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.
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: Hsp20: Hsp20 binds unfolded and aggregating substrate proteins in its dimeric active form, preventing stress-induced protein aggregation. 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: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 thΞ² encoding the Ξ² 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 |
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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?
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