HSP26 is the sole small heat shock protein (sHSP) of Saccharomyces cerevisiae, belonging to the conserved HSP20/alpha-crystallin superfamily. It functions as a temperature-regulated molecular chaperone with holdase activity: at heat shock temperatures, the 24-mer oligomeric storage complex dissociates, and the resulting smaller species bind non-native proteins to prevent their irreversible aggregation (PMID:10581247). This holdase activity is ATP-independent and mechanistically distinct from foldase chaperones (e.g., HSP70, GroEL). HSP26 does not actively refold substrates or escort them between compartments; rather, it maintains client proteins in a soluble, folding-competent state in situ until ATP-dependent chaperones (HSP70/HSP104) can mediate refolding. HSP26 forms large oligomeric complexes (>500 kD) and is induced by heat shock and other stress conditions. Its intracellular localization (cytoplasm vs. nucleus) varies with the metabolic state of the cell (PMID:2645298). HSP26 also localizes to stress granules during severe heat stress (PMID:24291094). Unexpectedly, Hsp26 was identified as a novel RNA-binding protein that co-purifies with a small set of specific mRNAs (PMID:20844764), though this activity is likely moonlighting rather than a core function.
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: HSP26 and other sHSPs (CRYAA, CRYAB, HSPB6 in human) are canonical in-situ holdases that cannot be properly annotated with existing GO terms. GO:0051082 is now formally obsolete, GO:0044183 implies active refolding, and GO:0140309 is carrier-specific. See UPB project and go-ontology#30552.
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006457 protein folding | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: IEA annotation from ARBA machine learning models mapping HSP26 to protein folding. HSP26 is an sHSP holdase that prevents aggregation of unfolded proteins but does not actively assist protein folding (refolding requires the HSP70/HSP104 disaggregase system). The term "protein folding" (GO:0006457) implies active assistance in acquiring correct tertiary structure, which does not match HSP26 holdase mechanism. However, this IEA is broader than the IDA annotation for the same term (see below), and both share the same concern. Reason: HSP26 is a holdase, not a foldase. It prevents aggregation of non-native proteins but does not actively catalyze protein folding. The GO:0006457 definition states "assisting in the covalent and noncovalent assembly of single chain polypeptides or multisubunit complexes into the correct tertiary structure," which better describes ATP-dependent foldase chaperones like GroEL/ES or HSP70. HSP26 maintains substrates in a folding-competent state for later refolding by other chaperones. The IEA mapping is too generous here. A more appropriate BP annotation would be to a process term reflecting prevention of protein aggregation under stress. Supporting Evidence: PMID:10581247 Hsp26, an sHsp from Saccharomyces cerevisiae, is a temperature-regulated molecular chaperone. Like other sHsps, Hsp26 forms large oligomeric complexes. At heat shock temperatures, however, the 24mer chaperone complex dissociates. ... Binding of non-native proteins to dissociated Hsp26 produces large globular assemblies |
| GO:0005515 protein binding | IPI PMID:16429126 Proteome survey reveals modularity of the yeast cell machine... | MARK AS OVER ANNOTATED | Summary: Protein binding annotation from the Gavin et al. (2006) large-scale TAP-tag affinity purification/mass spectrometry study of yeast protein complexes. HSP26 was identified as co-purifying with SSA2 (P10592) and VMA2 (P16140) in this study. While HSP26 does physically interact with proteins (both substrates and other chaperones), GO:0005515 "protein binding" is uninformative and does not capture the mechanistic nature of these interactions. Reason: GO:0005515 "protein binding" is too vague. For a chaperone like HSP26, the relevant molecular function is its holdase activity -- binding unfolded/misfolded proteins to prevent aggregation. The interactions detected in large-scale co-purification studies may reflect chaperone-substrate relationships (e.g., with SSA2, an HSP70 family member) or other associations. A more informative annotation would capture the holdase mechanism. Per curation guidelines, "protein binding" should be avoided in favor of more specific terms. Supporting Evidence: PMID:16429126 Protein complexes are key molecular entities that integrate multiple gene products to perform cellular functions. Here we report the first genome-wide screen for complexes in an organism, budding yeast, using affinity purification and mass spectrometry. |
| GO:0005515 protein binding | IPI PMID:16554755 Global landscape of protein complexes in the yeast Saccharom... | MARK AS OVER ANNOTATED | Summary: Protein binding annotation from Krogan et al. (2006) large-scale TAP-tag study of yeast protein complexes. Multiple interaction partners were detected (ADH3, EMW1, FUS3, NEW1, POL32, RNR1, RTT101, SGV1, SME1, TFB4, UPC2, VMA2). As a chaperone, HSP26 is expected to interact with many proteins, but GO:0005515 does not meaningfully describe the nature of these interactions. Reason: Same rationale as above. GO:0005515 is uninformative for a chaperone. Many of these interaction partners may represent chaperone substrates or indirect associations from large-scale co-purification. HSP26 was classified as a "specific chaperone" with fewer than 200 non-chaperone interactors in the comprehensive chaperone atlas (PMID:19536198), consistent with its selective holdase function. Supporting Evidence: PMID:19536198 the specific chaperones are (ordered from low to high number of interactors): Hsp32, Sno4, Hsp33, Tim14, Mdj2, Jac1, Cwc23, Jid1, Mcx1, Mdj1, Jjj2, Hlj1, Jjj3, Erj5, Ssc3, Kar2, Lhs1, Hsp26, Jem1, Jjj1, Xdj1, Hsp12, Hsp60 |
| GO:0005515 protein binding | IPI PMID:19536198 An atlas of chaperone-protein interactions in Saccharomyces ... | MARK AS OVER ANNOTATED | Summary: Protein binding annotation from Gong et al. (2009), a comprehensive TAP-tag based chaperone interaction atlas covering all 63 yeast chaperones. HSP26 was classified as a "specific chaperone" with fewer than 200 interactors. Multiple interaction partners were detected. This study provides excellent systems-level context for HSP26 as a chaperone but GO:0005515 remains uninformative. Reason: GO:0005515 does not capture the chaperone-substrate nature of these interactions. This study specifically characterizes HSP26 as a molecular chaperone with selective substrate binding. A more specific MF term reflecting holdase activity would be more appropriate. Supporting Evidence: PMID:19536198 Molecular chaperones are defined as a group of highly interactive proteins that modulate the folding and unfolding of other proteins ... We expect that most of the chaperone interactors represent putative substrates whose conformational stability is modulated by molecular chaperones |
| GO:0042802 identical protein binding | IPI PMID:16843901 Multiple distinct assemblies reveal conformational flexibili... | ACCEPT | Summary: Identical protein binding annotation based on White et al. (2006), which used cryo-EM to characterize Hsp26 oligomeric assemblies. The study demonstrated that Hsp26 forms 24-subunit assemblies with tetrahedral symmetry, composed of asymmetric dimers, confirming self-association. This homo-oligomerization is integral to Hsp26 chaperone function. Reason: HSP26 homo-oligomerization is well-established and functionally critical. The 24-mer complex is the storage form that dissociates at heat shock temperatures to become the active holdase species. The cryo-EM structural data directly demonstrates identical protein binding through dimer and trimer contacts. While GO:0042802 is somewhat generic, it correctly captures the self-association that is a defining structural feature of sHSPs. Supporting Evidence: PMID:16843901 Cryo-electron microscopy of yeast Hsp26 reveals two distinct forms, each comprising 24 subunits arranged in a porous shell with tetrahedral symmetry. The subunits form elongated, asymmetric dimers that assemble via trimeric contacts. PMID:10581247 Like other sHsps, Hsp26 forms large oligomeric complexes. At heat shock temperatures, however, the 24mer chaperone complex dissociates. |
| GO:0042802 identical protein binding | IPI PMID:18719252 High-quality binary protein interaction map of the yeast int... | ACCEPT | Summary: Identical protein binding annotation from Yu et al. (2008), a high-quality binary yeast interactome study. HSP26-HSP26 self-interaction was detected, consistent with the known oligomeric nature of Hsp26. Reason: This binary interaction study independently confirms HSP26 homo-oligomerization, which is consistent with the well-characterized 24-mer structure from cryo-EM (PMID:16843901). Duplicate evidence for the same GO term is appropriate when from independent experimental sources. Supporting Evidence: PMID:16843901 Cryo-electron microscopy of yeast Hsp26 reveals two distinct forms, each comprising 24 subunits arranged in a porous shell with tetrahedral symmetry. |
| GO:0010494 cytoplasmic stress granule | HDA PMID:26777405 ATPase-Modulated Stress Granules Contain a Diverse Proteome ... | ACCEPT | Summary: HDA annotation from Jain et al. (2016) large-scale proteomic analysis of stress granule cores. This study used super-resolution microscopy and mass spectrometry to characterize stress granule composition, identifying molecular chaperones as conserved stress granule components. Reason: Stress granule localization is consistent with HSP26 function as a stress-responsive chaperone. The Jain et al. study is a rigorous proteomic analysis of stress granule composition, and the finding is independently supported by direct IDA evidence from Cherkasov et al. (PMID:24291094), which showed heat stress granules contain molecular chaperones that coassemble with aggregates of misfolded proteins. Supporting Evidence: PMID:26777405 Proteomic analysis of stress granule cores reveals a dense network of protein-protein interactions and links between stress granules and human diseases and identifies ATP-dependent helicases and protein remodelers as conserved stress granule components. |
| GO:0005739 mitochondrion | HDA PMID:24769239 Quantitative variations of the mitochondrial proteome and ph... | KEEP AS NON CORE | Summary: HDA annotation from Renvoise et al. (2014) quantitative mitochondrial proteomics study. HSP26 was detected in isolated mitochondrial fractions by mass spectrometry. However, HSP26 is primarily a cytoplasmic protein, and mitochondrial detection in large-scale proteomics may reflect contamination or transient association rather than bona fide mitochondrial localization. Reason: HSP26 is primarily cytoplasmic and nuclear (PMID:2645298), with stress granule localization under heat stress. Detection in mitochondrial fractions from a quantitative proteomics study could represent genuine low-level mitochondrial association (e.g., chaperoning mitochondrial surface proteins during stress) or co-purification artifact. UniProt does not list mitochondrial localization. This is not a core localization but may represent a minor or transient association, so it should be kept as non-core rather than removed. Supporting Evidence: PMID:24769239 Label free quantitative analysis of protein accumulation revealed significant variation of 176 mitochondrial proteins PMID:2645298 When log-phase cells growing in glucose were heat shocked, hsp26 concentrated in nuclei ... in early stationary-phase cells hsp26 is induced at normal growth temperatures. This protein was generally distributed throughout the cells |
| GO:0005737 cytoplasm | HDA PMID:11914276 Subcellular localization of the yeast proteome. | ACCEPT | Summary: HDA annotation from Kumar et al. (2002) proteome-wide subcellular localization study using epitope-tagged proteins and immunolocalization. HSP26 was detected in the cytoplasm. Reason: Cytoplasmic localization is well-established for HSP26 and is its primary localization. This is confirmed by multiple independent studies including direct IDA evidence (PMID:2645298) and the UniProt record. The large-scale localization study provides additional support. Supporting Evidence: PMID:11914276 Here, we report the first proteome-scale analysis of protein localization within any eukaryote. ... we have determined the subcellular localization of 2744 yeast proteins. PMID:2645298 in early stationary-phase cells hsp26 is induced at normal growth temperatures. This protein was generally distributed throughout the cells |
| GO:0010494 cytoplasmic stress granule | IDA PMID:24291094 Coordination of translational control and protein homeostasi... | ACCEPT | Summary: IDA annotation from Cherkasov et al. (2013), which demonstrated that yeast heat stress granules contain molecular chaperones and coassemble with aggregates of misfolded proteins. The study showed that heat-SG disassembly requires Hsp104 and Hsp70 activity. Reason: Direct experimental evidence for HSP26 localization to stress granules during severe heat stress. This is mechanistically coherent with HSP26 holdase function, as stress granules form under conditions where protein aggregation occurs and translation is inhibited. The colocalization with misfolded protein aggregates is especially relevant to HSP26 chaperone activity. Supporting Evidence: PMID:24291094 both S. cerevisiae and D. melanogaster heat-SGs contain mRNA, translation machinery components (excluding ribosomes), and molecular chaperones and that heat-SGs coassemble with aggregates of misfolded, heat-labile proteins |
| GO:0034605 cellular response to heat | IDA PMID:10581247 Hsp26: a temperature-regulated chaperone. | ACCEPT | Summary: IDA annotation from Haslbeck et al. (1999), the seminal study demonstrating HSP26 is a temperature-regulated chaperone. The study showed that Hsp26 chaperone activity is temperature-dependent: the 24-mer dissociates at heat shock temperatures, and this dissociation is a prerequisite for efficient chaperone activity. Reason: HSP26 is one of the major proteins produced on heat shock (UniProt) and its chaperone activity is directly activated by elevated temperature through oligomer dissociation. This is a core biological process for HSP26 -- it is a heat shock protein whose functional activation is temperature-regulated. The evidence directly demonstrates temperature-dependent functional changes. Supporting Evidence: PMID:10581247 Hsp26, an sHsp from Saccharomyces cerevisiae, is a temperature-regulated molecular chaperone. ... the dissociation of the Hsp26 complex at heat shock temperatures is a prerequisite for efficient chaperone activity |
| GO:0003729 mRNA binding | IDA PMID:20844764 Proteome-wide search reveals unexpected RNA-binding proteins... | KEEP AS NON CORE | Summary: IDA annotation from Tsvetanova et al. (2010), a proteome-wide search for RNA-binding proteins using protein microarrays and IP-microarray validation. HSP26 was identified as a novel RBP that co-purified with 279 specific mRNAs (FDR less than 0.01%). The study describes HSP26 as a "chaperone; heat shock response" protein and notes it "co-purified with smaller sets of mRNAs, but these small sets of putative mRNA targets shared distinct functional and/or cytotopical themes." HSP26 lacks any known RNA-binding domain. Reason: While the experimental evidence is sound (validated by IP-microarray with replicate experiments and stringent FDR cutoffs), mRNA binding is likely a moonlighting activity rather than a core function of HSP26. HSP26 lacks canonical RNA-binding domains, and the authors themselves describe it among "unexpected" RNA-binding proteins. The primary function of HSP26 is as a holdase chaperone for unfolded proteins. The mRNA binding may reflect the broad substrate-binding properties of chaperones, or a secondary regulatory role. Many chaperones have been found to associate with RNA in proteome-wide screens. Supporting Evidence: PMID:20844764 Other candidate RBPs (Vtc1, Arc15, Hsp26, Arp8, Gis2) co-purified with smaller sets of mRNAs, but these small sets of putative mRNA targets shared distinct functional and/or cytotopical themes, increasing our confidence that the RNA-protein interactions we observed were genuine. |
| GO:0005634 nucleus | IDA PMID:2645298 The intracellular location of yeast heat-shock protein 26 va... | KEEP AS NON CORE | Summary: IDA annotation from Rossi and Lindquist (1989), which showed that HSP26 intracellular localization varies with the metabolic state of the cell. In log-phase cells growing in glucose after heat shock, HSP26 concentrated in nuclei. However, this nuclear concentration was condition-dependent and did not occur in cells grown in galactose or acetate, or in mitochondrial mutants. Reason: Nuclear localization of HSP26 is condition-dependent rather than constitutive. HSP26 concentrates in nuclei only in log-phase glucose-grown cells after heat shock; under other metabolic conditions it remains generally distributed throughout the cell. This represents a conditional localization rather than a core cellular component, so it should be retained but marked as non-core. Supporting Evidence: PMID:2645298 When log-phase cells growing in glucose were heat shocked, hsp26 concentrated in nuclei and continued to concentrate in nuclei when these cells were returned to normal temperatures for recovery. However, hsp26 did not concentrate in nuclei under a variety of other conditions. PMID:2645298 We conclude that the intracellular location of hsp26 in yeast depends upon the physiological state of the cell and not simply upon the presence or absence of heat stress. |
| GO:0005737 cytoplasm | IDA PMID:2645298 The intracellular location of yeast heat-shock protein 26 va... | ACCEPT | Summary: IDA annotation from Rossi and Lindquist (1989) demonstrating cytoplasmic localization of HSP26. The protein was generally distributed throughout the cytoplasm, particularly in stationary-phase cells and under most metabolic conditions. Reason: Cytoplasm is the primary and constitutive localization of HSP26. Under most physiological conditions, HSP26 is distributed throughout the cytoplasm. This is the core cellular component for HSP26 function as a cytoplasmic holdase chaperone. Supporting Evidence: PMID:2645298 in early stationary-phase cells hsp26 is induced at normal growth temperatures. This protein was generally distributed throughout the cells, even after heat shock. |
| GO:0006457 protein folding | IDA PMID:10581247 Hsp26: a temperature-regulated chaperone. | MODIFY | Summary: IDA annotation from Haslbeck et al. (1999) based on in vitro chaperone assays demonstrating that Hsp26 protects proteins from irreversible aggregation. The term "protein folding" is applied here, but the actual experimental evidence shows holdase activity (aggregation prevention), not active refolding. sHSPs maintain substrates in a folding-competent state but require HSP70/HSP104 for actual refolding. Reason: The experimental evidence in PMID:10581247 demonstrates that HSP26 prevents irreversible aggregation of non-native proteins. This is holdase activity, not protein folding per se. GO:0006457 "protein folding" implies active assistance in acquiring correct tertiary structure, which better describes ATP-dependent foldases. HSP26 functions upstream of refolding by maintaining substrates in a soluble, folding-competent state. A more accurate BP annotation would be "chaperone-mediated protein complex assembly" (GO:0051131) or ideally a BP term for "prevention of protein aggregation," though no such specific term currently exists. As an interim measure, "protein folding" is overly generous but not entirely wrong since HSP26 participates in the broader proteostasis pathway that includes folding. Proposed replacements: chaperone-mediated protein complex assembly Supporting Evidence: PMID:10581247 Several sHsps have been shown to exhibit chaperone activity and protect proteins from irreversible aggregation in vitro. ... Binding of non-native proteins to dissociated Hsp26 produces large globular assemblies ... In this complex one monomer of substrate is bound per Hsp26 dimer. |
| GO:0051082 unfolded protein binding | IDA PMID:10581247 Hsp26: a temperature-regulated chaperone. | MODIFY | Summary: IDA annotation from Haslbeck et al. (1999), the key study demonstrating HSP26 chaperone activity. The study showed that at heat shock temperatures, Hsp26 24-mer dissociates and the resulting active species binds non-native proteins, forming large chaperone-substrate complexes with a 1:2 substrate:Hsp26 stoichiometry. This is canonical holdase activity: binding unfolded/misfolded proteins to prevent their irreversible aggregation, without actively catalyzing refolding. HSP26 is ATP-independent and does not escort substrates between cellular compartments. Per the UPB project, GO:0051082 is now formally obsolete (go-ontology#30962). The appropriate replacement is a general "holdase chaperone activity" NTR. GO:0140309 "unfolded protein carrier activity" does NOT fit because it was created specifically for TIM carrier-holdases that escort proteins between compartments (go-ontology#30552), and HSP26 is an in-situ holdase. Reason: HSP26 is a canonical in-situ holdase chaperone. GO:0051082 "unfolded protein binding" is now formally obsolete and is too vague -- it is a binding term that does not distinguish holdase from foldase from sensor mechanisms. The best replacement would be the proposed "holdase chaperone activity" NTR (an activity term for ATP-independent prevention of protein aggregation in situ). GO:0140309 "unfolded protein carrier activity" does NOT fit because (1) its definition requires escort "between two different cellular components," (2) it is a child of GO:0140597 "protein carrier chaperone," and (3) HSP26 prevents aggregation in situ without transporting substrates between compartments. GO:0044183 "protein folding chaperone" is also inappropriate because HSP26 does not actively fold proteins. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:10581247 Several sHsps have been shown to exhibit chaperone activity and protect proteins from irreversible aggregation in vitro. Here we show that Hsp26, an sHsp from Saccharomyces cerevisiae, is a temperature-regulated molecular chaperone. PMID:10581247 Binding of non-native proteins to dissociated Hsp26 produces large globular assemblies with a structure that appears to be completely reorganized relative to the original Hsp26 oligomers. In this complex one monomer of substrate is bound per Hsp26 dimer. PMID:16843901 Hinge points between the domains allow a variety of assembly contacts, providing the flexibility required for formation of supercomplexes with non-native proteins. file:yeast/HSP26/HSP26-deep-research-falcon.md Hsp26 is an ATP-independent holdase that binds nonnative proteins and suppresses aggregation during heat stress. |
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Download this section (compressed HTML)Q: What are the specific substrate proteins that HSP26 binds during heat shock in vivo?
Q: Does HSP26 mRNA binding activity (PMID:20844764) have a physiological role, or is it an artifact of the broad substrate-binding properties of chaperones?
Q: How does HSP26 holdase activity coordinate with HSP104 disaggregase and HSP70 foldase during stress recovery?
Q: Is the temperature-dependent oligomer dissociation mechanism conserved across yeast sHSPs (e.g., Hsp42)?
Experiment: In vivo identification of HSP26 substrates by stress-induced cross-linking immunoprecipitation (e.g., crosslink-MS in heat-shocked cells) to define the physiological holdase clientele and compare with the substrate spectrum of HSP42 and HSP104.
Experiment: Reconstitute the HSP26 -> HSP70 (Ssa) -> HSP104 substrate hand-off in vitro with defined model substrates (e.g., luciferase, MDH) to determine whether HSP26 holdase complexes are productively channeled into the disaggregase/refolding pathway and to quantify the rate enhancement over spontaneous reactivation.
Experiment: Test whether HSP26 mRNA-binding (PMID:20844764) has functional consequences by CLIP-seq in heat-shocked cells with and without an HSP26 oligomer-dissociation mutant, and by measuring translation/stability of candidate target mRNAs in hsp26 deletion vs. wild-type.
Experiment: Use temperature-sensitive HSP26 oligomer-interface variants to test whether the 24-mer to active-species transition is required for stress granule recruitment (PMID:24291094) and whether stress granule HSP26 colocalizes with misfolded protein aggregates in real time by live-cell fluorescence imaging.
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