains (ACDs) assemble into a flattened tetrahedron file:human/HSPB3/HSPB3-deep-research-falcon.md A defining feature of HSPB3 is its obligate hetero-oligomerization with HSPB2. HSPB3 forms characteristic heterotetrameric complexes with HSPB2 in a specific 3:1 stoichiometric ratio |
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Download this section (compressed HTML)Q: Recombinant HSPB3 alone suppresses heat-induced aggregation of alcohol dehydrogenase but not DTT-induced insulin aggregation (PMID:22610661); what client features determine this selectivity, and does hetero-oligomerization with HSPB2 change it in muscle?
Q: How does the disease-causing R7S variant in the unique N-terminal domain alter HSPB3-HSPB2 complex assembly and client handling to cause motor neuron degeneration?
Q: Is HSPB3's reported inhibition of actin polymerization a direct molecular activity or an indirect consequence of cytoskeletal-client chaperoning?
Experiment: In vitro chaperone (holdase) assays measuring suppression of thermal/chemical aggregation of model clients (e.g. citrate synthase, luciferase) by recombinant HSPB3 alone versus the HSPB2/HSPB3 complex.
Experiment: Structural and biophysical characterization (e.g. SEC-MALS, cryo-EM, SAXS) of wild-type versus R7S HSPB3 in HSPB2/HSPB3 hetero-oligomers to define how the variant perturbs assembly.
Experiment: Patient-derived or knock-in motor neuron models of HSPB3 R7S to test for impaired proteostasis, cytoskeletal defects, and aggregate accumulation.
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