HSP17.7 (also known as At-HSP17.6A in older literature; locus At5g12030) is a 17.7 kDa cytosolic class II small heat shock protein (sHSP) in Arabidopsis thaliana. Like other sHSPs, it belongs to the HSP20/alpha-crystallin family characterized by a conserved alpha-crystallin domain (ACD). sHSPs function as ATP-independent holdase chaperones: they bind unfolding or denaturing substrate proteins to prevent irreversible aggregation, maintaining them in a soluble, refoldable state, but they do not actively refold substrates. Refolding requires subsequent transfer to ATP-dependent chaperone systems such as HSP70/DnaK. HSP17.7 forms oligomeric structures typical of sHSPs. Its expression is induced by heat stress and osmotic stress, and overexpression confers enhanced salt and drought tolerance. The chaperone (holdase) activity has been demonstrated in vitro (PMID:11576425).
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.
Justification: HSP17.7: HSP17.7 is an ATP-independent holdase chaperone that binds unfolding/denaturing proteins to prevent irreversible aggregation, maintaining them in a soluble, refoldable state for downstream ATP-dependent chaperones. 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.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009408 response to heat | IBA GO_REF:0000033 | ACCEPT | Summary: HSP17.7 is a small heat shock protein whose expression is induced by heat stress (PMID:11576425). The IBA annotation is well-supported by phylogenetic inference across the HSP20 family, with extensive evidence from multiple plant sHSP orthologs. Response to heat is a core biological process for all sHSPs. Reason: Heat shock response is a defining characteristic of sHSPs. The annotation is supported by phylogenetic inference (IBA) and corroborated by direct experimental evidence from PMID:11576425, which demonstrated heat-inducible expression of this gene. Supporting Evidence: PMID:11576425 The At-HSP17.6A expression was induced by heat and osmotic stress, as well as during seed development. file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md act as **ATP-independent molecular chaperones** that bind non-native proteins during stress, **prevent aggregation**, and maintain substrates in a folding-competent state for later refolding by ATP-dependent chaperones (e.g., HSP70 systems; HSP100/ClpB-type disaggregases). |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MODIFY | Summary: GO:0051082 (unfolded protein binding) is now formally obsolete (go-ontology#30962). HSP17.7 is a classic sHSP holdase that binds unfolding proteins to prevent aggregation but does not actively refold them (PMID:11576425). Per UPB project decision rules, sHSPs should be reannotated to a holdase chaperone activity NTR when created. GO:0140309 (unfolded protein carrier activity) does not fit because it was created for carrier-holdases (TIM chaperones) that escort substrates between compartments, whereas sHSPs are in-situ holdases. Reason: sHSPs like HSP17.7 are ATP-independent holdases: they bind denaturing proteins to prevent aggregation in situ but do not actively refold them. GO:0044183 (protein folding chaperone) is inappropriate because sHSPs do not catalyze folding. GO:0140309 does not fit because it is carrier-specific (created for TIM chaperones per go-ontology#30552). A general holdase chaperone activity NTR is needed. Propagation Review Root cause: SOURCE STALE OR MISSING Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000163021 Β· plant/fungal sHSP node SOURCE STALE OR MISSING The current PTHR11527 PAINT slice carries protein folding (GO:0006457), response-to-heat and oligomerization IBDs at this node but no GO:0051082; the unfolded-protein-binding IBD appears to have been withdrawn upstream. PomBase:SPBC3E7.02c Β· fission-yeast small heat shock protein SUPPORTS TRANSFER sHSP holdase-type client binding is real; the open issue is the lack of a GO term for ATP-independent holdase activity, tracked in the review reason. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:11576425 The chaperone activity of At-HSP17.6A was demonstrated in vitro. file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md Its best-supported primary function is as an **ATP-independent βholdaseβ chaperone** that binds stress-denatured client proteins to prevent irreversible aggregation and to support subsequent refolding/disaggregation by ATP-dependent chaperone systems (e.g., HSP70/HSP101). file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md act as **ATP-independent molecular chaperones** that bind non-native proteins during stress, **prevent aggregation**, and maintain substrates in a folding-competent state for later refolding by ATP-dependent chaperones (e.g., HSP70 systems; HSP100/ClpB-type disaggregases). |
| GO:0006457 protein folding | IBA GO_REF:0000033 | ACCEPT | Summary: The annotation of HSP17.7 to GO:0006457 (protein folding) via IBA is phylogenetically inferred. sHSPs participate in the broader protein folding pathway by preventing aggregation and maintaining substrates in a refoldable state, but they do not themselves catalyze folding (that is done by downstream ATP-dependent chaperones like HSP70). The involved_in relationship captures the broader pathway participation appropriately, though it should be understood that the sHSP contribution is specifically holdase/ aggregation-prevention rather than active folding. Reason: While sHSPs do not actively fold proteins, they participate in the protein folding process by preventing irreversible aggregation and maintaining substrates in a folding-competent state. The BP annotation (involved_in protein folding) is acceptable at the process level -- the sHSP step is part of the broader protein quality control/folding pathway. The IBA annotation is phylogenetically sound for the HSP20 family. Supporting Evidence: file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md act as **ATP-independent molecular chaperones** that bind non-native proteins during stress, **prevent aggregation**, and maintain substrates in a folding-competent state for later refolding by ATP-dependent chaperones (e.g., HSP70 systems; HSP100/ClpB-type disaggregases). |
| GO:0009651 response to salt stress | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: The IBA annotation for response to salt stress is supported by phylogenetic inference. PMID:11576425 demonstrated that overproduction of At-HSP17.6A (=HSP17.7) increases salt tolerance in Arabidopsis, providing direct experimental support for this annotation in this specific gene. However, salt stress response is secondary to the core chaperone function and represents a stress-responsive phenotype rather than the primary molecular role. Reason: Salt stress response is a downstream phenotypic consequence of the holdase chaperone function rather than a core molecular activity. The gene is induced under osmotic stress and overexpression confers salt tolerance (PMID:11576425), but this reflects the protective role of the holdase rather than a salt-specific function. Kept as non-core. Supporting Evidence: PMID:11576425 Overproduction of At-HSP17.6A could increase salt and drought tolerance in Arabidopsis. |
| GO:0042542 response to hydrogen peroxide | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: The IBA annotation for response to hydrogen peroxide is phylogenetically inferred from other plant sHSP orthologs. Oxidative stress can induce sHSP expression and sHSP holdase activity can protect against oxidative damage. However, there is no direct experimental evidence for HSP17.7 specifically in hydrogen peroxide response. This is a plausible but peripheral function. Reason: Hydrogen peroxide response is likely a secondary stress-responsive role. The IBA inference is phylogenetically reasonable for the sHSP family, but it represents a peripheral stress response rather than the core holdase chaperone function. No direct experimental evidence for HSP17.7/O81822 specifically. Falcon deep research corroborates that this gene is broadly induced by oxidative/chemical stress (e.g. gallic acid, SO2) as part of a general proteostasis response, consistent with -- but not specific to -- a hydrogen peroxide response. Supporting Evidence: file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md AtHsp17.7-CII is strongly inducible not only by heat but also by oxidative/chemical stress contexts, consistent with sHSP deployment as a general proteostasis mechanism under diverse proteotoxic stresses. |
| GO:0051259 protein complex oligomerization | IBA GO_REF:0000033 | ACCEPT | Summary: sHSPs characteristically form large oligomeric complexes (typically 12-40mers) that dynamically dissociate and reassociate in response to stress. Oligomerization is fundamental to sHSP function: substrate binding occurs when the oligomer dissociates under stress. UniProt notes for HSP17.7 that it "may form oligomeric structures." The IBA annotation is well-supported across the sHSP family. Reason: Oligomerization is a core structural and functional property of sHSPs. The IBA annotation is phylogenetically sound and consistent with UniProt annotation. This is integral to chaperone mechanism, not a peripheral function. Supporting Evidence: file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md characterized by a conserved **Ξ±-crystallin domain** and typically forming oligomers. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Cytoplasmic localization is inferred from UniProt subcellular location annotation. HSP17.7 is described as a cytosolic class II sHSP in the literature (PMID:11576425), and UniProt annotates it as cytoplasmic. The more specific cytosol annotation (GO:0005829) also exists via TAS. This broader cytoplasm annotation is acceptable and consistent. Reason: Cytoplasmic localization is well-established for this cytosolic class II sHSP. The IEA annotation is consistent with the TAS annotation for cytosol and with the published characterization of this protein as a cytosolic sHSP (PMID:11576425). |
| GO:0006950 response to stress | IEA GO_REF:0000117 | ACCEPT | Summary: This IEA annotation to the general term GO:0006950 (response to stress) is computed by ARBA machine learning models. It is a very broad term, and more specific stress response terms (response to heat GO:0009408, response to salt stress GO:0009651) are already annotated. While not wrong, this general term adds little information beyond the more specific annotations. Reason: The broad IEA annotation is correct -- HSP17.7 is a stress-responsive protein. While more specific stress annotations exist, the general term from an automated pipeline is acceptable and not misleading. IEA annotations at broader levels than what is determined by IBA or literature are acceptable. |
| GO:0071456 cellular response to hypoxia | HEP PMID:31519798 Integrative Analysis from the Epigenome to Translatome Uncov... | KEEP AS NON CORE | Summary: This HEP (high-throughput expression pattern) annotation is based on PMID:31519798, a multi-omic study of Arabidopsis seedlings under hypoxia. The paper found that hypoxia promoted "a progressive upregulation of heat stress transcripts, as evidenced by RNAPII binding and increased nuclear RNA." HSP17.7 is a heat stress transcript that was progressively upregulated under hypoxia. This is an expression-based annotation with the weaker "acts_upstream_of_or_within" qualifier, reflecting that the gene is transcriptionally responsive to hypoxia rather than having a demonstrated functional role in hypoxia response. Reason: The HEP annotation reflects transcriptional upregulation during hypoxia rather than a demonstrated functional role. The paper describes progressive upregulation of heat stress genes during hypoxia (PMID:31519798), which is a secondary stress cross-talk phenomenon. This is a peripheral annotation, not a core function. Supporting Evidence: PMID:31519798 Hypoxia promoted a progressive upregulation of heat stress transcripts, as evidenced by RNAPII binding and increased nuclear RNA, with polyadenylated RNA levels only elevated after prolonged stress or reoxygenation. |
| GO:0005737 cytoplasm | ISM GO_REF:0000122 | ACCEPT | Summary: This ISM annotation is based on AtSubP (Arabidopsis Subcellular Proteome Prediction) computational analysis. Cytoplasmic localization is well-established for this cytosolic class II sHSP and is consistent with the IEA and TAS annotations. Duplicate of the IEA cytoplasm annotation but from a different computational method. Reason: Consistent with multiple other lines of evidence for cytoplasmic localization. The ISM prediction agrees with UniProt subcellular annotation, the TAS cytosol annotation, and the characterization as a cytosolic sHSP (PMID:11576425). |
| GO:0006457 protein folding | IDA PMID:11576425 At-HSP17.6A, encoding a small heat-shock protein in Arabidop... | ACCEPT | Summary: The IDA annotation to protein folding is based on the in vitro chaperone activity demonstrated in PMID:11576425. The paper showed that At-HSP17.6A (=HSP17.7, O81822) possesses chaperone activity in vitro. However, the sHSP mechanism is holdase-type: it prevents aggregation of denaturing substrates but does not actively refold them. The "acts_upstream_of_or_within" qualifier used in the GOA is appropriate, as the sHSP participates in the protein folding pathway upstream of the actual folding step. Reason: The BP annotation with "acts_upstream_of_or_within" qualifier is appropriate. sHSPs participate in the protein quality control and folding pathway by preventing aggregation and maintaining substrates for subsequent refolding by HSP70-type foldases. The in vitro chaperone assay in PMID:11576425 demonstrated this activity. While sHSPs do not themselves fold proteins, involvement in the folding process at the BP level is accurate. Supporting Evidence: PMID:11576425 The chaperone activity of At-HSP17.6A was demonstrated in vitro. |
| GO:0051082 unfolded protein binding | IDA PMID:11576425 At-HSP17.6A, encoding a small heat-shock protein in Arabidop... | MODIFY | Summary: The IDA annotation to GO:0051082 (unfolded protein binding) is based on the in vitro chaperone assay in PMID:11576425, which demonstrated that At-HSP17.6A (=HSP17.7) binds denaturing substrate proteins and prevents their aggregation. This is holdase activity -- the sHSP binds unfolding proteins without actively refolding them. Per UPB project rules, GO:0051082 is now formally obsolete and should be replaced with a holdase chaperone activity NTR when available. GO:0044183 (protein folding chaperone) is NOT appropriate for pure holdases. GO:0140309 is carrier-specific and does not fit in-situ holdases. Reason: GO:0051082 is now formally obsolete (go-ontology#30962). HSP17.7 is a classic sHSP holdase: it prevents aggregation of denaturing proteins in situ without active refolding. Per UPB project decision rules for sHSPs/holdases, it should be replaced by a holdase chaperone activity NTR. GO:0044183 is not appropriate (sHSPs do not actively fold). GO:0140309 does not fit (carrier-specific, created for TIM chaperones per go-ontology#30552). Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:11576425 The chaperone activity of At-HSP17.6A was demonstrated in vitro. |
| GO:0006972 hyperosmotic response | IMP PMID:11576425 At-HSP17.6A, encoding a small heat-shock protein in Arabidop... | KEEP AS NON CORE | Summary: The IMP annotation is based on PMID:11576425, which demonstrated that overexpression of At-HSP17.6A (=HSP17.7) enhanced salt and drought tolerance in Arabidopsis. The gene expression is induced by osmotic stress at the mRNA level, though protein accumulation was not detected under osmotic stress alone (only under heat). Overexpression conferred increased tolerance to hyperosmotic conditions, supporting the IMP evidence. The "acts_upstream_of_or_within" qualifier is appropriate. Reason: The hyperosmotic response represents a stress-protective phenotype mediated by the holdase chaperone activity rather than a core molecular function. The gene is transcriptionally induced by osmotic stress, and overexpression confers osmotolerance (PMID:11576425), but this is a downstream consequence of the general proteostasis function. Kept as non-core. Supporting Evidence: PMID:11576425 Overproduction of At-HSP17.6A could increase salt and drought tolerance in Arabidopsis. PMID:11576425 The At-HSP17.6A expression was induced by heat and osmotic stress, as well as during seed development. Accumulation of At-HSP17.6A proteins could be detected with heat and at a late stage of seed development, but not with osmotic stress |
| GO:0005829 cytosol | TAS PMID:11576425 At-HSP17.6A, encoding a small heat-shock protein in Arabidop... | ACCEPT | Summary: The TAS annotation to cytosol is based on the characterization of At-HSP17.6A (=HSP17.7) as a "cytosolic class II smHSP" in PMID:11576425. The literature consistently describes class II plant sHSPs as cytosolic proteins. This is a more specific localization than the broader cytoplasm annotations (IEA, ISM) and is well-supported. Reason: Cytosolic localization is a core feature of this class II sHSP. The TAS evidence from PMID:11576425 is consistent with the classification of At-HSP17.6A as a cytosolic class II small heat shock protein, and with the broader cytoplasm annotations from UniProt and AtSubP prediction. Supporting Evidence: PMID:11576425 the gene that encoded the cytosolic class II smHSP in Arabidopsis thaliana (At-HSP17.6A) was characterized. file:ARATH/HSP17.7/HSP17.7-deep-research-falcon.md AtHsp17.7 is consistently categorized as **cytosolic class II**, which supports a **cytosolic** site of action |
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Download this section (compressed HTML)Q: Does AtHsp17.7-CII (At5g12030) function as a regulator of the heat shock response by repressing HsfA2 in planta, and is this repression isoform-specific relative to other cytosolic class II sHSPs? Falcon deep research surfaced gene-specific reporter evidence (Port et al. 2004) that AtHsp17.7-CII selectively represses AtHsfA2 transcriptional activity while the closely related AtHsp17.6-CII does not, but a direct physical AtHsp17.7-CII/AtHsfA2 interaction was not detected in non-reporter assays. If confirmed in Arabidopsis, a GO:0043433 (negative regulation of DNA-binding transcription factor activity) annotation may be warranted.
Q: What is the developmental role of AtHsp17.7-CII in seed maturation downstream of the ABI3 -> HsfA9 cascade? Falcon evidence indicates the protein accumulates from mid- through late seed maturation and dry seeds and that accumulation is abolished in the abi3 mutant, suggesting a GO:0010431 (seed maturation) involvement that is not currently annotated.
Experiment: Test AtHsfA2-dependent reporter activity in Arabidopsis protoplasts (rather than tobacco) with titrated AtHsp17.7-CII vs AtHsp17.6-CII co-expression, and assay direct physical interaction (co-IP, BiFC, or in vitro pulldown) between purified AtHsp17.7-CII and AtHsfA2 to determine whether repression is via direct binding or an indirect mechanism.
Hypothesis: AtHsp17.7-CII acts as an isoform-specific negative regulator of the heat shock transcription factor HsfA2 in Arabidopsis.
Type: reporter assay and protein interaction assay
Experiment: Generate At5g12030 loss-of-function (T-DNA/CRISPR) and overexpression lines and assess seed viability, desiccation tolerance, and longevity, alongside immunoblot quantification of AtHsp17.7-CII accumulation across seed maturation stages in WT vs abi3 and hsfA9 backgrounds.
Hypothesis: AtHsp17.7-CII contributes to seed desiccation tolerance proteostasis as a downstream target of the ABI3/HsfA9 maturation program.
Type: reverse genetics and developmental immunoblot
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