HSP-16.2 is a small heat shock protein (sHSP) belonging to the alpha-crystallin/HSP20 family. It functions as an ATP-independent molecular chaperone with holdase activity, binding to unfolded or misfolded proteins to prevent their aggregation under stress conditions. The protein is strongly induced by heat shock and other stressors, and is expressed in multiple tissues including pharynx, muscle, hypodermis, and intestine. HSP-16.2 maintains client proteins in a refolding-competent state until ATP-dependent chaperones (like HSP70) can catalyze their refolding. It is part of a family of four hsp16 genes in C. elegans arranged in divergently transcribed pairs at two loci.
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: hsp-16.2: HSP-16.2 binds to unfolded proteins through its conserved alpha-crystallin domain, maintaining them in a refolding-competent state. 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:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasmic localization is well-supported for sHSPs across species. The IBA annotation is based on phylogenetic inference from multiple orthologs including mammalian alpha-crystallins and HSP27/HSPB1. Direct experimental evidence in C. elegans also confirms cytoplasmic localization (PMID:11001875). Reason: Small heat shock proteins are predominantly cytoplasmic, where they function as holdase chaperones to prevent protein aggregation. This is consistent with phylogenetic inference and direct experimental evidence. Supporting Evidence: PMID:11001875 Immunohistochemical data on 10 of the 14 small heat-shock (smHSPs) proteins in fourth larval stage and adult Caenorhabditis elegans show that the tissues expressing the greatest number of smHSPs are vulva (HSP12s, HSP43 and, under stress, HSP16s) and spermatheca (HSP12s, HSP25, HSP43 and, under stress, HSP16s) file:worm/hsp-16.2/hsp-16.2-deep-research-falcon.md model: Edison Scientific Literature |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Nuclear localization has been reported for some mammalian sHSPs such as HSPB1 and alpha-crystallins under stress conditions. The IBA annotation propagates this from orthologs. However, direct evidence for nuclear localization of hsp-16.2 specifically in C. elegans is limited. Reason: While the primary localization is cytoplasmic, nuclear translocation of sHSPs under stress is a conserved feature across species. The phylogenetic inference from mammalian orthologs is reasonable, though direct C. elegans data would strengthen this annotation. |
| GO:0009408 response to heat | IBA GO_REF:0000033 | ACCEPT | Summary: Heat shock inducibility is a defining characteristic of the hsp16 gene family. Multiple studies confirm that hsp16 genes are transcriptionally induced by heat shock through heat shock elements in their promoters regulated by HSF-1. Reason: Response to heat is a core, conserved function of the hsp16 family. This is supported by extensive phylogenetic data and direct experimental evidence in C. elegans (PMID:1550963, PMID:28198373). Supporting Evidence: PMID:1550963 Transcription of the hsp16-lacZ transgenes was totally heat-shock dependent and resulted in the rapid synthesis of detectable levels of beta-galactosidase PMID:3017958 Each gene encodes a 16-kDa polypeptide which is expressed following heat induction |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | REMOVE | Summary: This annotation is mechanistically INCORRECT. Small heat shock proteins like hsp-16.2 do NOT catalyze protein refolding. They function as ATP-independent holdases that bind unfolded proteins to prevent aggregation, maintaining substrates in a refolding-competent state. Actual protein refolding requires ATP-dependent chaperones like HSP70 (DnaK) working with co-chaperones. The sHSP acts upstream, sequestering substrates until the refolding machinery becomes available. This annotation incorrectly describes sHSP function. Reason: sHSPs are ATP-independent holdases, not foldases. They prevent aggregation and maintain proteins in a refolding-competent state, but do not directly catalyze refolding. The holdase molecular function (proposed holdase chaperone activity NTR) and the biological process contributions to heat stress response already capture the accurate functional role. The "protein refolding" annotation should be removed as it is mechanistically inaccurate. |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MODIFY | Summary: Unfolded protein binding is the core molecular function of small heat shock proteins. sHSPs bind to partially unfolded or misfolded proteins through hydrophobic interactions, preventing their aggregation. This is a well-conserved function across the alpha-crystallin/HSP20 family. Reason: GO:0051082 is now formally obsolete. HSP-16.2 is an sHSP holdase that binds unfolded proteins to prevent aggregation in an ATP-independent manner. GO:0140309 (unfolded protein carrier activity) is not appropriate because it is carrier-specific (per go-ontology#30552). The replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). Proposed replacements: holdase chaperone activity |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine learning prediction of cytoplasmic localization, which is consistent with the IBA and IDA annotations for the same term. Reason: Redundant with other evidence but correct. Cytoplasmic localization is well-established for this protein family. |
| GO:0009408 response to heat | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine learning prediction consistent with experimental evidence. Reason: Redundant with IBA and IEP annotations but correct. Heat shock response is a core function. |
| GO:0009408 response to heat | IEP PMID:28198373 Hormetic heat stress and HSF-1 induce autophagy to improve s... | ACCEPT | Summary: This annotation is based on expression pattern data showing hsp-16.2 induction following hormetic heat shock. The study demonstrates that heat stress induces autophagy and HSP expression for improved survival and proteostasis. Reason: Heat-inducible expression is well-documented for the hsp16 gene family in C. elegans. IEP (inferred from expression pattern) is appropriate evidence for this biological process annotation. Supporting Evidence: PMID:28198373 Hormetic heat stress and HSF-1 induce autophagy to improve survival and proteostasis in C. elegans |
| GO:0009408 response to heat | IEP PMID:1550963 Temporal and spatial expression patterns of the small heat s... | ACCEPT | Summary: Classic study demonstrating heat shock-dependent expression of hsp16 genes using transgenic reporter constructs. Shows spatial and temporal expression patterns of hsp16 genes following heat shock. Reason: Foundational study establishing heat-inducible expression of hsp16 genes. The hsp16-lacZ fusion experiments clearly demonstrate heat shock dependence. Supporting Evidence: PMID:1550963 Transcription of the hsp16-lacZ transgenes was totally heat-shock dependent and resulted in the rapid synthesis of detectable levels of beta-galactosidase |
| GO:0051082 unfolded protein binding | ISS PMID:3017958 Structure, expression, and evolution of a heat shock gene lo... | MODIFY | Summary: Sequence similarity-based annotation. The hsp16 genes belong to the well-characterized small heat shock protein family with conserved alpha-crystallin domain. While PMID:3017958 primarily describes gene structure and expression, the annotation is based on sequence similarity to proteins with established unfolded protein binding activity. Reason: The ISS inference is sound (highly conserved alpha-crystallin domain, and recombinant HSP16-2 suppresses thermal aggregation of citrate synthase, PMID:9305934), but GO:0051082 is now formally obsolete and GO:0140309 keeps a carrier-specific definition (go-ontology#30552). As for the IBA row, the replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:3017958 the results presented here define a family of four distinct, related small heat shock protein genes |
| GO:0005737 cytoplasm | IDA PMID:11001875 Association of several small heat-shock proteins with reprod... | ACCEPT | Summary: Direct experimental evidence for cytoplasmic localization from immunohistochemical studies. The study examined tissue distribution of multiple sHSPs in C. elegans using antibodies, providing direct evidence for localization patterns. Reason: IDA evidence based on immunohistochemistry provides the strongest support for cytoplasmic localization. This annotation anchors the other cytoplasm annotations based on computational inference. Supporting Evidence: PMID:11001875 Immunohistochemical data on 10 of the 14 small heat-shock (smHSPs) proteins in fourth larval stage and adult Caenorhabditis elegans show that the tissues expressing the greatest number of smHSPs are vulva (HSP12s, HSP43 and, under stress, HSP16s) and spermatheca (HSP12s, HSP25, HSP43 and, under stress, HSP16s) |
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Download this section (compressed HTML)Q: Does hsp-16.2 form oligomeric complexes and if so what is the stoichiometry?
Q: What is the substrate specificity of hsp-16.2 compared to other C. elegans sHSPs?
Experiment: In vitro holdase assay using purified hsp-16.2 to directly demonstrate prevention of protein aggregation (e.g., citrate synthase or luciferase aggregation assays).
Hypothesis: HSP-16.2 prevents aggregation of thermosensitive client proteins in an ATP-independent manner, functioning as a holdase chaperone.
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