Hsp20 (Saci_0922) is a 173-amino acid (19.9 kDa) small heat shock protein (sHSP) of the alpha-crystallin/Hsp20 family from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. It exhibits remarkable oligomeric plasticity, existing as a ~24-mer at room temperature that shifts to higher oligomeric forms at elevated temperature and low pH, while the dimer is the functional substrate-binding conformation. Hsp20 protects against stress-induced protein aggregation and additionally interacts with membrane lipids via hydrophobic interactions to stabilize membranes by lowering the propensity of lipid phase transitions. Together with Hsp14 and the group II chaperonin (thermosome/Hsp60), Hsp20 constitutes the core chaperone machinery of S. acidocaldarius, which lacks Hsp70, Hsp90, and Hsp100.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0051082
unfolded protein binding
|
IDA
PMID:30293966 The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldar... |
NEW |
Summary: Hsp20 binds unfolded and aggregating substrate proteins in its dimeric active form. The ~24-mer is a storage form and the dimer is the active substrate-binding conformation, with oligomeric plasticity regulated by the hydrophobic microenvironment.
Reason: Hsp20 directly binds aggregating substrate proteins as demonstrated by in vitro chaperone activity assays showing the dimer as the functional conformation.
Supporting Evidence:
PMID:30293966
we identified a dimeric form of protein as the functional conformation in the presence of aggregating substrate proteins
PMID:30293966
it plays a key role in the protection of stress-induced protein aggregation
|
|
GO:0008289
lipid binding
|
IDA
PMID:30293966 The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldar... |
NEW |
Summary: Hsp20 interacts with membrane lipids via hydrophobic interactions and lowers the propensity of lipid phase transitions, stabilizing membranes under stress conditions. This is distinct from its protein chaperone activity.
Reason: Direct lipid binding was demonstrated by biophysical assays showing hydrophobic interaction with membrane lipids and modulation of membrane fluidity.
Supporting Evidence:
PMID:30293966
Hsp20 interacts with membrane lipids via a hydrophobic interaction
PMID:30293966
it lowers the propensity of in vitro phase transition of bacterial and archaeal lipids
|
|
GO:0006457
protein folding
|
IDA
PMID:30293966 The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldar... |
NEW |
Summary: Hsp20 participates in protein folding by capturing aggregation-prone substrates, which are subsequently transferred via Hsp14 to the thermosome (Hsp60) for ATP-dependent refolding.
Reason: Hsp20 is part of the sHSP-to-thermosome protein folding pathway in S. acidocaldarius.
Supporting Evidence:
PMID:30293966
it plays a key role in the protection of stress-induced protein aggregation
PMID:34637594
Hsp14 could transfer sHsp-captured substrate proteins to Hsp60, which then refolds them back to their active form
|
|
GO:0034605
cellular response to heat
|
IDA
PMID:37516156 Heat shock response in Sulfolobus acidocaldarius and first i... |
NEW |
Summary: Hsp20 is upregulated under heat shock and plays a crucial role in the heat stress response of S. acidocaldarius.
Reason: Transcriptomic and qRT-PCR analyses demonstrated upregulation of hsp20 under heat shock (92 degrees C) and other stresses.
Supporting Evidence:
PMID:37516156
The results demonstrated that the gene thβ encoding the β subunit of the thermosome, as well as hsp14 and hsp20, play crucial roles in the majority of stress conditions
PMID:32562000
a dynamic increase in mRNA levels of all relevant heat shock proteins
|
|
GO:0005737
cytoplasm
|
IDA
PMID:30293966 The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldar... |
NEW |
Summary: Hsp20 is a cytoplasmic sHSP that interacts with other cytoplasmic chaperones (Hsp14 and Hsp60/thermosome).
Reason: sHSPs are cytoplasmic proteins; Hsp20 was purified from and characterized in the cytoplasmic fraction.
Supporting Evidence:
PMID:30293966
we identified a dimeric form of protein as the functional conformation in the presence of aggregating substrate proteins
|
Q: What is the physiological significance of Hsp20 in secretory vesicles - does it stabilize vesicle membranes or protect cargo proteins?
Q: Does Hsp20 have holdase activity independent of the Hsp14-mediated transfer pathway, or are all captured substrates ultimately routed through Hsp14 to the thermosome?
Hsp20 (UniProt Q4JA95) in Sulfolobus acidocaldarius is a small heat shock protein (sHSP) of approximately 20 kDa belonging to the HSP20 family. Small heat shock proteins are ubiquitous ATP-independent molecular chaperones found across all domains of life (www.frontiersin.org). They characteristically contain an α-crystallin domain (≈90 amino acids) and typically form oligomeric complexes (often 9–50 subunits) (pmc.ncbi.nlm.nih.gov). Despite their small size, sHSPs play a crucial role in cellular stress responses by binding unfolded or misfolded proteins and preventing their irreversible aggregation (www.frontiersin.org) (pmc.ncbi.nlm.nih.gov). This “holdase” activity keeps client proteins in a folding-competent state until they can be refolded by ATP-dependent chaperones. In the thermoacidophilic archaeon S. acidocaldarius, which thrives at ~75–80 °C and pH 2–3, maintaining protein homeostasis under extreme heat and acidity is vital (www.sciencedirect.com). The Hsp20 protein is one of the primary stress-responsive chaperones in this organism, acting as a first line of defense against proteotoxic stress (www.sciencedirect.com). It is a cytosolic protein (no signal peptide for secretion), but notably has also been detected in membrane-derived extracellular vesicles of Sulfolobus, hinting at additional roles beyond the cytosol (www.frontiersin.org).
Like other HSP20-family members, S. acidocaldarius Hsp20 contains the conserved α-crystallin domain flanked by flexible N- and C-terminal regions. These regions enable oligomerization and client binding. Biophysical studies (Roy et al., 2018) showed that purified Hsp20 predominantly assembles into a large oligomer (~24 subunits) under moderate conditions (pubmed.ncbi.nlm.nih.gov). This oligomeric “storage” form is highly dynamic: subunits exchange in and out, and the complex can dissociate into smaller species depending on environmental conditions (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). At higher temperatures (50–70 °C) or lower pH (3–5) – conditions that increase solvent hydrophobicity – Hsp20 undergoes “oligomeric plasticity,” dissociating from the 24-mer into smaller assemblies (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). Strikingly, the active chaperone form appears to be a dimer: in the presence of unfolding substrate proteins, Hsp20 was observed to shift into dimeric units that expose hydrophobic interfaces for client binding (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). This reversible oligomerization mechanism allows Hsp20 to rapidly adjust its conformation and activity in response to stress-induced changes in the cellular environment. Such environment-induced oligomeric transitions are now recognized as a crucial feature of archaeal sHSP function (www.frontiersin.org).
Hsp20 in S. acidocaldarius is one of at least two sHSPs in this species – the other being a ~14 kDa Hsp14. Recent research indicates these two chaperones can interact. At normal growth temperatures (~75 °C), Hsp20 and Hsp14 exist mostly as separate homo-oligomers. However, at severe heat stress (≥50–70 °C) they are capable of exchanging subunits to form hetero-oligomeric complexes (pubmed.ncbi.nlm.nih.gov). Roy et al. (2022) demonstrated that Hsp14 and Hsp20 rapidly form mixed oligomers at high temperatures, suggesting a cooperative defense mechanism under extreme conditions (pubmed.ncbi.nlm.nih.gov). Both chaperones store as oligomers and activate as dimers, and this partnership may enhance the capacity to capture a wide range of unfolding proteins during acute stress (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). The ability to form hetero-oligomers is relatively unique and highlights the plasticity of the archaeal sHSP system in adapting to escalating stress.
Hsp20’s primary function is as an ATP-independent molecular chaperone safeguarding the proteome during stress. Upon heat shock or other stress, proteins begin to unfold or misfold, risking aggregation. Hsp20 recognizes and binds these exposed hydrophobic regions on unfolding proteins, sequestering them in soluble complexes (www.sciencedirect.com). By doing so, Hsp20 prevents the formation of large insoluble aggregates that would otherwise be toxic to the cell (www.sciencedirect.com) (www.sciencedirect.com). This protective “holding” action is critical in S. acidocaldarius, where thermal and oxidative stresses are frequent in its hot acidic habitats (www.sciencedirect.com). Notably, Hsp20 cannot by itself refold proteins into their active conformation – it lacks ATPase activity and acts only as an initial holding chaperone (www.sciencedirect.com). Instead, it works in concert with ATP-driven chaperones. In archaea, the major ATP-dependent refolding machine is the group II chaperonin complex known as the thermosome (an Hsp60 family complex composed of α/β subunits) (www.sciencedirect.com). Hsp20 (and Hsp14) serve as the first line of defense: they bind unfolding proteins and stabilize them, then transfer these clients to the thermosome for final refolding when conditions permit (www.sciencedirect.com) (pubmed.ncbi.nlm.nih.gov). This two-step pathway – sHSP holds the substrate, thermosome refolds it – substitutes for the Hsp70/Hsp40 system that is absent in hyperthermophilic archaea (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Recent biochemical experiments have shed light on how this transfer might occur. Roy et al. (2022, FEBS J.) found that Hsp14 is able to physically interact with the thermosome (direct binding to Hsp60) and can act as a shuttle, receiving captured substrates (possibly from Hsp20) and delivering them to the chaperonin for refolding (pubmed.ncbi.nlm.nih.gov). In this model, Hsp20 and Hsp14 work together: Hsp20 largely buffers the misfolded protein pool (especially under high stress), and Hsp14, which can form mixed complexes with Hsp20, then facilitates handing off those protein clients to the thermosome (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). This cooperative mechanism enables S. acidocaldarius to maintain protein quality without Hsp70, using a minimal chaperone network of just sHSPs and the thermosome (pubmed.ncbi.nlm.nih.gov).
Several lines of evidence underscore Hsp20’s importance in protecting proteins. Expression profiles: S. acidocaldarius sharply induces the hsp20 gene under stress. In a 2023 transcriptomic study, hsp20 was one of the most upregulated genes during acute heat shock (92 °C): its mRNA levels rose with a log₂ fold-change of ~5.6 (~50-fold increase) after 60 minutes of heat stress (www.sciencedirect.com). This was confirmed by qRT-PCR, highlighting Hsp20 as a major heat shock protein of this archaeon. Importantly, hsp20 induction is not limited to heat: the gene is also significantly upregulated under other stress conditions, indicating a broad role in cross-stress adaptation. For example, oxidative stress (paraquat-induced, 60 min) caused nearly a 4-fold increase in hsp20 expression (www.sciencedirect.com), and nutrient starvation stress induced hsp20 by about 10-fold (log₂FC ~3.3 after 90 min) (www.sciencedirect.com). In fact, among the stress-response genes, hsp20 (along with the thermosome subunit Thβ) was found to be consistently overexpressed across all tested stress types, underlining its versatile protective role (www.sciencedirect.com). Mutational or heterologous expression data also support its function: an earlier study of the S. solfataricus homolog (sharing the Hsp20/α-crystallin family) showed that expressing this sHSP in E. coli increased the bacterium’s survival at high temperature (50 °C) and even under cold shock (pmc.ncbi.nlm.nih.gov). Purified archaeal Hsp20 was able to bind and prevent aggregation of model substrate proteins (like citrate synthase) in vitro (pmc.ncbi.nlm.nih.gov). These results illustrate that Hsp20 indeed acts as an effective chaperone, enhancing cellular thermotolerance and general stress resilience.
Beyond protein folding, S. acidocaldarius Hsp20 has a remarkable additional function in stabilizing cell membranes during stress. This role has come to light from recent research and appears to be a unique adaptation in some archaeal sHSPs (www.frontiersin.org). High-temperature stress not only causes protein unfolding but can also disrupt membrane integrity – lipid bilayers may become too fluid or even start to denature at extreme heat, leading to leakage or phase separation (www.sciencedirect.com). Hsp20 helps guard against this form of damage. Roy et al. (2018, published in Biochim. Biophys. Acta - Biomembranes, Dec 2018) reported that Hsp20 can directly interact with lipid membranes and prevent heat-induced membrane destabilization (www.frontiersin.org). In vitro experiments showed Hsp20 binds to lipids via hydrophobic interactions and can measurably decrease the propensity of model membranes (both archaeal and bacterial liposomes) to undergo harmful phase transitions at high temperature (pubmed.ncbi.nlm.nih.gov). In essence, Hsp20 behaves like a molecular “shield” for the cell membrane under extreme conditions, helping maintain proper membrane fluidity and integrity (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Interestingly, membrane perturbation (such as heat-induced lipid disorder) creates a more hydrophobic milieu, similar to the effect of protein unfolding (www.frontiersin.org). Hsp20 appears to sense this change: the increased hydrophobic environment triggers Hsp20 oligomers to dissociate into active dimers (analogous to how unfolding proteins trigger its activation) (www.frontiersin.org) (www.frontiersin.org). The activated Hsp20 then associates with the membrane, its exposed hydrophobic surfaces stabilizing lipid regions that might otherwise coalesce or collapse under stress (www.frontiersin.org) (www.frontiersin.org). Through this mechanism, Hsp20 mitigates heat-induced membrane phase transitions and damage.
This membrane-protective function has also been observed in vivo. Although Hsp20 does not possess a secretion signal, studies in Sulfolobus have found Hsp20 enriched in extracellular membrane vesicles that the cell releases (www.frontiersin.org). These secretory vesicles, which bud off from the archaeal cell envelope, often contain stress-related proteins. The presence of Hsp20 in vesicles suggests it may stabilize the vesicle membrane or protect any proteins within the vesicles during their extracellular journey (www.frontiersin.org). It remains an open question whether Hsp20’s vesicle association serves a communication role (delivering protective chaperones to other cells) or simply reflects a cellular strategy to sequester and stabilize damaged proteins/lipids during stress. Nonetheless, the ability of Hsp20 to operate at the membrane – in addition to its traditional chaperone role in the cytosol – demonstrates its dual functionality. A recent 2023 analysis pointed out that the upregulation of hsp20 under oxidative stress likely benefits the cell by protecting membranes from oxidative damage, in line with Hsp20’s known membrane-stabilizing activity (www.sciencedirect.com). This versatile chaperone thus safeguards both major classes of macromolecules in the cell: proteins and membranes.
Hsp20 is a key component of the heat shock response (HSR) and general stress response network in S. acidocaldarius. Under normal conditions, expression of hsp20 is low, but it is strongly and rapidly induced when the cells are exposed to stress such as heat, oxidative agents, or starvation (www.sciencedirect.com) (www.sciencedirect.com). The promoter of hsp20 is expected to contain archaeal heat shock regulator elements (upstream promoter motifs where repressors bind). In other archaea, specialized regulators (e.g. HSR1 in Archaeoglobus or Phr in Pyrococcus) bind heat-shock gene promoters at normal temperatures to repress transcription, and dissociate upon stress allowing HSP gene expression (www.frontiersin.org). A similar regulatory paradigm may govern Saci_0922 (hsp20), ensuring Hsp20 is produced precisely when needed. Once translated, Hsp20 localizes mainly to the cytosol, ready to engage unfolding cytosolic proteins. During acute stress, some Hsp20 also associates with cell membranes (peripherally) as described, and a fraction is packaged into membrane vesicles that get released from the cell (www.frontiersin.org). There is no evidence of Hsp20 having a signal peptide or transmembrane domain, so any membrane association is likely peripheral and reversible, driven by hydrophobic contacts. Hsp20 does not have enzymatic activity or a classic metabolic “pathway.” Instead, it functions in the protein quality control pathway. It works alongside other chaperones: primarily the thermosome (Hsp60 family) and another small HSP (Hsp14), and possibly with proteases for disposing of proteins that cannot be rescued. Collectively, these elements comprise the archaeal proteostasis system that counters stress-induced damage. In S. acidocaldarius, Hsp20 and Hsp14 are the front-line responders for different stress conditions, capturing a wide array of denatured proteins (www.sciencedirect.com) (www.sciencedirect.com). After the stress subsides or in areas of the cell where conditions normalize, the thermosome (which operates ATP-dependently) refolds the stabilized proteins back to functional states (www.sciencedirect.com). This cooperation prevents cell death from protein aggregation and helps the organism quickly recover from extreme fluctuations in temperature or other environmental challenges.
Understanding Hsp20’s function has practical implications in biotechnology and research. One application of small heat shock proteins is in engineering stress tolerance in other organisms. As mentioned, expression of archaeal Hsp20 in E. coli significantly improved the bacterium’s survival at high and low temperature extremes (pmc.ncbi.nlm.nih.gov). This suggests that heterologous expression of such stable chaperones could be used to create thermotolerant or stress-resistant microbial strains for industrial processes. For example, industries that use bacteria or yeasts at elevated temperatures (biofuel production, composting, etc.) might benefit from introducing a robust archaeal sHSP to protect host enzymes from heat denaturation. There is also interest in using small HSPs as additives or stabilizers for enzyme preparations: since Hsp20 can prevent aggregation of proteins like citrate synthase in vitro (pmc.ncbi.nlm.nih.gov), adding it to enzyme formulations might prolong enzyme activity under stress (e.g. in PCR mixes or in harsh chemical environments). Another emerging application is in medicine and protein folding diseases. Human small heat shock proteins (like HSPB5, HSPB1) are implicated in neurodegenerative diseases and other protein aggregation disorders. Archaeal Hsp20 shares a remarkable structural and functional analogy to human sHSPs, but with much higher stability (www.frontiersin.org). Researchers have noted that archaeal sHSPs can serve as model systems to study “chaperonopathies” (diseases caused by chaperone malfunction) (www.frontiersin.org). Because S. acidocaldarius Hsp20 is stable and amenable to in vitro analysis, it can be used to investigate the fundamental mechanics of small HSP function (oligomerization, client binding, etc.) and even to screen small molecules that might modulate sHSP activity. Any insights gained could be translated to the human context, given the conserved features. In sum, Hsp20 is not only crucial for the survival of S. acidocaldarius in extreme environments but is also a valuable tool and target in biotechnological innovation and fundamental protein-folding research.
Leading scientists in the field highlight Hsp20 as an exemplar of the minimalist yet effective stress defense in archaea. In a 2022 review, Roy and Ghosh and colleagues emphasized that the archaeal heat shock system has a “strikingly simplified” set of chaperones, and Hsp20-type proteins are indispensable in this pared-down machinery (www.frontiersin.org). Despite their simplicity, archaeal sHSPs exhibit sophisticated behavior (like oligomeric switching and membrane interaction) that parallels features seen in more complex eukaryotic systems (www.frontiersin.org) (www.frontiersin.org). The authors note that archaeal sHSPs share “several features with [their] highly sophisticated eukaryotic counterpart,” underlining the deep evolutionary conservation of the chaperone mechanism (www.frontiersin.org). At the same time, the “minimal nature” of the archaeal system – having essentially only sHSPs and a chaperonin – provides a clean model to study how chaperones function and are regulated without the redundancy found in other organisms (www.frontiersin.org). Because S. acidocaldarius Hsp20 is extremely stable, it has been pointed out as an excellent model to investigate small HSP dynamics and protein aggregation diseases (www.frontiersin.org). Overall, expert analyses concur that Hsp20 in Sulfolobus is a multi-faceted chaperone: it protects proteins and membranes, adapts via oligomeric changes, and compensates for the absence of Hsp70 in extreme thermophiles (www.frontiersin.org) (pubmed.ncbi.nlm.nih.gov). Its versatile roles in helping the cell survive multiple stresses (heat, oxidation, starvation) make it a linchpin of the stress response in S. acidocaldarius (www.sciencedirect.com). As research continues (with recent studies in 2018–2023 shedding new light on its mechanisms), Hsp20 stands out as a model for understanding how life can persist under extreme conditions and how minimal chaperone systems can be leveraged in biotechnology and medicine.
References: Key sources include Roy et al. 2018 (BBA Biomembranes) (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org), Roy et al. 2022 (Front. Mol. Biosci.) (www.frontiersin.org) (www.frontiersin.org), Roy et al. 2022 (FEBS J.) (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov), a 2023 transcriptomic study in Research in Microbiology (www.sciencedirect.com) (www.sciencedirect.com), and Li et al. 2012 (Cell Stress Chaperones) on Sulfolobus Hsp20 function (pmc.ncbi.nlm.nih.gov). These and other recent studies provide a comprehensive understanding of Hsp20’s function, regulation, and importance in the cell.
Hsp20 is a small heat shock protein (sHSP) of the alpha-crystallin/Hsp20 family from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. It is 173 amino acids (19.9 kDa). Together with Hsp14 and Hsp60 (group II chaperonin/thermosome), Hsp20 constitutes the core chaperone machinery of S. acidocaldarius, which lacks Hsp70, Hsp90, and Hsp100.
"The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldarius protects environment-induced protein aggregation and membrane destabilization"
Key findings:
- ~24-mer at room temperature (25 degrees C); higher oligomeric forms at higher temperature (50-70 degrees C) and lower pH (3.0-5.0) PMID:30293966
- Dimer is the functional conformation in the presence of aggregating substrate proteins PMID:30293966
- Hydrophobic microenvironment regulates oligomeric plasticity PMID:30293966
- Protects against stress-induced protein aggregation PMID:30293966
- Interacts with membrane lipids via hydrophobic interaction PMID:30293966
- Lowers the propensity of in vitro phase transition of bacterial and archaeal lipids PMID:30293966
- Found in secretory vesicles PMID:30293966
"Archaeal Hsp14 drives substrate shuttling between small heat shock proteins and thermosome"
Key findings relevant to Hsp20:
- Hsp20 forms hetero-oligomers with Hsp14 at 50-70 degrees C PMID:34637594
- Hsp20-captured substrates can be transferred via Hsp14 to the thermosome PMID:34637594
"Heat shock response in Sulfolobus acidocaldarius and first implications for cross-stress adaptation"
Key findings:
- hsp20 plays crucial roles in the majority of stress conditions (heat, oxidative, nutrient) PMID:37516156
- Highest increase in abundance after 60 min of heat shock treatment (from Frontiers review)
"Defining heat shock response for the thermoacidophilic model crenarchaeon Sulfolobus acidocaldarius"
id: Q4JA95
gene_symbol: Hsp20
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:330779
label: Sulfolobus acidocaldarius (strain ATCC 33909 / DSM 639 / JCM 8929 / NBRC
15157 / NCIMB 11770)
description: Hsp20 (Saci_0922) is a 173-amino acid (19.9 kDa) small heat shock protein
(sHSP) of the alpha-crystallin/Hsp20 family from the thermoacidophilic crenarchaeon
Sulfolobus acidocaldarius. It exhibits remarkable oligomeric plasticity, existing
as a ~24-mer at room temperature that shifts to higher oligomeric forms at elevated
temperature and low pH, while the dimer is the functional substrate-binding conformation.
Hsp20 protects against stress-induced protein aggregation and additionally interacts
with membrane lipids via hydrophobic interactions to stabilize membranes by lowering
the propensity of lipid phase transitions. Together with Hsp14 and the group II
chaperonin (thermosome/Hsp60), Hsp20 constitutes the core chaperone machinery of
S. acidocaldarius, which lacks Hsp70, Hsp90, and Hsp100.
existing_annotations:
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IDA
original_reference_id: PMID:30293966
review:
summary: Hsp20 binds unfolded and aggregating substrate proteins in its dimeric
active form. The ~24-mer is a storage form and the dimer is the active substrate-binding
conformation, with oligomeric plasticity regulated by the hydrophobic microenvironment.
action: NEW
reason: Hsp20 directly binds aggregating substrate proteins as demonstrated by
in vitro chaperone activity assays showing the dimer as the functional conformation.
supported_by:
- reference_id: PMID:30293966
supporting_text: we identified a dimeric form of protein as the functional conformation
in the presence of aggregating substrate proteins
- reference_id: PMID:30293966
supporting_text: it plays a key role in the protection of stress-induced protein
aggregation
- term:
id: GO:0008289
label: lipid binding
evidence_type: IDA
original_reference_id: PMID:30293966
review:
summary: Hsp20 interacts with membrane lipids via hydrophobic interactions and
lowers the propensity of lipid phase transitions, stabilizing membranes under
stress conditions. This is distinct from its protein chaperone activity.
action: NEW
reason: Direct lipid binding was demonstrated by biophysical assays showing
hydrophobic interaction with membrane lipids and modulation of membrane fluidity.
supported_by:
- reference_id: PMID:30293966
supporting_text: Hsp20 interacts with membrane lipids via a hydrophobic interaction
- reference_id: PMID:30293966
supporting_text: it lowers the propensity of in vitro phase transition of bacterial
and archaeal lipids
- term:
id: GO:0006457
label: protein folding
evidence_type: IDA
original_reference_id: PMID:30293966
review:
summary: Hsp20 participates in protein folding by capturing aggregation-prone
substrates, which are subsequently transferred via Hsp14 to the thermosome
(Hsp60) for ATP-dependent refolding.
action: NEW
reason: Hsp20 is part of the sHSP-to-thermosome protein folding pathway in
S. acidocaldarius.
supported_by:
- reference_id: PMID:30293966
supporting_text: it plays a key role in the protection of stress-induced protein
aggregation
- reference_id: PMID:34637594
supporting_text: Hsp14 could transfer sHsp-captured substrate proteins to Hsp60,
which then refolds them back to their active form
- term:
id: GO:0034605
label: cellular response to heat
evidence_type: IDA
original_reference_id: PMID:37516156
review:
summary: Hsp20 is upregulated under heat shock and plays a crucial role in the
heat stress response of S. acidocaldarius.
action: NEW
reason: Transcriptomic and qRT-PCR analyses demonstrated upregulation of hsp20
under heat shock (92 degrees C) and other stresses.
supported_by:
- reference_id: PMID:37516156
supporting_text: The results demonstrated that the gene thβ encoding the β subunit
of the thermosome, as well as hsp14 and hsp20, play crucial roles in the majority
of stress conditions
- reference_id: PMID:32562000
supporting_text: a dynamic increase in mRNA levels of all relevant heat shock
proteins
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IDA
original_reference_id: PMID:30293966
review:
summary: Hsp20 is a cytoplasmic sHSP that interacts with other cytoplasmic chaperones
(Hsp14 and Hsp60/thermosome).
action: NEW
reason: sHSPs are cytoplasmic proteins; Hsp20 was purified from and characterized
in the cytoplasmic fraction.
supported_by:
- reference_id: PMID:30293966
supporting_text: we identified a dimeric form of protein as the functional conformation
in the presence of aggregating substrate proteins
references:
- id: PMID:30293966
title: The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldarius protects
environment-induced protein aggregation and membrane destabilization.
findings:
- statement: Hsp20 exists as a ~24-mer at room temperature and forms higher oligomeric
forms at higher temperature (50-70 degrees C) and lower pH (3.0-5.0).
supporting_text: Our data suggested the existence of a ~24-mer of Hsp20 at room
temperature (25 °C) and a higher oligomeric form at higher temperature (50 °C-70 °C)
and lower pH (3.0-5.0)
- statement: The dimer is the functional conformation in the presence of aggregating
substrate proteins.
supporting_text: we identified a dimeric form of protein as the functional conformation
in the presence of aggregating substrate proteins
- statement: Hsp20 protects against stress-induced protein aggregation.
supporting_text: it plays a key role in the protection of stress-induced protein
aggregation
- statement: Hsp20 interacts with membrane lipids via hydrophobic interaction and
stabilizes membranes.
supporting_text: Hsp20 interacts with membrane lipids via a hydrophobic interaction
and it lowers the propensity of in vitro phase transition of bacterial and archaeal
lipids
- statement: Hsp20 has been found in secretory vesicles despite being a non-secreted
protein.
supporting_text: Hsp20, despite being a non-secreted protein, has been reported
to be present in secretory vesicles
- id: PMID:34637594
title: 'Archaeal Hsp14 drives substrate shuttling between small heat shock proteins
and thermosome: insights into a novel substrate transfer pathway.'
findings:
- statement: Hsp20 forms hetero-oligomers with Hsp14 at 50-70 degrees C.
supporting_text: We observed hetero-oligomer formation only at higher temperatures
(50 °C-70 °C)
- statement: Hsp20-captured substrates can be transferred via Hsp14 to the thermosome
for refolding.
supporting_text: Hsp14 could transfer sHsp-captured substrate proteins to Hsp60
- id: PMID:37516156
title: Heat shock response in Sulfolobus acidocaldarius and first implications for
cross-stress adaptation.
findings:
- statement: hsp20 plays crucial roles in the majority of stress conditions including
heat, oxidative, and nutrient stress.
supporting_text: The results demonstrated that the gene thβ encoding the β subunit
of the thermosome, as well as hsp14 and hsp20, play crucial roles in the majority
of stress conditions
- id: PMID:32562000
title: Defining heat shock response for the thermoacidophilic model crenarchaeon
Sulfolobus acidocaldarius.
findings:
- statement: Dynamic increase in mRNA levels of heat shock proteins upon heat shock.
supporting_text: a dynamic increase in mRNA levels of all relevant heat shock
proteins
- id: PMID:15995215
title: The genome of Sulfolobus acidocaldarius, a model organism of the Crenarchaeota.
findings: []
core_functions:
- molecular_function:
id: GO:0051082
label: unfolded protein binding
description: Hsp20 binds unfolded and aggregating substrate proteins in its dimeric
active form, preventing stress-induced protein aggregation. The hydrophobic
microenvironment regulates its oligomeric plasticity, with the ~24-mer serving
as a storage form and the dimer as the active substrate-binding conformation.
Hsp20 functions primarily as a holdase, with captured substrates subsequently
transferred via Hsp14 to the thermosome (Hsp60) for ATP-dependent refolding.
directly_involved_in:
- id: GO:0006457
label: protein folding
- id: GO:0034605
label: cellular response to heat
locations:
- id: GO:0005737
label: cytoplasm
supported_by:
- reference_id: PMID:30293966
supporting_text: we identified a dimeric form of protein as the functional conformation
in the presence of aggregating substrate proteins
- reference_id: PMID:30293966
supporting_text: it plays a key role in the protection of stress-induced protein
aggregation
- reference_id: PMID:34637594
supporting_text: Hsp14 could transfer sHsp-captured substrate proteins to Hsp60,
which then refolds them back to their active form
- molecular_function:
id: GO:0008289
label: lipid binding
description: Hsp20 interacts with membrane lipids via hydrophobic interactions and
lowers the propensity of in vitro phase transition of bacterial and archaeal lipids,
thereby stabilizing membranes under stress conditions. This membrane-stabilizing
function is distinct from its protein chaperone activity and may explain its
presence in secretory vesicles.
directly_involved_in:
- id: GO:0034605
label: cellular response to heat
locations:
- id: GO:0005737
label: cytoplasm
supported_by:
- reference_id: PMID:30293966
supporting_text: Hsp20 interacts with membrane lipids via a hydrophobic interaction
- reference_id: PMID:30293966
supporting_text: it lowers the propensity of in vitro phase transition of bacterial
and archaeal lipids
suggested_questions:
- question: What is the physiological significance of Hsp20 in secretory vesicles -
does it stabilize vesicle membranes or protect cargo proteins?
- question: Does Hsp20 have holdase activity independent of the Hsp14-mediated transfer
pathway, or are all captured substrates ultimately routed through Hsp14 to the
thermosome?