SSA4 (YER103W; UniProt P22202) encodes Ssa4, a stress-inducible cytosolic Hsp70 chaperone of the budding yeast Ssa subfamily (paralogs Ssa1-Ssa4). Like all Hsp70s, it has the canonical two-domain architecture of an N-terminal nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD), and acts as an ATP-dependent molecular chaperone that binds exposed hydrophobic segments of non-native polypeptides to assist folding/refolding, prevent aggregation, support protein translocation, and triage damaged proteins toward sequestration or degradation (cooperating with cochaperones, the Hsp104 disaggregase, the ubiquitin-proteasome system, and autophagy). Whereas SSA1/SSA2 are constitutively expressed, SSA3/SSA4 are stress-inducible: SSA4 is a direct Hsf1 target gene and a component of the Hsf1-Hsp70 negative feedback loop controlling heat shock response dynamics, and its output is additionally tuned post-transcriptionally via codon usage and Asc1/Hel2-dependent ribosome quality control. Ssa4 is predominantly cytosolic; nuclear accumulation occurs under starvation/stress conditions. Function inference is supported by falcon deep research (file:yeast/SSA4/SSA4-deep-research-falcon.md).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005634
nucleus
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: Phylogenetically inferred nuclear localization. For Ssa4 this is consistent with
the experimentally observed, but conditional, starvation-induced nuclear
accumulation (PMID:11279056); it is not the default site of chaperone function.
Retained as non-core, as falcon describes Ssa4 as a cytosolic/cytoplasmic Hsp70.
Reason: Nuclear localization is a real but stress/starvation-conditional relocalization, not the core cytosolic site of function.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0005737
cytoplasm
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Cytoplasm is the core localization of Ssa4, consistent with UniProt and falcon
synthesis classifying Ssa4 as a cytosolic/cytoplasmic Hsp70.
Reason: Core cytoplasmic localization, well supported across sources.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0005886
plasma membrane
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: Plasma membrane is a phylogenetic (IBA) inference not specifically supported for
Ssa4 in the falcon synthesis, which consistently localizes Ssa4 to the
cytosol/cytoplasm. Some cytosolic Hsp70s are peripherally membrane-associated, so
this is retained as a non-core, low-confidence localization rather than removed.
Reason: Not supported as a primary site for Ssa4; plausible peripheral association only, so kept as non-core.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0016887
ATP hydrolysis activity
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: ATP hydrolysis drives the Hsp70 substrate-binding/release cycle. Ssa4 is an
ATP-dependent molecular chaperone with the canonical Hsp70 nucleotide-binding
domain (NBD); ATP hydrolysis activity captures the catalytic component of the
chaperone cycle. Note that the unified molecular function is better expressed as
ATP-dependent protein folding chaperone (GO:0140662; see core_functions).
Reason: Core catalytic activity underlying the ATP-dependent chaperone cycle of this Hsp70.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
|
|
GO:0031072
heat shock protein binding
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Ssa4 engages other heat shock proteins and cochaperones (e.g., the J-protein/Hsp40
Sis1, the NEF Sse1, and paralogs Ssa2/Ssa3, all recorded as UniProt INTERACTION
partners) and cooperates with the Hsp104 disaggregase. Falcon notes inter-isoform
variation is concentrated in an NBD surface implicated in J-protein cochaperone
interactions, supporting cochaperone/heat-shock-protein binding.
Reason: Cochaperone/Hsp partner binding is integral to Hsp70 function and supported by interactome data.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
cooperation with disaggregation machinery (notably Hsp104 in yeast).
|
|
GO:0044183
protein folding chaperone
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Core molecular function. Ssa4 is a protein folding chaperone that binds exposed
hydrophobic segments of client proteins to assist folding/refolding of nascent or
stress-damaged polypeptides. Falcon supports this directly; the ATP-dependent
child term GO:0140662 (ATP-dependent protein folding chaperone) is the most precise
MF and is used in core_functions.
Reason: Core molecular function of an Hsp70 chaperone, strongly supported by literature synthesis.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
assisting folding/refolding of nascent or stress-damaged proteins;
|
|
GO:0140662
ATP-dependent protein folding chaperone
|
IC
file:yeast/SSA4/SSA4-deep-research-falcon.md |
NEW |
Summary: Proposed NEW annotation capturing the unified core molecular function of Ssa4 as
an Hsp70: ATP-dependent protein folding chaperone. This integrates the existing
ATP binding (GO:0005524), ATP hydrolysis (GO:0016887), and protein folding
chaperone (GO:0044183) annotations into the precise activity selected for in
evolution - ATP-driven cycling of client binding/release to assist folding. The
term is already present in UniProt (GO:0140662; IEA:InterPro) but absent from the
curated GOA set. Inferred by curator (IC) from the cited annotations and falcon
synthesis describing Ssa4 as an ATP-dependent molecular chaperone.
Reason: Most precise representation of the core enabled molecular function of this Hsp70; synthesizes the existing ATP-binding/hydrolysis and chaperone annotations.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
PMID:9789005
refolding of OTC diluted from denaturant was assisted by crude yeast cytosol and
ATP and found to be directed by SSA1/2.
|
|
GO:0005829
cytosol
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Cytosol is the primary site of action for Ssa4. Falcon deep research confirms
Ssa4 is consistently classified as a cytosolic/cytoplasmic Hsp70, consistent with
UniProt (SUBCELLULAR LOCATION: Cytoplasm).
Reason: Core localization. Cytosolic Ssa-family Hsp70 strongly supported by literature synthesis.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0042026
protein refolding
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Core biological process. As a stress-inducible Hsp70, Ssa4 contributes to
refolding of stress-denatured proteins and cooperates with the Hsp104 disaggregase
to recover proteins from aggregates following heat shock.
Reason: Core process for a stress-inducible chaperone; well supported by Hsp70/Hsp104 disaggregation biology.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
cooperation with disaggregation machinery (notably Hsp104 in yeast).
|
|
GO:0000166
nucleotide binding
|
IEA
GO_REF:0000043 |
ACCEPT |
Summary: Nucleotide (ATP/ADP) binding at the Hsp70 N-terminal nucleotide-binding domain.
A more specific, equivalent annotation (GO:0005524 ATP binding) is also present.
Consistent with the conserved Hsp70 NBD architecture.
Reason: Generic but accurate; the conserved Hsp70 NBD binds adenine nucleotides. Subsumed by the ATP binding annotation.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
an **N-terminal nucleotide-binding domain (NBD)**
|
|
GO:0005524
ATP binding
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: ATP binding at the Hsp70 N-terminal nucleotide-binding domain (NBD) is required
for the chaperone substrate-binding/release cycle. Supported by UniProt ATP-binding
keyword and the conserved Hsp70 NBD architecture.
Reason: Well-supported molecular function intrinsic to the Hsp70 NBD.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Hsp70 proteins have a canonical domain architecture consisting of an **N-terminal nucleotide-binding domain (NBD)** and a **C-terminal substrate-binding domain (SBD)** connected by a flexible linker.
|
|
GO:0005737
cytoplasm
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: Cytoplasm localization, consistent with UniProt and falcon synthesis (cytosolic
Hsp70). Duplicates the IBA/IDA cytoplasm annotations.
Reason: Core cytoplasmic localization, consistent across automated and manual sources.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0006457
protein folding
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: Core biological process. Ssa4 assists folding/refolding of nascent and
stress-damaged proteins as part of the cytosolic Hsp70 chaperone network.
Reason: Core chaperone process strongly supported by Hsp70 family biology and literature synthesis.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
assisting folding/refolding of nascent or stress-damaged proteins;
|
|
GO:0006616
SRP-dependent cotranslational protein targeting to membrane, translocation
|
IEA
GO_REF:0000117 |
MODIFY |
Summary: Same mechanistic correction as the IMP GO:0006616 entry. Cytosolic Ssa Hsp70 (with
Ydj1) functions in the SRP-INDEPENDENT post-translational translocation pathway, not
the SRP-dependent cotranslational route. Modify to GO:0031204 (post-translational
protein targeting to membrane, translocation). This is a non-core, ancillary role
relative to the protein's core folding/refolding chaperone function.
Reason: ARBA-inferred SRP-dependent cotranslational term is the wrong mechanism for yeast Ssa Hsp70, which mediates post-translational translocation.
Proposed replacements:
post-translational protein targeting to membrane, translocation
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
The Ssa family broadly supports folding, translocation, and degradation
|
|
GO:0016887
ATP hydrolysis activity
|
IEA
GO_REF:0000002 |
ACCEPT |
Summary: ATP hydrolysis activity of the Hsp70 NBD; duplicates the IBA GO:0016887 annotation.
The unified core molecular function is better captured as GO:0140662
(ATP-dependent protein folding chaperone).
Reason: Accurate catalytic activity intrinsic to the Hsp70 chaperone cycle.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
|
|
GO:0051082
unfolded protein binding
|
IEA
GO_REF:0000117 |
MODIFY |
Summary: Unfolded protein binding accurately describes the Hsp70 substrate-binding domain
engaging non-native polypeptides exposing hydrophobic segments. It is retained but
modified toward the ATP-dependent chaperone activity that unifies binding with the
productive folding outcome (the catalytic role is what is evolutionarily selected).
Reason: Unfolded protein binding is correct but is the binding component of the broader ATP-dependent chaperone activity; the protein folding chaperone term better captures the core enabled function.
Proposed replacements:
protein folding chaperone
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Its “substrate specificity” is primarily **proteins/peptides exposing hydrophobic segments**, typical of unfolded or partially folded polypeptides.
|
|
GO:0005515
protein binding
|
IPI
PMID:16429126 Proteome survey reveals modularity of the yeast cell machine... |
MARK AS OVER ANNOTATED |
Summary: Manual review: protein binding is too generic or over-extended for SSA4.
Reason: Marked over-annotated because more specific terms capture the biology more accurately.
|
|
GO:0005515
protein binding
|
IPI
PMID:17892321 Structure-templated predictions of novel protein interaction... |
MARK AS OVER ANNOTATED |
Summary: Manual review: protein binding is too generic or over-extended for SSA4.
Reason: Marked over-annotated because more specific terms capture the biology more accurately.
|
|
GO:0005515
protein binding
|
IPI
PMID:19536198 An atlas of chaperone-protein interactions in Saccharomyces ... |
MARK AS OVER ANNOTATED |
Summary: Manual review: protein binding is too generic or over-extended for SSA4.
Reason: Marked over-annotated because more specific terms capture the biology more accurately.
|
|
GO:0005515
protein binding
|
IPI
PMID:31454312 The role of structural pleiotropy and regulatory evolution i... |
MARK AS OVER ANNOTATED |
Summary: Manual review: protein binding is too generic or over-extended for SSA4.
Reason: Marked over-annotated because more specific terms capture the biology more accurately.
|
|
GO:0005515
protein binding
|
IPI
PMID:37968396 The social and structural architecture of the yeast protein ... |
MARK AS OVER ANNOTATED |
Summary: Manual review: protein binding is too generic or over-extended for SSA4.
Reason: Marked over-annotated because more specific terms capture the biology more accurately.
|
|
GO:0005634
nucleus
|
IDA
PMID:11279056 Starvation promotes nuclear accumulation of the hsp70 Ssa4p ... |
KEEP AS NON CORE |
Summary: Nuclear localization of Ssa4 is real but conditional: Chughtai et al. (2001) show
Ssa4p concentrates in nuclei specifically upon starvation (reversible, with active
export on refeeding) via an N-terminal Star sequence and the beta-importin Nmd5p.
This is a stress/starvation-dependent relocalization rather than the default site
of chaperone action, so it is retained as non-core. Falcon confirms Ssa4 is
otherwise classified as a cytosolic/cytoplasmic Hsp70.
Reason: Starvation-induced, reversible nuclear accumulation is a real but context-specific localization, not the core cytosolic site of chaperone function.
Supporting Evidence:
PMID:11279056
the hsp70 Ssa4p
concentrates in nuclei upon starvation. Nuclear concentration of Ssa4p in
starving cells is reversible
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0005737
cytoplasm
|
IDA
PMID:11279056 Starvation promotes nuclear accumulation of the hsp70 Ssa4p ... |
ACCEPT |
Summary: Cytoplasm is the default, core localization of Ssa4 in non-starved cells (nuclear
accumulation occurs only upon starvation; see GO:0005634). Consistent with UniProt
(SUBCELLULAR LOCATION: Cytoplasm) and falcon synthesis.
Reason: Core cytoplasmic/cytosolic localization for this Hsp70, directly observed and consistent across sources.
Supporting Evidence:
file:yeast/SSA4/SSA4-deep-research-falcon.md
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
|
|
GO:0006457
protein folding
|
IGI
PMID:9789005 Folding in vivo of a newly translated yeast cytosolic enzyme... |
ACCEPT |
Summary: Kim et al. (1998) show the SSA class of cytosolic Hsp70 mediates folding of newly
translated cytosolic enzymes (e.g., OTC); in vitro refolding was specifically
directed by Ssa1/2, while the in vivo SSA-deficiency phenotype is a class-level
result. The annotation to SSA4 reflects membership in the functionally redundant
Ssa class rather than a distinct Ssa4-specific mechanism, but the chaperone folding
role is genuine for the inducible paralog.
Reason: Core chaperone folding role; SSA-class genetic evidence applies to Ssa4 as a member of the redundant subfamily.
Supporting Evidence:
PMID:9789005
we observe that yeast cytosolic OTC is assisted to its
native state by the SSA class of yeast cytosolic Hsp70 proteins.
file:yeast/SSA4/SSA4-deep-research-falcon.md
assisting folding/refolding of nascent or stress-damaged proteins;
|
|
GO:0006616
SRP-dependent cotranslational protein targeting to membrane, translocation
|
IMP
PMID:8754838 Functional interaction of cytosolic hsp70 and a DnaJ-related... |
MODIFY |
Summary: The cited evidence (Becker et al. 1996) shows that the SSA class of cytosolic
Hsp70, together with the Hsp40/DnaJ cochaperone Ydj1, supports POST-TRANSLATIONAL
translocation of precursors (e.g., prepro-alpha-factor, proteinase A, F1-beta) into
the ER and mitochondria - the SRP-INDEPENDENT pathway. The assigned term
"SRP-dependent cotranslational protein targeting" is therefore mislabeled with
respect to mechanism: yeast Ssa Hsp70/Ydj1 act in the post-translational, not the
SRP-dependent cotranslational, route. The annotation should be modified to
GO:0031204 (post-translational protein targeting to membrane, translocation), and
remains a non-core, class-level role (the experiments used a ssa1ts ssa2 ssa3 ssa4
background, so the role reflects the redundant Ssa class rather than Ssa4 alone).
Reason: Cited evidence describes SRP-independent post-translational translocation via Ssa Hsp70/Ydj1; the current term asserts the SRP-dependent cotranslational pathway, which is the wrong mechanism.
Proposed replacements:
post-translational protein targeting to membrane, translocation
Supporting Evidence:
PMID:8754838
Functional interaction of cytosolic hsp70 and a DnaJ-related protein, Ydj1p, in
protein translocation in vivo.
PMID:8754838
The processing of
prepro-alpha-factor was inhibited within 2 min of the shift to 37 degrees C,
suggesting a direct effect of the hsp70 defect on translocation.
|
|
GO:0051082
unfolded protein binding
|
IGI
PMID:9789005 Folding in vivo of a newly translated yeast cytosolic enzyme... |
MODIFY |
Summary: Same rationale as the IEA GO:0051082 entry: binding of non-native/unfolded clients
is accurate (Kim et al. show SSA-class Hsp70 assists folding of newly translated
cytosolic enzymes) but is best represented as the unified ATP-dependent protein
folding chaperone activity.
Reason: Unfolded protein binding is the binding component of the broader chaperone activity; protein folding chaperone better captures the enabled core function.
Proposed replacements:
protein folding chaperone
Supporting Evidence:
PMID:9789005
we observe that yeast cytosolic OTC is assisted to its
native state by the SSA class of yeast cytosolic Hsp70 proteins.
file:yeast/SSA4/SSA4-deep-research-falcon.md
binding exposed hydrophobic segments in client proteins;
|
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The gene symbol SSA4 in this report refers specifically to the heat shock protein SSA4 (Ssa4) from Saccharomyces cerevisiae strain S288c, encoded by YER103W and corresponding to UniProt P22202. This identity match is explicitly stated in a focused review of yeast cytosolic Hsp70 isoforms, which lists Ssa4 (YER103W, UniProt P22202) as one of the four cytosolic Ssa Hsp70 paralogs. (lotz2019notquitethe pages 1-3)
SSA4 encodes Ssa4, a member of the cytosolic Hsp70 (70 kDa heat shock protein) family. In yeast, the Ssa subfamily comprises four paralogs (Ssa1–Ssa4) that support essential proteostasis functions, including:
- binding exposed hydrophobic segments in client proteins;
- assisting folding/refolding of nascent or stress-damaged proteins;
- cooperating with other chaperone systems to route clients toward refolding, sequestration, or degradation. (verghese2012biologyofthe pages 13-13, farley2023effectsofhsp70 pages 2-4)
Hsp70 proteins have a canonical domain architecture consisting of an N-terminal nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD) connected by a flexible linker. Sequence differences among Ssa paralogs are concentrated in the SBD “lid” and an NBD surface implicated in J-protein cochaperone interactions, supporting the idea that paralogs can differ in cochaperone/client preferences even when broadly redundant. (lotz2019notquitethe pages 1-3)
A major organizing principle for the Ssa paralogs is expression regulation:
- SSA1/SSA2 are largely constitutively expressed.
- SSA3/SSA4 are stress-inducible (e.g., induced during heat shock) and can also be induced in strains lacking SSA1/SSA2. (verghese2012biologyofthe pages 13-13, lotz2019notquitethe pages 1-3)
Although any single Ssa isoform can support viability, ssa1Δ ssa2Δ double mutants show severe growth defects and thermosensitivity, suggesting SSA3/SSA4 do not fully replace all Ssa1/Ssa2 functions under normal growth. (verghese2012biologyofthe pages 13-13)
SSA4 encodes an ATP-dependent molecular chaperone (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate. Its “substrate specificity” is primarily proteins/peptides exposing hydrophobic segments, typical of unfolded or partially folded polypeptides. The Ssa family contributes to:
- folding/refolding,
- prevention of aggregation,
- triage of damaged proteins toward degradation,
- and cooperation with disaggregation machinery (notably Hsp104 in yeast). (verghese2012biologyofthe pages 13-13, lotz2019notquitethe pages 1-3, jawed2023balancedactivitiesof pages 1-2)
A central pathway involving SSA4 is the yeast heat shock response, governed by Hsf1, where Hsp70 provides negative feedback:
- Hsf1 activates transcription of Hsp70 genes including SSA3/SSA4.
- Hsp70 represses Hsf1, establishing a two-component negative feedback loop.
This architecture was experimentally tested by genetically decoupling Hsf1 regulation from cytosolic Hsp70 paralogs (SSA1/2/3/4), demonstrating that without heat-induced Hsp70 transcription, Hsf1 fails to deactivate efficiently after heat shock. (krakowiak2018hsf1andhsp70 pages 2-4, krakowiak2018hsf1andhsp70 pages 1-2)
SSA4 is also directly assayed as an Hsf1 target gene in mechanistic studies of Hsf1 regulation by Hsp70–Hsf1 physical contacts, where SSA4 transcript induction is tracked by RNA-seq/qRT-PCR under heat shock and altered when Hsp70–Hsf1 regulation is disrupted. (peffer2019regulationofthe pages 18-20)
Ssa-family Hsp70s cooperate with degradation pathways:
- The Ssa Hsp70 system interfaces with the ubiquitin–proteasome system (UPS) and helps balance refolding vs degradation demands; proteostasis defects can trigger UPS induction, and inappropriate UPS upregulation can worsen growth in some low-Hsp70-capacity contexts. (jawed2023balancedactivitiesof pages 1-2)
- In disease-protein models, stress-inducible Ssa isoforms including Ssa4 can reduce toxicity of aggregation-prone proteins by promoting clearance mechanisms, including autophagy (see §4). (gupta2018theyeaststress pages 1-2)
In the evidence retrieved here, Ssa4 is consistently classified as a cytosolic/cytoplasmic Hsp70 (Ssa family). (lotz2019notquitethe pages 1-3, verghese2012biologyofthe pages 13-13)
More granular stress-dependent relocalization is most clearly developed at the Hsp70-system level (rather than SSA4 specifically) in recent work suggesting spatiotemporal control of Hsp70 partitioning (nuclear vs cytosolic condensates) contributes to heat shock feedback. SSA paralogs (including SSA4) are included among feedback effector genes in that framework, but the excerpted evidence does not isolate Ssa4 localization dynamics from other paralogs. (garde2024feedbackcontrolof pages 1-4)
A major recent SSA4-specific advance is the discovery that SSA4 is regulated not only transcriptionally but also post-transcriptionally at the mRNA translation/decay level.
In Nucleic Acids Research (publication date: May 2023), Boopathy et al. show that:
- SSA4 coding sequence is enriched in low-frequency codons, promoting ribosome stalling during heat stress.
- Stalled ribosomes on SSA4 are recognized by RQC factors Asc1 and Hel2, and by ribosomal proteins identified as novel components (Rps28A, Rps19B).
- This RQC engagement downregulates Ssa4 protein synthesis during heat shock, preventing overproduction.
- Unexpectedly, RQC recognition does not trigger canonical No-Go Decay of SSA4 mRNA; instead, Asc1 promotes rapid SSA4 mRNA destabilization during recovery via an off-ribosome mechanism (independent of Asc1 ribosome binding and independent of SSA4 codon optimality). (boopathy2023theribosomequality pages 1-2)
Quantitative SSA4 data (Boopathy et al., 2023):
- WT SSA4 mRNA half-life ~30 min.
- Asc1 M1X mutant increases SSA4 mRNA half-life to 84 min.
- The M1X strain produces ~7-fold more Ssa4 protein than WT upon heat shock.
- Deleting ASC1 increased SSA4 mRNA half-life ~2.5× in a comparison described in the excerpt; RPS28A or EDC3 deletions prolonged half-life by ~1.5×. (boopathy2023theribosomequality pages 12-13)
These results update the prevailing “HSF1 transcription induces Hsp70” view by showing SSA4 output is also tuned by translational and mRNA-decay control during stress and recovery.
A 2024 systems analysis preprint (bioRxiv; publication date: Jan 2024) proposes that negative feedback in the HSR is organized around a core loop controlling Hsp70 expression, reinforced by an auxiliary loop controlling Hsp70 localization/condensate interactions.
Key quantitative statements from the excerpt include:
- the yeast Hsf1 regulon is a compact set of 42 target genes;
- induction magnitudes across these targets span <10% to >8-fold;
- Hsp70 repression of Hsf1 is restored on the order of ~15 minutes after heat shock (rebinding/repression kinetics described in the excerpt). (garde2024feedbackcontrolof pages 1-4)
While this excerpt is not SSA4-exclusive, it places SSA paralogs (including SSA4) in the modern feedback-control framing of the HSR.
Evidence supports both redundancy and specialization among Ssa paralogs:
- Physical interactome coverage under standard conditions suggests inducible Ssa3/Ssa4 have far smaller reported interactomes (e.g., Ssa4: 57 proteins) than constitutive Ssa1/Ssa2 (e.g., Ssa1: 717), though this may reflect lower abundance at 30°C and condition dependence. Only ~1.5% of interactions are shared across all Ssa isoforms in the cited global datasets. (lotz2019notquitethe pages 3-4)
- Protein stability differs markedly among paralogs: reported half-lives include Ssa4 >100 h, much longer than Ssa1–3 in the same dataset. (lotz2019notquitethe pages 1-3)
These data support a model where Ssa4 is an inducible but long-lived cytosolic Hsp70 that may be deployed as a durable proteostasis component following stress.
In a yeast model of proteotoxicity (publication date: Oct 2018), cells expressing stress-inducible Ssa3 or Ssa4 as the sole Ssa isoform showed reduced α-synuclein toxicity, and protection extended to other inclusion-forming proteins (e.g., polyQ). The mechanism supported in the excerpt is promotion of α-synuclein degradation through autophagy, not simply induction of a generalized stress response, highlighting isoform-dependent proteostasis control. (gupta2018theyeaststress pages 1-2)
Although SSA4 itself is a basic cell-biology gene, the literature supports several applied uses of SSA4/Ssa4 biology:
Synthetic biology / strain engineering for stress tolerance and production stability (conceptual and emerging): The ability of Hsf1–Hsp70 networks to manage unfolded-protein stress is being leveraged in biotechnology contexts; studies of Hsf1/Hsp networks in yeast production settings suggest chaperone networks can influence yields under high expression burden. While not SSA4-specific in the excerpt, SSA4 is explicitly an Hsf1-regulated Hsp70 effector in the HSR network. (krakowiak2018hsf1andhsp70 pages 2-4, garde2024feedbackcontrolof pages 1-4)
Proteostasis engineering in yeast as a platform for human disease protein aggregation: SSA4’s inducible isoform properties and its link to autophagic clearance in α-synuclein models make it relevant to yeast-based screening and mechanistic dissection of aggregation and clearance pathways. (gupta2018theyeaststress pages 1-2)
mRNA design/translation control under stress: The 2023 findings that SSA4 output is tuned via codon usage, ribosome stalling, and Asc1/Hel2 pathways provides a concrete framework for engineering stress-responsive expression systems where translation persists under heat shock but is shut off rapidly during recovery. (boopathy2023theribosomequality pages 1-2, boopathy2023theribosomequality pages 12-13)
The following table summarizes SSA4 findings, separating SSA4-specific evidence from Ssa-family/system-level statements and providing URLs/DOIs and publication dates.
| Aspect | Key finding | Evidence type (review/primary) | System/condition | Quantitative/statistics (if any) | Citation (context id) | Publication (authors, year, journal) | URL/DOI |
|---|---|---|---|---|---|---|---|
| Identity/domains | SSA4 is verified as Ssa4, a cytosolic Hsp70 of Saccharomyces cerevisiae, encoded by YER103W and corresponding to UniProt P22202; canonical Hsp70 architecture includes an NBD and SBD linked by a flexible linker, with inter-isoform variation enriched in the SBD lid and an NBD surface implicated in J-protein interactions. | Review | Budding yeast cytosolic Hsp70 family | Ssa1 shares ~85% identity with Ssa4 | (lotz2019notquitethe pages 1-3) | Lotz et al., 2019, Current Genetics | https://doi.org/10.1007/s00294-019-00978-8 |
| Identity/regulation | Ssa1–4 are the four cytosolic Ssa Hsp70s; SSA3/SSA4 are stress-inducible, whereas SSA1/SSA2 are constitutive. Any single Ssa can support viability, but ssa1Δ ssa2Δ cells are slow-growing and thermosensitive, indicating inducible paralogs do not fully replace constitutive ones. | Review | Yeast heat-shock/proteostasis network | No explicit SSA4 fold-change in excerpt | (verghese2012biologyofthe pages 13-13) | Verghese et al., 2012, Microbiology and Molecular Biology Reviews | https://doi.org/10.1128/MMBR.05018-11 |
| Regulation | SSA4 is an Hsf1-induced gene; Hsf1-mediated induction of SSA3/SSA4 is central to the Hsp70–Hsf1 negative-feedback loop. A strain with all four SSA genes decoupled from Hsf1 regulation failed to induce Hsp70 during heat shock despite elevated basal Hsp70. | Primary | Heat shock response; feedback-severed DFBL yeast strain | Qualitative result: no heat-induced Hsp70 induction in DFBL | (krakowiak2018hsf1andhsp70 pages 2-4) | Krakowiak et al., 2018, eLife | https://doi.org/10.7554/eLife.31668 |
| Regulation | SSA4 was assayed as an Hsf1 target by RNA-seq and qRT-PCR; disruption of bipartite Hsp70–Hsf1 contacts caused constitutive Hsf1 activation, slow growth, and dysregulated target-gene expression including SSA4. | Primary | 37°C heat shock; Hsf1 mutant backgrounds | RNA-seq at 37°C for 15 min; qRT-PCR statistics reported but numeric SSA4 values not given in excerpt | (peffer2019regulationofthe pages 18-20) | Peffer et al., 2019, Journal of Biological Chemistry | https://doi.org/10.1074/jbc.RA119.008822 |
| Regulation/localization | Recent systems analysis places cytosolic Hsp70 paralogs (SSA1, SSA3, SSA4) in the heat-shock feedback architecture, where Hsp70 rebinding represses Hsf1 after stress and auxiliary loops regulate Hsp70 partitioning between nucleus and cytosolic condensates. | Primary preprint | Yeast heat shock response dynamics | Hsf1 regulon: 42 genes; induction magnitudes range from <10% to >8-fold across targets (not SSA4-specific) | (garde2024feedbackcontrolof pages 1-4) | Garde et al., 2024, bioRxiv | https://doi.org/10.1101/2024.01.09.574867 |
| Quantitative data/regulation | SSA4 mRNA is uniquely enriched in low-frequency codons that promote ribosome stalling during heat shock; Asc1/Hel2 and RQC factors tune SSA4 translation and recovery-phase decay rather than triggering standard NGD. | Primary | Heat shock and recovery | WT SSA4 mRNA t1/2 ~30 min; Asc1 M1X t1/2 84 min; ASC1 deletion increased half-life ~2.5× in one comparison; M1X produced ~7-fold more Ssa4 protein after heat shock | (boopathy2023theribosomequality pages 12-13, boopathy2023theribosomequality pages 7-8, boopathy2023theribosomequality pages 1-2) | Boopathy et al., 2023, Nucleic Acids Research | https://doi.org/10.1093/nar/gkad338 |
| Localization | Ssa4 is a cytosolic Hsp70; inducible Ssa3/4 are low at 30°C, likely biasing interactome studies toward constitutive isoforms under non-stress conditions. | Review | Standard growth vs stress conditions | Reported physical interactors: Ssa4 57 vs Ssa1 717, Ssa2 375, Ssa3 69; only ~1.5% shared across isoforms | (lotz2019notquitethe pages 3-4) | Lotz et al., 2019, Current Genetics | https://doi.org/10.1007/s00294-019-00978-8 |
| Function/pathways | Cytosolic Ssa Hsp70s including Ssa4 function in protein folding/refolding, prevention of aggregation, disaggregation with Hsp104, and triage of damaged proteins for degradation. | Review | General proteostasis network | Ssa4 half-life reported as >100 h | (lotz2019notquitethe pages 1-3) | Lotz et al., 2019, Current Genetics | https://doi.org/10.1007/s00294-019-00978-8 |
| Function/pathways | The Ssa family broadly supports folding, translocation, and degradation; depletion/inactivation causes client-folding defects, while stress-inducible SSA3/SSA4 are induced by heat shock or SSA1/2 loss. | Review | General yeast proteostasis | Any single Ssa supports viability, but inducible paralogs incompletely complement Ssa1/2 loss | (verghese2012biologyofthe pages 13-13) | Verghese et al., 2012, Microbiology and Molecular Biology Reviews | https://doi.org/10.1128/MMBR.05018-11 |
| Function/pathways | Cells expressing stress-inducible Ssa3 or Ssa4 as the sole Ssa isoform show reduced α-synuclein toxicity and protection against other aggregation-prone proteins; mechanism implicated is promotion of autophagic degradation, not simply general stress induction. | Primary | Yeast models of α-synuclein/polyQ proteotoxicity | No exact fold-change in excerpt | (gupta2018theyeaststress pages 1-2) | Gupta et al., 2018, PLoS Genetics | https://doi.org/10.1371/journal.pgen.1007751 |
| Function/pathways | Ssa1–4 are essential cytosolic Hsp70s; Hsp70 capacity must be balanced with the ubiquitin–proteasome system (UPS) and sequestration pathways. Hsp70 cooperates with Hsp104 for disaggregation and with Hsp90 for client folding. | Primary review-style research | Proteostasis mutants with low Hsp70 capacity | Simultaneous deletion of SSA1–SSA4 is lethal; reducing 26S proteasome levels improved growth/refolding in low-Hsp70-capacity mutants | (jawed2023balancedactivitiesof pages 1-2) | Jawed et al., 2023, Frontiers in Molecular Biosciences | https://doi.org/10.3389/fmolb.2022.1106477 |
| Function/pathways | The Ssa family is described as managing irretrievably misfolded proteins—including aggregates and prions—through proteasomal or lysosomal/autophagic routes; however, Farley et al. chiefly establish Ssa1/Ssa2 roles and do not provide a distinct SSA4 mechanism in the excerpt. | Primary | Ste5 scaffold quality-control context; mating MAPK pathway | Qualitative family-level statement; no SSA4-specific statistic | (farley2023effectsofhsp70 pages 2-4, farley2023effectsofhsp70 pages 1-2) | Farley et al., 2023, PLOS ONE | https://doi.org/10.1371/journal.pone.0289339 |
| Comparative pathway evidence | During arsenite stress, aggregate clearance depends on Hsp104/Hsp70/Hsp40 systems; the excerpt directly supports Ssa1/2 rather than Ssa4, but it places Ssa1–Ssa4 within the cytoplasmic Hsp70 disaggregation framework. | Primary | Arsenite-induced proteotoxic stress | ssa1Δ ssa2Δ mutants accumulated more Sis1-GFP foci and cleared aggregates more slowly | (hua2022differentialcontributionsof pages 5-6) | Hua et al., 2022, Journal of Biological Chemistry | https://doi.org/10.1016/j.jbc.2022.102680 |
| Regulation/redox sensing | Heat-inducible Ssa3/4 are distinguished from constitutive Ssa1/2 by cysteine content; Ssa3/4 lack the Ssa1/Ssa2 C264 but contain C303, supporting biochemical divergence among paralogs in stress sensing/regulation. | Primary | Thiol-reactive compound response; Hsf1 regulation context | No SSA4-specific fold-change in excerpt | (wang2012theyeasthsp70 pages 6-7) | Wang et al., 2012, Molecular Biology of the Cell | https://doi.org/10.1091/mbc.E12-06-0447 |
Table: This table summarizes identity, regulation, localization, function, and quantitative findings for yeast SSA4/Ssa4 (YER103W; UniProt P22202) from key reviews and primary studies. It highlights where evidence is SSA4-specific versus family-level, and emphasizes recent 2023-2024 mechanistic work on SSA4 mRNA regulation.
SSA4 (YER103W; UniProt P22202) encodes Ssa4, a stress-inducible cytosolic Hsp70 chaperone that supports proteostasis by binding non-native polypeptides and cooperating with cochaperones and downstream quality-control systems to refold, sequester, or degrade damaged proteins. SSA4 is a direct part of the Hsf1 heat shock transcriptional program and contributes to the Hsf1–Hsp70 negative feedback loop controlling heat shock response dynamics. Beyond transcriptional induction, SSA4 output is now understood to be post-transcriptionally tuned: the SSA4 coding sequence promotes ribosome stalling and engages Asc1/Hel2-dependent ribosome quality control that limits Ssa4 overproduction during heat shock and drives rapid SSA4 mRNA clearance during recovery. (lotz2019notquitethe pages 1-3, verghese2012biologyofthe pages 13-13, krakowiak2018hsf1andhsp70 pages 2-4, boopathy2023theribosomequality pages 1-2, boopathy2023theribosomequality pages 12-13)
References
(lotz2019notquitethe pages 1-3): Sarah K. Lotz, Laura E. Knighton, Nitika, Gary W. Jones, and Andrew W. Truman. Not quite the ssame: unique roles for the yeast cytosolic hsp70s. Current Genetics, 65:1127-1134, Apr 2019. URL: https://doi.org/10.1007/s00294-019-00978-8, doi:10.1007/s00294-019-00978-8. This article has 52 citations and is from a peer-reviewed journal.
(verghese2012biologyofthe pages 13-13): Jacob Verghese, Jennifer Abrams, Yanyu Wang, and Kevin A. Morano. Biology of the heat shock response and protein chaperones: budding yeast (saccharomyces cerevisiae) as a model system. Microbiology and Molecular Biology Reviews, 76:115-158, Jun 2012. URL: https://doi.org/10.1128/mmbr.05018-11, doi:10.1128/mmbr.05018-11. This article has 768 citations and is from a domain leading peer-reviewed journal.
(farley2023effectsofhsp70 pages 2-4): Francis W. Farley, Ryan R. McCully, Paul B. Maslo, Lu Yu, Mark A. Sheff, Homayoun Sadeghi, and Elaine A. Elion. Effects of hsp70 chaperones ssa1 and ssa2 on ste5 scaffold and the mating mitogen-activated protein kinase (mapk) pathway in saccharomyces cerevisiae. PLOS ONE, 18:e0289339, Oct 2023. URL: https://doi.org/10.1371/journal.pone.0289339, doi:10.1371/journal.pone.0289339. This article has 2 citations and is from a peer-reviewed journal.
(jawed2023balancedactivitiesof pages 1-2): Areeb Jawed, Chi-Ting Ho, Tomas Grousl, Aseem Shrivastava, Thomas Ruppert, Bernd Bukau, and Axel Mogk. Balanced activities of hsp70 and the ubiquitin proteasome system underlie cellular protein homeostasis. Frontiers in Molecular Biosciences, Jan 2023. URL: https://doi.org/10.3389/fmolb.2022.1106477, doi:10.3389/fmolb.2022.1106477. This article has 14 citations.
(krakowiak2018hsf1andhsp70 pages 2-4): Joanna Krakowiak, Xu Zheng, Nikit Patel, Jayamani Anandhakumar, Kendra Valerius, David S. Gross, Ahmad S. Khalil, and David Pincus. Hsf1 and hsp70 constitute a two-component feedback loop that regulates the yeast heat shock response. eLife, Aug 2018. URL: https://doi.org/10.7554/elife.31668, doi:10.7554/elife.31668. This article has 189 citations and is from a domain leading peer-reviewed journal.
(krakowiak2018hsf1andhsp70 pages 1-2): Joanna Krakowiak, Xu Zheng, Nikit Patel, Jayamani Anandhakumar, Kendra Valerius, David S. Gross, Ahmad S. Khalil, and David Pincus. Hsf1 and hsp70 constitute a two-component feedback loop that regulates the yeast heat shock response. eLife, Aug 2018. URL: https://doi.org/10.7554/elife.31668, doi:10.7554/elife.31668. This article has 189 citations and is from a domain leading peer-reviewed journal.
(peffer2019regulationofthe pages 18-20): Sara Peffer, Davi Gonçalves, and Kevin A. Morano. Regulation of the hsf1-dependent transcriptome via conserved bipartite contacts with hsp70 promotes survival in yeast. Aug 2019. URL: https://doi.org/10.1074/jbc.ra119.008822, doi:10.1074/jbc.ra119.008822. This article has 77 citations and is from a domain leading peer-reviewed journal.
(gupta2018theyeaststress pages 1-2): Arpit Gupta, Anuradhika Puri, Prashant Singh, Surabhi Sonam, Richa Pandey, and Deepak Sharma. The yeast stress inducible ssa hsp70 reduces α-synuclein toxicity by promoting its degradation through autophagy. PLOS Genetics, 14:e1007751, Oct 2018. URL: https://doi.org/10.1371/journal.pgen.1007751, doi:10.1371/journal.pgen.1007751. This article has 27 citations and is from a domain leading peer-reviewed journal.
(garde2024feedbackcontrolof pages 1-4): Rania Garde, Annisa Dea, Madeline F. Herwig, and David Pincus. Feedback control of the heat shock response by spatiotemporal regulation of hsp70. bioRxiv, Jan 2024. URL: https://doi.org/10.1101/2024.01.09.574867, doi:10.1101/2024.01.09.574867. This article has 17 citations.
(boopathy2023theribosomequality pages 1-2): Lokha R Alagar Boopathy, Emma Beadle, Alan RuoChen Xiao, Aitana Garcia-Bueno Rico, Celia Alecki, Irene Garcia de-Andres, Kyla Edelmeier, Luca Lazzari, Mehdi Amiri, and Maria Vera. The ribosome quality control factor asc1 determines the fate of hsp70 mrna on and off the ribosome. Nucleic Acids Research, 51:6370-6388, May 2023. URL: https://doi.org/10.1093/nar/gkad338, doi:10.1093/nar/gkad338. This article has 13 citations and is from a highest quality peer-reviewed journal.
(boopathy2023theribosomequality pages 12-13): Lokha R Alagar Boopathy, Emma Beadle, Alan RuoChen Xiao, Aitana Garcia-Bueno Rico, Celia Alecki, Irene Garcia de-Andres, Kyla Edelmeier, Luca Lazzari, Mehdi Amiri, and Maria Vera. The ribosome quality control factor asc1 determines the fate of hsp70 mrna on and off the ribosome. Nucleic Acids Research, 51:6370-6388, May 2023. URL: https://doi.org/10.1093/nar/gkad338, doi:10.1093/nar/gkad338. This article has 13 citations and is from a highest quality peer-reviewed journal.
(lotz2019notquitethe pages 3-4): Sarah K. Lotz, Laura E. Knighton, Nitika, Gary W. Jones, and Andrew W. Truman. Not quite the ssame: unique roles for the yeast cytosolic hsp70s. Current Genetics, 65:1127-1134, Apr 2019. URL: https://doi.org/10.1007/s00294-019-00978-8, doi:10.1007/s00294-019-00978-8. This article has 52 citations and is from a peer-reviewed journal.
(boopathy2023theribosomequality pages 7-8): Lokha R Alagar Boopathy, Emma Beadle, Alan RuoChen Xiao, Aitana Garcia-Bueno Rico, Celia Alecki, Irene Garcia de-Andres, Kyla Edelmeier, Luca Lazzari, Mehdi Amiri, and Maria Vera. The ribosome quality control factor asc1 determines the fate of hsp70 mrna on and off the ribosome. Nucleic Acids Research, 51:6370-6388, May 2023. URL: https://doi.org/10.1093/nar/gkad338, doi:10.1093/nar/gkad338. This article has 13 citations and is from a highest quality peer-reviewed journal.
(farley2023effectsofhsp70 pages 1-2): Francis W. Farley, Ryan R. McCully, Paul B. Maslo, Lu Yu, Mark A. Sheff, Homayoun Sadeghi, and Elaine A. Elion. Effects of hsp70 chaperones ssa1 and ssa2 on ste5 scaffold and the mating mitogen-activated protein kinase (mapk) pathway in saccharomyces cerevisiae. PLOS ONE, 18:e0289339, Oct 2023. URL: https://doi.org/10.1371/journal.pone.0289339, doi:10.1371/journal.pone.0289339. This article has 2 citations and is from a peer-reviewed journal.
(hua2022differentialcontributionsof pages 5-6): Sansan Hua, Agnieszka Kłosowska, Joana I. Rodrigues, Gabriel Petelski, Lidia A. Esquembre, Emma Lorentzon, Lars F. Olsen, Krzysztof Liberek, and Markus J. Tamás. Differential contributions of the proteasome, autophagy, and chaperones to the clearance of arsenite-induced protein aggregates in yeast. Journal of Biological Chemistry, 298:102680, Dec 2022. URL: https://doi.org/10.1016/j.jbc.2022.102680, doi:10.1016/j.jbc.2022.102680. This article has 11 citations and is from a domain leading peer-reviewed journal.
(wang2012theyeasthsp70 pages 6-7): Yanyu Wang, Patrick A. Gibney, James D. West, and Kevin A. Morano. The yeast hsp70 ssa1 is a sensor for activation of the heat shock response by thiol-reactive compounds. Molecular Biology of the Cell, 23:3290-3298, Sep 2012. URL: https://doi.org/10.1091/mbc.e12-06-0447, doi:10.1091/mbc.e12-06-0447. This article has 88 citations and is from a domain leading peer-reviewed journal.
id: P22202
gene_symbol: SSA4
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:559292
label: Saccharomyces cerevisiae
description: |-
SSA4 (YER103W; UniProt P22202) encodes Ssa4, a stress-inducible cytosolic Hsp70
chaperone of the budding yeast Ssa subfamily (paralogs Ssa1-Ssa4). Like all Hsp70s,
it has the canonical two-domain architecture of an N-terminal nucleotide-binding
domain (NBD) and a C-terminal substrate-binding domain (SBD), and acts as an
ATP-dependent molecular chaperone that binds exposed hydrophobic segments of
non-native polypeptides to assist folding/refolding, prevent aggregation, support
protein translocation, and triage damaged proteins toward sequestration or
degradation (cooperating with cochaperones, the Hsp104 disaggregase, the
ubiquitin-proteasome system, and autophagy). Whereas SSA1/SSA2 are constitutively
expressed, SSA3/SSA4 are stress-inducible: SSA4 is a direct Hsf1 target gene and a
component of the Hsf1-Hsp70 negative feedback loop controlling heat shock response
dynamics, and its output is additionally tuned post-transcriptionally via codon
usage and Asc1/Hel2-dependent ribosome quality control. Ssa4 is predominantly
cytosolic; nuclear accumulation occurs under starvation/stress conditions.
Function inference is supported by falcon deep research (file:yeast/SSA4/SSA4-deep-research-falcon.md).
existing_annotations:
- term:
id: GO:0005634
label: nucleus
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Phylogenetically inferred nuclear localization. For Ssa4 this is consistent with
the experimentally observed, but conditional, starvation-induced nuclear
accumulation (PMID:11279056); it is not the default site of chaperone function.
Retained as non-core, as falcon describes Ssa4 as a cytosolic/cytoplasmic Hsp70.
action: KEEP_AS_NON_CORE
reason: Nuclear localization is a real but stress/starvation-conditional relocalization, not the core cytosolic site of function.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Cytoplasm is the core localization of Ssa4, consistent with UniProt and falcon
synthesis classifying Ssa4 as a cytosolic/cytoplasmic Hsp70.
action: ACCEPT
reason: Core cytoplasmic localization, well supported across sources.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Plasma membrane is a phylogenetic (IBA) inference not specifically supported for
Ssa4 in the falcon synthesis, which consistently localizes Ssa4 to the
cytosol/cytoplasm. Some cytosolic Hsp70s are peripherally membrane-associated, so
this is retained as a non-core, low-confidence localization rather than removed.
action: KEEP_AS_NON_CORE
reason: Not supported as a primary site for Ssa4; plausible peripheral association only, so kept as non-core.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0016887
label: ATP hydrolysis activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
ATP hydrolysis drives the Hsp70 substrate-binding/release cycle. Ssa4 is an
ATP-dependent molecular chaperone with the canonical Hsp70 nucleotide-binding
domain (NBD); ATP hydrolysis activity captures the catalytic component of the
chaperone cycle. Note that the unified molecular function is better expressed as
ATP-dependent protein folding chaperone (GO:0140662; see core_functions).
action: ACCEPT
reason: Core catalytic activity underlying the ATP-dependent chaperone cycle of this Hsp70.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
reference_section_type: RESULTS
- term:
id: GO:0031072
label: heat shock protein binding
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Ssa4 engages other heat shock proteins and cochaperones (e.g., the J-protein/Hsp40
Sis1, the NEF Sse1, and paralogs Ssa2/Ssa3, all recorded as UniProt INTERACTION
partners) and cooperates with the Hsp104 disaggregase. Falcon notes inter-isoform
variation is concentrated in an NBD surface implicated in J-protein cochaperone
interactions, supporting cochaperone/heat-shock-protein binding.
action: ACCEPT
reason: Cochaperone/Hsp partner binding is integral to Hsp70 function and supported by interactome data.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
cooperation with disaggregation machinery (notably Hsp104 in yeast).
reference_section_type: RESULTS
- term:
id: GO:0044183
label: protein folding chaperone
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Core molecular function. Ssa4 is a protein folding chaperone that binds exposed
hydrophobic segments of client proteins to assist folding/refolding of nascent or
stress-damaged polypeptides. Falcon supports this directly; the ATP-dependent
child term GO:0140662 (ATP-dependent protein folding chaperone) is the most precise
MF and is used in core_functions.
action: ACCEPT
reason: Core molecular function of an Hsp70 chaperone, strongly supported by literature synthesis.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
assisting folding/refolding of nascent or stress-damaged proteins;
reference_section_type: RESULTS
- term:
id: GO:0140662
label: ATP-dependent protein folding chaperone
evidence_type: IC
original_reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
review:
summary: |-
Proposed NEW annotation capturing the unified core molecular function of Ssa4 as
an Hsp70: ATP-dependent protein folding chaperone. This integrates the existing
ATP binding (GO:0005524), ATP hydrolysis (GO:0016887), and protein folding
chaperone (GO:0044183) annotations into the precise activity selected for in
evolution - ATP-driven cycling of client binding/release to assist folding. The
term is already present in UniProt (GO:0140662; IEA:InterPro) but absent from the
curated GOA set. Inferred by curator (IC) from the cited annotations and falcon
synthesis describing Ssa4 as an ATP-dependent molecular chaperone.
action: NEW
reason: Most precise representation of the core enabled molecular function of this Hsp70; synthesizes the existing ATP-binding/hydrolysis and chaperone annotations.
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
reference_section_type: OTHER
- reference_id: PMID:9789005
supporting_text: |-
refolding of OTC diluted from denaturant was assisted by crude yeast cytosol and
ATP and found to be directed by SSA1/2.
reference_section_type: ABSTRACT
- term:
id: GO:0005829
label: cytosol
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Cytosol is the primary site of action for Ssa4. Falcon deep research confirms
Ssa4 is consistently classified as a cytosolic/cytoplasmic Hsp70, consistent with
UniProt (SUBCELLULAR LOCATION: Cytoplasm).
action: ACCEPT
reason: Core localization. Cytosolic Ssa-family Hsp70 strongly supported by literature synthesis.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0042026
label: protein refolding
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: |-
Core biological process. As a stress-inducible Hsp70, Ssa4 contributes to
refolding of stress-denatured proteins and cooperates with the Hsp104 disaggregase
to recover proteins from aggregates following heat shock.
action: ACCEPT
reason: Core process for a stress-inducible chaperone; well supported by Hsp70/Hsp104 disaggregation biology.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
cooperation with disaggregation machinery (notably Hsp104 in yeast).
reference_section_type: RESULTS
- term:
id: GO:0000166
label: nucleotide binding
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: |-
Nucleotide (ATP/ADP) binding at the Hsp70 N-terminal nucleotide-binding domain.
A more specific, equivalent annotation (GO:0005524 ATP binding) is also present.
Consistent with the conserved Hsp70 NBD architecture.
action: ACCEPT
reason: Generic but accurate; the conserved Hsp70 NBD binds adenine nucleotides. Subsumed by the ATP binding annotation.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
an **N-terminal nucleotide-binding domain (NBD)**
reference_section_type: RESULTS
- term:
id: GO:0005524
label: ATP binding
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: |-
ATP binding at the Hsp70 N-terminal nucleotide-binding domain (NBD) is required
for the chaperone substrate-binding/release cycle. Supported by UniProt ATP-binding
keyword and the conserved Hsp70 NBD architecture.
action: ACCEPT
reason: Well-supported molecular function intrinsic to the Hsp70 NBD.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Hsp70 proteins have a canonical domain architecture consisting of an **N-terminal nucleotide-binding domain (NBD)** and a **C-terminal substrate-binding domain (SBD)** connected by a flexible linker.
reference_section_type: RESULTS
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: |-
Cytoplasm localization, consistent with UniProt and falcon synthesis (cytosolic
Hsp70). Duplicates the IBA/IDA cytoplasm annotations.
action: ACCEPT
reason: Core cytoplasmic localization, consistent across automated and manual sources.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0006457
label: protein folding
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: |-
Core biological process. Ssa4 assists folding/refolding of nascent and
stress-damaged proteins as part of the cytosolic Hsp70 chaperone network.
action: ACCEPT
reason: Core chaperone process strongly supported by Hsp70 family biology and literature synthesis.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
assisting folding/refolding of nascent or stress-damaged proteins;
reference_section_type: RESULTS
- term:
id: GO:0006616
label: SRP-dependent cotranslational protein targeting to membrane, translocation
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: |-
Same mechanistic correction as the IMP GO:0006616 entry. Cytosolic Ssa Hsp70 (with
Ydj1) functions in the SRP-INDEPENDENT post-translational translocation pathway, not
the SRP-dependent cotranslational route. Modify to GO:0031204 (post-translational
protein targeting to membrane, translocation). This is a non-core, ancillary role
relative to the protein's core folding/refolding chaperone function.
action: MODIFY
reason: ARBA-inferred SRP-dependent cotranslational term is the wrong mechanism for yeast Ssa Hsp70, which mediates post-translational translocation.
proposed_replacement_terms:
- id: GO:0031204
label: post-translational protein targeting to membrane, translocation
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
The Ssa family broadly supports folding, translocation, and degradation
reference_section_type: RESULTS
- term:
id: GO:0016887
label: ATP hydrolysis activity
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: |-
ATP hydrolysis activity of the Hsp70 NBD; duplicates the IBA GO:0016887 annotation.
The unified core molecular function is better captured as GO:0140662
(ATP-dependent protein folding chaperone).
action: ACCEPT
reason: Accurate catalytic activity intrinsic to the Hsp70 chaperone cycle.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
reference_section_type: RESULTS
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: |-
Unfolded protein binding accurately describes the Hsp70 substrate-binding domain
engaging non-native polypeptides exposing hydrophobic segments. It is retained but
modified toward the ATP-dependent chaperone activity that unifies binding with the
productive folding outcome (the catalytic role is what is evolutionarily selected).
action: MODIFY
reason: Unfolded protein binding is correct but is the binding component of the broader ATP-dependent chaperone activity; the protein folding chaperone term better captures the core enabled function.
proposed_replacement_terms:
- id: GO:0044183
label: protein folding chaperone
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Its “substrate specificity” is primarily **proteins/peptides exposing hydrophobic segments**, typical of unfolded or partially folded polypeptides.
reference_section_type: RESULTS
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:16429126
review:
summary: 'Manual review: protein binding is too generic or over-extended for SSA4.'
action: MARK_AS_OVER_ANNOTATED
reason: Marked over-annotated because more specific terms capture the biology more accurately.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:17892321
review:
summary: 'Manual review: protein binding is too generic or over-extended for SSA4.'
action: MARK_AS_OVER_ANNOTATED
reason: Marked over-annotated because more specific terms capture the biology more accurately.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:19536198
review:
summary: 'Manual review: protein binding is too generic or over-extended for SSA4.'
action: MARK_AS_OVER_ANNOTATED
reason: Marked over-annotated because more specific terms capture the biology more accurately.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:31454312
review:
summary: 'Manual review: protein binding is too generic or over-extended for SSA4.'
action: MARK_AS_OVER_ANNOTATED
reason: Marked over-annotated because more specific terms capture the biology more accurately.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:37968396
review:
summary: 'Manual review: protein binding is too generic or over-extended for SSA4.'
action: MARK_AS_OVER_ANNOTATED
reason: Marked over-annotated because more specific terms capture the biology more accurately.
- term:
id: GO:0005634
label: nucleus
evidence_type: IDA
original_reference_id: PMID:11279056
review:
summary: |-
Nuclear localization of Ssa4 is real but conditional: Chughtai et al. (2001) show
Ssa4p concentrates in nuclei specifically upon starvation (reversible, with active
export on refeeding) via an N-terminal Star sequence and the beta-importin Nmd5p.
This is a stress/starvation-dependent relocalization rather than the default site
of chaperone action, so it is retained as non-core. Falcon confirms Ssa4 is
otherwise classified as a cytosolic/cytoplasmic Hsp70.
action: KEEP_AS_NON_CORE
reason: Starvation-induced, reversible nuclear accumulation is a real but context-specific localization, not the core cytosolic site of chaperone function.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: PMID:11279056
supporting_text: |-
the hsp70 Ssa4p
concentrates in nuclei upon starvation. Nuclear concentration of Ssa4p in
starving cells is reversible
reference_section_type: ABSTRACT
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IDA
original_reference_id: PMID:11279056
review:
summary: |-
Cytoplasm is the default, core localization of Ssa4 in non-starved cells (nuclear
accumulation occurs only upon starvation; see GO:0005634). Consistent with UniProt
(SUBCELLULAR LOCATION: Cytoplasm) and falcon synthesis.
action: ACCEPT
reason: Core cytoplasmic/cytosolic localization for this Hsp70, directly observed and consistent across sources.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70**
reference_section_type: RESULTS
- term:
id: GO:0006457
label: protein folding
evidence_type: IGI
original_reference_id: PMID:9789005
review:
summary: |-
Kim et al. (1998) show the SSA class of cytosolic Hsp70 mediates folding of newly
translated cytosolic enzymes (e.g., OTC); in vitro refolding was specifically
directed by Ssa1/2, while the in vivo SSA-deficiency phenotype is a class-level
result. The annotation to SSA4 reflects membership in the functionally redundant
Ssa class rather than a distinct Ssa4-specific mechanism, but the chaperone folding
role is genuine for the inducible paralog.
action: ACCEPT
reason: Core chaperone folding role; SSA-class genetic evidence applies to Ssa4 as a member of the redundant subfamily.
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: PMID:9789005
supporting_text: |-
we observe that yeast cytosolic OTC is assisted to its
native state by the SSA class of yeast cytosolic Hsp70 proteins.
reference_section_type: ABSTRACT
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
assisting folding/refolding of nascent or stress-damaged proteins;
reference_section_type: RESULTS
- term:
id: GO:0006616
label: SRP-dependent cotranslational protein targeting to membrane, translocation
evidence_type: IMP
original_reference_id: PMID:8754838
review:
summary: |-
The cited evidence (Becker et al. 1996) shows that the SSA class of cytosolic
Hsp70, together with the Hsp40/DnaJ cochaperone Ydj1, supports POST-TRANSLATIONAL
translocation of precursors (e.g., prepro-alpha-factor, proteinase A, F1-beta) into
the ER and mitochondria - the SRP-INDEPENDENT pathway. The assigned term
"SRP-dependent cotranslational protein targeting" is therefore mislabeled with
respect to mechanism: yeast Ssa Hsp70/Ydj1 act in the post-translational, not the
SRP-dependent cotranslational, route. The annotation should be modified to
GO:0031204 (post-translational protein targeting to membrane, translocation), and
remains a non-core, class-level role (the experiments used a ssa1ts ssa2 ssa3 ssa4
background, so the role reflects the redundant Ssa class rather than Ssa4 alone).
action: MODIFY
reason: Cited evidence describes SRP-independent post-translational translocation via Ssa Hsp70/Ydj1; the current term asserts the SRP-dependent cotranslational pathway, which is the wrong mechanism.
proposed_replacement_terms:
- id: GO:0031204
label: post-translational protein targeting to membrane, translocation
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: PMID:8754838
supporting_text: |-
Functional interaction of cytosolic hsp70 and a DnaJ-related protein, Ydj1p, in
protein translocation in vivo.
reference_section_type: TITLE
- reference_id: PMID:8754838
supporting_text: |-
The processing of
prepro-alpha-factor was inhibited within 2 min of the shift to 37 degrees C,
suggesting a direct effect of the hsp70 defect on translocation.
reference_section_type: ABSTRACT
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IGI
original_reference_id: PMID:9789005
review:
summary: |-
Same rationale as the IEA GO:0051082 entry: binding of non-native/unfolded clients
is accurate (Kim et al. show SSA-class Hsp70 assists folding of newly translated
cytosolic enzymes) but is best represented as the unified ATP-dependent protein
folding chaperone activity.
action: MODIFY
reason: Unfolded protein binding is the binding component of the broader chaperone activity; protein folding chaperone better captures the enabled core function.
proposed_replacement_terms:
- id: GO:0044183
label: protein folding chaperone
additional_reference_ids:
- file:yeast/SSA4/SSA4-deep-research-falcon.md
supported_by:
- reference_id: PMID:9789005
supporting_text: |-
we observe that yeast cytosolic OTC is assisted to its
native state by the SSA class of yeast cytosolic Hsp70 proteins.
reference_section_type: ABSTRACT
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
binding exposed hydrophobic segments in client proteins;
reference_section_type: RESULTS
references:
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with GO terms
findings: []
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000043
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
findings: []
- id: GO_REF:0000117
title: Electronic Gene Ontology annotations created by ARBA machine learning models
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods
findings: []
- id: PMID:11279056
title: Starvation promotes nuclear accumulation of the hsp70 Ssa4p in yeast cells.
findings: []
- id: PMID:16429126
title: Proteome survey reveals modularity of the yeast cell machinery.
findings: []
- id: PMID:17892321
title: Structure-templated predictions of novel protein interactions from sequence information.
findings: []
- id: PMID:19536198
title: 'An atlas of chaperone-protein interactions in Saccharomyces cerevisiae: implications to protein folding pathways in the cell.'
findings: []
- id: PMID:31454312
title: The role of structural pleiotropy and regulatory evolution in the retention of heteromers of paralogs.
findings: []
- id: PMID:37968396
title: The social and structural architecture of the yeast protein interactome.
findings: []
- id: PMID:8754838
title: Functional interaction of cytosolic hsp70 and a DnaJ-related protein, Ydj1p, in protein translocation in vivo.
findings: []
- id: PMID:9789005
title: Folding in vivo of a newly translated yeast cytosolic enzyme is mediated by the SSA class of cytosolic yeast Hsp70 proteins.
findings: []
- id: file:yeast/SSA4/SSA4-deep-research-falcon.md
title: Falcon deep research report on SSA4 (Saccharomyces cerevisiae, UniProt P22202, YER103W)
findings:
- statement: |
SSA4 (YER103W; UniProt P22202) encodes Ssa4, a stress-inducible cytosolic Hsp70
chaperone of the budding yeast Ssa subfamily (paralogs Ssa1-Ssa4) that binds
non-native polypeptides and cooperates with cochaperones and quality-control
systems to refold, sequester, or degrade damaged proteins.
supporting_text: |-
**SSA4 (YER103W; UniProt P22202)** encodes **Ssa4**, a **stress-inducible cytosolic Hsp70 chaperone** that supports proteostasis by binding non-native polypeptides and cooperating with cochaperones and downstream quality-control systems to refold, sequester, or degrade damaged proteins.
reference_section_type: OTHER
- statement: |
Ssa4 has the canonical Hsp70 two-domain architecture: an N-terminal
nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD)
connected by a flexible linker.
supporting_text: |-
Hsp70 proteins have a canonical domain architecture consisting of an **N-terminal nucleotide-binding domain (NBD)** and a **C-terminal substrate-binding domain (SBD)** connected by a flexible linker.
reference_section_type: OTHER
- statement: |
Ssa4 is an ATP-dependent molecular chaperone whose substrate specificity is
proteins/peptides exposing hydrophobic segments (unfolded or partially folded
polypeptides), rather than a discrete small-molecule substrate.
supporting_text: |-
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate. Its “substrate specificity” is primarily **proteins/peptides exposing hydrophobic segments**, typical of unfolded or partially folded polypeptides.
reference_section_type: OTHER
- statement: |
Whereas SSA1/SSA2 are constitutively expressed, SSA3/SSA4 are stress-inducible
(e.g., induced during heat shock and in strains lacking SSA1/SSA2).
supporting_text: |-
- **SSA1/SSA2** are largely **constitutively expressed**.
- **SSA3/SSA4** are **stress-inducible** (e.g., induced during heat shock) and can also be induced in strains lacking SSA1/SSA2.
reference_section_type: OTHER
- statement: |
SSA4 is a direct Hsf1 target gene and contributes to the Hsf1-Hsp70 negative
feedback loop that controls heat shock response dynamics.
supporting_text: |-
- Hsf1 activates transcription of Hsp70 genes including **SSA3/SSA4**.
- Hsp70 represses Hsf1, establishing a two-component negative feedback loop.
reference_section_type: OTHER
- statement: |
Ssa4 is consistently classified as a cytosolic/cytoplasmic Hsp70.
supporting_text: |-
In the evidence retrieved here, Ssa4 is consistently classified as a **cytosolic/cytoplasmic Hsp70** (Ssa family).
reference_section_type: OTHER
- statement: |
Cytosolic Ssa Hsp70s including Ssa4 cooperate with the Hsp104 disaggregase and
triage damaged proteins toward degradation (ubiquitin-proteasome system and
autophagy), with stress-inducible Ssa3/Ssa4 able to reduce toxicity of
aggregation-prone proteins (e.g., alpha-synuclein) by promoting autophagic clearance.
supporting_text: |-
cells expressing stress-inducible **Ssa3 or Ssa4** as the sole Ssa isoform showed reduced **α-synuclein toxicity**, and protection extended to other inclusion-forming proteins (e.g., polyQ). The mechanism supported in the excerpt is promotion of **α-synuclein degradation through autophagy**
reference_section_type: OTHER
- statement: |
The Ssa family broadly supports protein folding, translocation, and degradation;
yeast Ssa Hsp70 (with the Hsp40 Ydj1) mediates SRP-independent post-translational
translocation of precursors into the ER and mitochondria.
supporting_text: |-
The Ssa family broadly supports folding, translocation, and degradation; depletion/inactivation causes client-folding defects, while stress-inducible **SSA3/SSA4** are induced by heat shock or **SSA1/2** loss.
reference_section_type: OTHER
core_functions:
- description: |-
Stress-inducible cytosolic Hsp70 chaperone: Ssa4 binds exposed hydrophobic
segments of non-native polypeptides and uses ATP-driven cycling of its
nucleotide-binding domain to assist folding/refolding of nascent and
stress-denatured proteins, prevent aggregation, and (with the Hsp104
disaggregase and cochaperones) recover proteins from aggregates following
heat shock.
molecular_function:
id: GO:0140662
label: ATP-dependent protein folding chaperone
directly_involved_in:
- id: GO:0006457
label: protein folding
- id: GO:0042026
label: protein refolding
locations:
- id: GO:0005829
label: cytosol
- id: GO:0005737
label: cytoplasm
supported_by:
- reference_id: file:yeast/SSA4/SSA4-deep-research-falcon.md
supporting_text: |-
SSA4 encodes an **ATP-dependent molecular chaperone** (Hsp70 family) rather than an enzyme with a discrete small-molecule substrate.
reference_section_type: OTHER
- reference_id: PMID:9789005
supporting_text: |-
we observe that yeast cytosolic OTC is assisted to its
native state by the SSA class of yeast cytosolic Hsp70 proteins.
reference_section_type: ABSTRACT