SSB2

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005634 nucleus	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Ssb2 functions on cytosolic translating ribosomes; a nuclear pool is at most transient/peripheral (e.g., association with pre-ribosomes during ribosome biogenesis). Not a core localization. Reason: Ssb is overwhelmingly cytosolic and ribosome-associated. The falcon report localizes Ssb to the cytosol and 60S tunnel exit with no evidence for an autonomous nuclear function; any nuclear signal is best treated as peripheral/context-dependent. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb proteins (Ssb1/Ssb2) are cytosolic and ribosome-associated, positioned at the 60S tunnel exit
GO:0005737 cytoplasm	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Ssb2 is a cytoplasmic/cytosolic chaperone. Cytoplasm is correct but generic; the more informative localization is cytosol/ribosome-associated. Reason: Correct but non-specific. The functionally meaningful localization is the cytosolic translating ribosome (see cytosol annotation). Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb proteins (Ssb1/Ssb2) are cytosolic and ribosome-associated, positioned at the 60S tunnel exit
GO:0005886 plasma membrane	IBA GO_REF:0000033	REMOVE	Summary: Plasma membrane is not a site of Ssb2 function. This likely reflects high-throughput proteome surveys detecting the abundant cytosolic Ssb pool. Reason: No experimental or literature support for plasma membrane function. The falcon report and UniProt localize Ssb to the cytosol and ribosome; a plasma membrane assignment for this abundant cytosolic Hsp70 is an over-annotation from high-throughput localization datasets. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb proteins (Ssb1/Ssb2) are cytosolic and ribosome-associated, positioned at the 60S tunnel exit
GO:0016887 ATP hydrolysis activity	IBA GO_REF:0000033	ACCEPT	Summary: Ssb2 is an Hsp70 ATPase (EC 3.6.4.10); ATP hydrolysis powers its chaperone cycle and is stimulated by the Zuo1 J-domain of RAC. Reason: Directly supported. The Ssb ATPase activity is experimentally characterized and the ATP-driven conformational cycle is central to its function. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md J-domain protein Zuo1 stimulates ATP hydrolysis of Ssb1/2, driving this high-affinity substrate engagement on nascent chains
GO:0031072 heat shock protein binding	IBA GO_REF:0000033	ACCEPT	Summary: Ssb2 interacts with co-chaperone partners of the Hsp70 system, notably the RAC J-protein Zuo1/Ssz1 and the Hsp110 nucleotide exchange factor Sse1. Reason: Consistent with the documented Ssb-RAC and Ssb-Sse1 (Hsp110) functional interactions that constitute the ribosome-associated Hsp70 chaperone system. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md RAC is an obligate Zuo1–Ssz1 heterodimer attached to the ribosome (via Zuo1)
GO:0044183 protein folding chaperone	IBA GO_REF:0000033	ACCEPT	Summary: Ssb2 is a protein folding chaperone that assists cotranslational folding of nascent chains. The more precise MF for this ATP-driven Hsp70 is ATP-dependent protein folding chaperone (GO:0140662). Reason: Core molecular function. Accepted; a more specific child term (ATP-dependent protein folding chaperone, GO:0140662) is captured in core_functions to reflect the ATP-dependent Hsp70 mechanism. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb1/2 (including Ssb2) act as the direct nascent-chain binders during co-translational folding in yeast
GO:0005829 cytosol	IBA GO_REF:0000033	ACCEPT	Summary: Ssb2 is a cytosolic chaperone; ~50% is ribosome-associated and the remainder is free cytosolic Ssb. This is the core localization. Reason: Strongly supported. The cytosol (and specifically cytosolic translating ribosomes) is where Ssb2 carries out its function. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md only about ~50% of total cellular Ssb is ribosome-associated at steady state
GO:0042026 protein refolding	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Ssb's primary, best-supported role is de novo cotranslational folding of nascent chains rather than refolding of pre-existing denatured proteins, though as an Hsp70 it can contribute to general proteostasis/aggregation prevention. Reason: Plausible Hsp70 activity but not the core, distinguishing function of Ssb. The falcon report and primary literature emphasize cotranslational folding of nascent chains; refolding is a generic Hsp70 capability kept as non-core. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb suppresses formation and/or inheritance of multiple amyloid/prion-like elements
GO:0000054 ribosomal subunit export from nucleus	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Ssb/RAC participates in a ribosome-anchored chaperone network linked to ribosome biogenesis; a role in ribosomal subunit export was reported by genetic interaction. This is downstream/ancillary to the core folding role. Reason: Supported by genetic interaction (PMID:20368619) but ancillary to the core cotranslational chaperone function. Kept as non-core.
GO:0000166 nucleotide binding	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: Generic parent of ATP binding. Ssb2 binds ATP via its NBD; the more specific ATP binding term is preferred. Reason: Too generic. ATP binding (GO:0005524) captures the actual ligand more precisely; nucleotide binding is an uninformative parent.
GO:0005524 ATP binding	IEA GO_REF:0000120	ACCEPT	Summary: Ssb2's N-terminal nucleotide-binding domain binds ATP, the basis of its ATP-driven Hsp70 chaperone cycle. Reason: Directly supported by domain architecture and the ATP-driven conformational cycle of the chaperone. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md core biochemistry is an ATP-driven conformational cycle
GO:0005737 cytoplasm	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: Cytoplasm is correct but generic; cytosol/ribosome-associated is the informative localization (duplicate of the IBA cytoplasm annotation). Reason: Correct but non-specific localization, retained as non-core.
GO:0006364 rRNA processing	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Ssb/RAC is part of a ribosome-anchored chaperone network implicated in ribosome biogenesis (rRNA processing reported by genetic interaction). This is ancillary to the core cotranslational folding role. Reason: Indirect/ancillary role via the ribosome-anchored chaperone network (PMID:20368619); not the core function.
GO:0006412 translation	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: Generic parent. Ssb acts on translating ribosomes; the more specific cytoplasmic translation and de novo cotranslational protein folding terms better capture its role. Reason: Too broad. Ssb is not a core translation factor; its role is cotranslational chaperoning. More specific terms (cytoplasmic translation, cotranslational protein folding) are preferred.
GO:0006450 regulation of translational fidelity	IEA GO_REF:0000117	ACCEPT	Summary: Loss of Ssb1/2 (or RAC) impairs translational fidelity, primarily at translation termination. A genuine, experimentally supported downstream role. Reason: Supported experimentally (PMID:15456889): RAC and Ssb1/2p are crucial for translational fidelity, with the principal defect in translation termination.
GO:0006452 translational frameshifting	IEA GO_REF:0000117	ACCEPT	Summary: Deletion of Ssb1/2 (or RAC) specifically inhibits -1 programmed ribosomal frameshifting and impairs Killer virus maintenance. Reason: Supported experimentally (PMID:16607023). Note the effect is specific to -1 PRF (no effect on +1 PRF), a downstream consequence of Ssb's role at the translating ribosome.
GO:0016787 hydrolase activity	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: Uninformative parent term. Ssb2's relevant activity is ATP hydrolysis (GO:0016887) as part of its Hsp70 ATPase cycle. Reason: Too generic. ATP hydrolysis activity (GO:0016887) is the specific and accurate term.
GO:0016887 ATP hydrolysis activity	IEA GO_REF:0000120	ACCEPT	Summary: Ssb2 hydrolyzes ATP as part of its Hsp70 chaperone cycle (EC 3.6.4.10), stimulated by the RAC J-protein Zuo1. Reason: Directly supported; duplicate of the IBA/IDA ATP hydrolysis annotations.
GO:0051082 unfolded protein binding	IEA GO_REF:0000117	MODIFY	Summary: Ssb2 binds exposed hydrophobic segments of unfolded/nascent polypeptides. The chaperone activity is better captured by ATP-dependent protein folding chaperone (GO:0140662). Reason: The binding term is accurate but a holdase/binding-only term understates the ATP-driven Hsp70 mechanism. Modify to the ATP-dependent protein folding chaperone MF. Proposed replacements: ATP-dependent protein folding chaperone Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md J-domain protein Zuo1 stimulates ATP hydrolysis of Ssb1/2, driving this high-affinity substrate engagement on nascent chains
GO:0051083 'de novo' cotranslational protein folding	IEA GO_REF:0000117	ACCEPT	Summary: This is the core biological process of Ssb2: direct binding to nascent chains at the ribosomal tunnel exit to promote de novo cotranslational folding, activated by RAC. Reason: Core function, strongly supported by both the falcon report and primary literature (PMID:9670014, PMID:23332755). Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb1/2 (including Ssb2) act as the direct nascent-chain binders during co-translational folding in yeast
GO:0005515 protein binding	IPI PMID:16429126 Proteome survey reveals modularity of the yeast cell machine...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding from a high-throughput proteome survey; provides no specific functional information. Reason: Uninformative protein binding term per curation guidelines; the specific Ssb-RAC/Ssb-nascent chain interactions are captured by chaperone MF terms.
GO:0005515 protein binding	IPI PMID:16554755 Global landscape of protein complexes in the yeast Saccharom...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding from a high-throughput complex landscape study; no specific functional information. Reason: Uninformative protein binding term per curation guidelines.
GO:0005515 protein binding	IPI PMID:19536198 An atlas of chaperone-protein interactions in Saccharomyces ...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding from a chaperone-interaction atlas; no specific functional information beyond the chaperone network role. Reason: Uninformative protein binding term per curation guidelines.
GO:0005515 protein binding	IPI PMID:23332755 The cotranslational function of ribosome-associated Hsp70 in...	MARK AS OVER ANNOTATED	Summary: This reference actually documents Ssb's cotranslational nascent-chain substrate binding and RAC modulation; the generic protein binding term understates it. Reason: Uninformative as protein binding; the specific function from this paper (cotranslational chaperoning of nascent chains) is captured by the de novo cotranslational protein folding and chaperone MF annotations.
GO:0005515 protein binding	IPI PMID:37070168 RNA-dependent interactome allows network-based assignment of...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding from an RNA-dependent interactome study; no specific functional information. Reason: Uninformative protein binding term per curation guidelines.
GO:0005515 protein binding	IPI PMID:37968396 The social and structural architecture of the yeast protein ...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding from a global interactome architecture study; no specific functional information. Reason: Uninformative protein binding term per curation guidelines.
GO:0010494 cytoplasmic stress granule	HDA PMID:26777405 ATPase-Modulated Stress Granules Contain a Diverse Proteome ...	KEEP AS NON CORE	Summary: As an abundant cytosolic Hsp70, Ssb2 is detected in stress granules; this is a stress-condition localization, not the core function. Reason: Plausible stress-condition localization detected by high-throughput proteomics; peripheral to the core cotranslational folding function.
GO:0005737 cytoplasm	HDA PMID:11914276 Subcellular localization of the yeast proteome.	KEEP AS NON CORE	Summary: Cytoplasm localization confirmed by genome-wide GFP localization; correct but generic relative to cytosol/ribosome-associated. Reason: Correct but non-specific localization, retained as non-core.
GO:0005886 plasma membrane	HDA PMID:16622836 The plasma membrane proteome of Saccharomyces cerevisiae and...	REMOVE	Summary: Plasma membrane proteome detection of the abundant cytosolic Ssb; not a genuine functional localization. Reason: Over-annotation from a plasma-membrane proteome survey. Ssb is a cytosolic ribosome-associated Hsp70 with no functional role at the plasma membrane. Supporting Evidence: file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb proteins (Ssb1/Ssb2) are cytosolic and ribosome-associated, positioned at the 60S tunnel exit
GO:0006452 translational frameshifting	IMP PMID:16607023 Specific effects of ribosome-tethered molecular chaperones o...	ACCEPT	Summary: Deletion of Ssb1p/Ssb2p (or RAC) specifically inhibits -1 programmed ribosomal frameshifting and impairs Killer virus maintenance, with no effect on +1 PRF. Reason: Strong direct IMP evidence (PMID:16607023). A genuine, specific downstream consequence of Ssb function at the translating ribosome. Supporting Evidence: PMID:16607023 deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of -1
GO:0000054 ribosomal subunit export from nucleus	IGI PMID:20368619 A ribosome-anchored chaperone network that facilitates eukar...	KEEP AS NON CORE	Summary: Genetic interaction evidence places Ssb/RAC in a ribosome-anchored chaperone network facilitating ribosome biogenesis, including subunit export. Ancillary to the core cotranslational folding role. Reason: Supported by genetic interaction (PMID:20368619) but ancillary; kept as non-core.
GO:0002181 cytoplasmic translation	IMP PMID:1394434 The translation machinery and 70 kd heat shock protein coope...	ACCEPT	Summary: Ssb1/2 are associated with translating ribosomes; ssb1 ssb2 mutants grow slowly, have fewer translating ribosomes, and are hypersensitive to protein synthesis inhibitors, linking Ssb to cytoplasmic translation. Reason: Supported by IMP (PMID:1394434). Ssb participates in cytoplasmic translation as a ribosome-associated cotranslational chaperone. Supporting Evidence: PMID:1394434 Mutant ssb1 ssb2 file:yeast/SSB2/SSB2-deep-research-falcon.md Single-gene loss has little obvious phenotype, whereas combined ssb1/2Δ causes broad defects
GO:0002181 cytoplasmic translation	IPI PMID:1394434 The translation machinery and 70 kd heat shock protein coope...	ACCEPT	Summary: Ssb1/2p associate with translating ribosomes and the association is disrupted by puromycin, suggesting direct binding to the nascent polypeptide during cytoplasmic translation. Reason: Supported (PMID:1394434). Consistent IPI/IMP support for Ssb's role at the translating cytoplasmic ribosome (same action as the paired IMP annotation). Supporting Evidence: PMID:1394434 The SSB hsp70s (Ssb1/2p) are associated with
GO:0006364 rRNA processing	IGI PMID:20368619 A ribosome-anchored chaperone network that facilitates eukar...	KEEP AS NON CORE	Summary: Genetic interaction places Ssb/RAC in a ribosome-anchored chaperone network facilitating ribosome biogenesis (rRNA processing). Ancillary to the core cotranslational folding role. Reason: Indirect/ancillary role via the ribosome-anchored chaperone network (PMID:20368619); not the core function.
GO:0006450 regulation of translational fidelity	IMP PMID:15456889 The ribosome-bound chaperones RAC and Ssb1/2p are required f...	ACCEPT	Summary: Absence of RAC or Ssb1/2p impairs translational fidelity in vivo and in vitro, primarily through a defect in translation termination, enhanced by paromomycin. Reason: Strong direct IMP evidence (PMID:15456889) that Ssb1/2p are crucial for translational fidelity beyond their chaperone role for nascent chains. Supporting Evidence: PMID:15456889 Translational fidelity was impaired in the absence of functional RAC or Ssb1/2p
GO:0016887 ATP hydrolysis activity	IDA PMID:9860955 The biochemical properties of the ATPase activity of a 70-kD...	ACCEPT	Summary: Ssb has direct, biochemically characterized ATPase activity with unusual kinetics (low steady-state affinity for ATP, higher Vmax, K+-independent) governed by its C-terminal domains. Reason: Strong direct IDA biochemical evidence (PMID:9860955) for the Ssb ATPase activity underlying its Hsp70 chaperone cycle. Supporting Evidence: PMID:9860955 Ssb, however, has an unusually low steady-state affinity for ATP but a file:yeast/SSB2/SSB2-deep-research-falcon.md core biochemistry is an ATP-driven conformational cycle
GO:0042149 cellular response to glucose starvation	IGI PMID:19723765 The Hsp70 homolog Ssb is essential for glucose sensing via t...	ACCEPT	Summary: Ssb is required for glucose sensing via the SNF1 kinase network: the chaperone keeps SNF1 in the nonphosphorylated state in the presence of glucose, and Deltassb1 Deltassb2 cells resemble glucose-repression mutants. Reason: Supported by genetic interaction (PMID:19723765). A genuine downstream physiological role connecting Ssb chaperone function to glucose/SNF1 signaling. Supporting Evidence: PMID:19723765 the chaperone Ssb is required to keep SNF1 in the
GO:0051082 unfolded protein binding	IDA PMID:9670014 The molecular chaperone Ssb from Saccharomyces cerevisiae is...	MODIFY	Summary: Ssb can be cross-linked to nascent chains and is released with nascent chains upon puromycin treatment, demonstrating direct binding to unfolded/nascent polypeptides. The ATP-driven chaperone mechanism is better captured by ATP-dependent protein folding chaperone (GO:0140662). Reason: The binding term is directly supported (PMID:9670014) but a binding-only term understates the ATP-dependent Hsp70 mechanism. Modify to the ATP-dependent protein folding chaperone MF. Proposed replacements: ATP-dependent protein folding chaperone Supporting Evidence: PMID:9670014 Ssb could be cross-linked to nascent chains file:yeast/SSB2/SSB2-deep-research-falcon.md J-domain protein Zuo1 stimulates ATP hydrolysis of Ssb1/2, driving this high-affinity substrate engagement on nascent chains
GO:0051083 'de novo' cotranslational protein folding	IDA PMID:9670014 The molecular chaperone Ssb from Saccharomyces cerevisiae is...	ACCEPT	Summary: Ssb is a core component of the translating ribosome that interacts with both the nascent polypeptide and the ribosome, functioning as a chaperone to prevent misfolding of newly synthesized proteins. This is the core process. Reason: Core function with direct IDA evidence (PMID:9670014). The defining biological role of Ssb2. Supporting Evidence: PMID:9670014 Ssb to function as a chaperone on the ribosome, preventing the misfolding of file:yeast/SSB2/SSB2-deep-research-falcon.md Ssb2 belongs to an Hsp70 triad at the exit tunnel

Core Functions

ATP-dependent Hsp70 molecular chaperone that binds short, largely hydrophobic segments of nascent polypeptides emerging from the ribosomal tunnel exit, using an ATP-driven NBD/SBD conformational cycle (stimulated by the RAC J-protein Zuo1) to promote de novo cotranslational protein folding.

Molecular Function:

ATP-dependent protein folding chaperone

Supporting Evidence:

file:yeast/SSB2/SSB2-deep-research-falcon.md
Ssb1/2 (including Ssb2) act as the **direct nascent-chain binders** during co-translational folding in yeast
PMID:9670014
Ssb to function as a chaperone on the ribosome, preventing the misfolding of

Cotranslational chaperone acting on cytosolic translating 80S ribosomes near the 60S tunnel exit, directly engaging nascent chains as part of the RAC-Ssb system to support de novo folding of a large fraction of the nascent proteome.

Molecular Function:

ATP-dependent protein folding chaperone

Supporting Evidence:

file:yeast/SSB2/SSB2-deep-research-falcon.md
Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
PMID:23332755
define the cotranslational substrate

References

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:11914276

Subcellular localization of the yeast proteome.

PMID:1394434

The translation machinery and 70 kd heat shock protein cooperate in protein synthesis.

The SSB Hsp70s (Ssb1/2p) are associated with translating ribosomes, an association disrupted by puromycin, suggesting Ssb binds directly to the nascent polypeptide chain.

"The SSB hsp70s (Ssb1/2p) are associated with translating ribosomes. This association is disrupted by puromycin, suggesting that Ssb1/2p may bind directly to the nascent polypeptide."
Mutant ssb1 ssb2 strains grow slowly, contain fewer translating ribosomes, and are hypersensitive to protein synthesis inhibitors.

"Mutant ssb1 ssb2 strains grow slowly, contain a low number of translating ribosomes, and are hypersensitive to several inhibitors of protein synthesis."

PMID:15456889

The ribosome-bound chaperones RAC and Ssb1/2p are required for accurate translation in Saccharomyces cerevisiae.

Translational fidelity is impaired in the absence of functional RAC or Ssb1/2p, with the principal defect in translation termination.

"Translational fidelity was impaired in the absence of functional RAC or Ssb1/2p, and the effect was further enhanced by paromomycin. The mutant strains suffered primarily from a defect in translation termination"

PMID:16429126

Proteome survey reveals modularity of the yeast cell machinery.

PMID:16554755

Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.

PMID:16607023

Specific effects of ribosome-tethered molecular chaperones on programmed -1 ribosomal frameshifting.

Deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p (RAC) specifically inhibits -1 programmed ribosomal frameshifting and impairs Killer virus maintenance, with no effect on +1 PRF.

"deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of -1 PRF and defects in Killer virus maintenance, while no effects were observed on +1 PRF."

PMID:16622836

The plasma membrane proteome of Saccharomyces cerevisiae and its response to the antifungal calcofluor.

PMID:19536198

An atlas of chaperone-protein interactions in Saccharomyces cerevisiae: implications to protein folding pathways in the cell.

PMID:19723765

The Hsp70 homolog Ssb is essential for glucose sensing via the SNF1 kinase network.

The chaperone Ssb is required to keep SNF1 in the nonphosphorylated state in the presence of glucose; Deltassb1 Deltassb2 cells display features reminiscent of glucose-repression mutants.

"the chaperone Ssb is required to keep SNF1 in the nonphosphorylated state in the presence of glucose."

PMID:20368619

A ribosome-anchored chaperone network that facilitates eukaryotic ribosome biogenesis.

PMID:23332755

The cotranslational function of ribosome-associated Hsp70 in eukaryotic protein homeostasis.

The yeast Hsp70 SSB binds a subset of nascent polypeptides whose intrinsic properties and slow translation rates hinder cotranslational folding; the SSB-ribosome cycle and substrate recognition are modulated by RAC.

"SSB binds to a subset of nascent polypeptides whose intrinsic properties and slow translation rates hinder efficient cotranslational folding. The SSB-ribosome cycle and substrate recognition is modulated by its ribosome-bound cochaperone, RAC."
Deletion of SSB leads to widespread aggregation of newly synthesized polypeptides, demonstrating its proteome-wide cotranslational folding role.

"Deletion of SSB leads to widespread aggregation of newly synthesized polypeptides."

PMID:26777405

ATPase-Modulated Stress Granules Contain a Diverse Proteome and Substructure.

PMID:37070168

RNA-dependent interactome allows network-based assignment of RNA-binding protein function.

PMID:37968396

The social and structural architecture of the yeast protein interactome.

PMID:9670014

The molecular chaperone Ssb from Saccharomyces cerevisiae is a component of the ribosome-nascent chain complex.

Ssb can be cross-linked to nascent chains and is released together with nascent chains upon puromycin treatment, demonstrating direct interaction with the nascent polypeptide.

"Ssb could be cross-linked to nascent chains containing a modified lysine residue with a photoactivatable cross-linker."
Ssb is a core component of the translating ribosome that interacts with both the nascent chain and the ribosome, functioning as a chaperone that prevents misfolding of newly synthesized proteins.

"Ssb is a core component of the translating ribosome which interacts with both the nascent polypeptide chain and the ribosome. These interactions allow Ssb to function as a chaperone on the ribosome, preventing the misfolding of newly synthesized proteins."

PMID:9860955

The biochemical properties of the ATPase activity of a 70-kDa heat shock protein (Hsp70) are governed by the C-terminal domains.

Ssb has an unusually low steady-state affinity for ATP but a higher maximal velocity, and its ATPase (unlike Ssa) is K+-independent; the peptide-binding domain shapes these properties.

"Ssb, however, has an unusually low steady-state affinity for ATP but a higher maximal velocity. In addition, the ATPase activity of Hsp70s, like that of Ssa1, depends on the addition of K+ whereas Ssb activity does not."

file:yeast/SSB2/SSB2-deep-research-falcon.md

Falcon deep research report on SSB2 (yeast, UniProt P40150 / YNL209W)

SSB2 (P40150/YNL209W) is one of two nearly identical ribosome-associated cytosolic Hsp70s in S. cerevisiae; SSB1 and SSB2 differ by ~4 residues and are generally studied together as "Ssb", confirming this is the genuine Ssb-type Hsp70 (not an unrelated same-symbol gene).

"Ssb is encoded by **two paralogous genes, SSB1 and SSB2**"
Ssb2 is a canonical Hsp70 with an N-terminal ATPase/nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD), driven by an ATP-dependent conformational cycle.

"core biochemistry is an **ATP-driven conformational cycle**"
The RAC J-domain protein Zuo1 stimulates Ssb1/2 ATP hydrolysis, driving the high-affinity substrate state that stabilizes nascent-chain binding.

"J-domain protein **Zuo1** stimulates ATP hydrolysis of **Ssb1/2**, driving this high-affinity substrate engagement on nascent chains"
Ssb belongs to an Hsp70 triad at the ribosomal tunnel exit (RAC = Zuo1 + Ssz1, plus Ssb), with RAC recruiting and activating Ssb to directly bind nascent chains for cotranslational folding.

"Ssb2 belongs to an **Hsp70 triad at the exit tunnel**"
Ssb1/2 are the direct nascent-chain binders during cotranslational folding in yeast; RAC is an obligate Zuo1-Ssz1 heterodimer anchored to the ribosome via Zuo1.

"RAC is an obligate Zuo1–Ssz1 heterodimer attached to the ribosome (via Zuo1)"
Ssb is cytosolic and ribosome-associated at the 60S tunnel exit; about 50% of total cellular Ssb is ribosome-bound at steady state (~1:1 with ribosomes), with the remainder free cytosolic.

"only about **~50% of total cellular Ssb** is ribosome-associated at steady state"
Ssb's cotranslational substrate coverage is broad, engaging ~80% of cytosolic/nuclear nascent proteins, supporting a proteome-wide cotranslational folding role.

"Ssb’s co-translational substrate coverage is broad"
Single SSB1 or SSB2 deletion gives little phenotype owing to redundancy, whereas combined ssb1/2 deletion causes broad defects.

"Single-gene loss has little obvious phenotype, whereas combined **ssb1/2Δ** causes broad defects"
Ssb suppresses formation and/or inheritance of multiple amyloid/prion-like heritable elements, linking it to proteostasis and aggregation control.

"Ssb suppresses formation and/or inheritance of multiple **amyloid/prion-like elements**"
The RAC/Ssb system is required for proper translational downregulation and proteostasis during TORC1 inhibition, connecting tunnel-exit chaperoning to nutrient/stress signaling.

"RAC/Ssb system is required for proper translational downregulation and proteostasis during TORC1 inhibition"
Ssb/RAC is linked to recruitment of the ubiquitin ligase Ltn1, connecting cotranslational chaperoning to ribosome-associated quality control.

"Ssb/RAC is linked to recruitment of the ubiquitin ligase **Ltn1**"

Deep Research

Falcon

(SSB2-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 21 citations 2 artifacts 2026-05-30T17:44:54.894214

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000008 Figure 4 on page 6 is the main model figure showing the positioning of the RAC complex (Zuo1 in orange, Ssz1 in blue) and its inter (image/png)

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.

Research report: Saccharomyces cerevisiae SSB2 (UniProt P40150; YNL209W) — ribosome-associated Hsp70 chaperone

0) Identity verification (critical disambiguation)

The target protein is Ssb2, a ribosome-associated cytosolic Hsp70 in Saccharomyces cerevisiae (S288c), encoded by SSB2 and commonly discussed together with its near-identical paralog SSB1 as “Ssb”. Experimental literature explicitly states that Ssb is encoded by two paralogous genes, SSB1 and SSB2, and functions as the ribosome-associated Hsp70 chaperone system in yeast, supporting that this report is about the intended UniProt P40150 protein family/context. (jaygarcia2023yeastchaperonehsp70ssb pages 2-3, ziegelhoffer2024nacandzuotinhsp70 pages 1-2)

1) Key concepts and definitions (current understanding)

1.1 Hsp70 chaperone cycle (what Ssb2 does)

Ssb2 is a canonical Hsp70-family molecular chaperone. As with other Hsp70s, its core biochemistry is an ATP-driven conformational cycle coupling an N-terminal nucleotide-binding/ATPase domain (NBD) to a C-terminal substrate-binding domain (SBD). In the ATP-bound state, Hsp70s typically exhibit lower substrate affinity and higher exchange; J-domain co-chaperones stimulate ATP hydrolysis, shifting Hsp70 to an ADP-bound high-affinity state that stabilizes client binding. In the yeast RAC–Ssb system, the J-domain protein Zuo1 stimulates ATP hydrolysis of Ssb1/2, driving this high-affinity substrate engagement on nascent chains. (chen2022structuralremodelingof pages 1-2, zhang2026thecotranslationalcycle pages 1-2)

1.2 Co-translational protein folding and the “tunnel-exit chaperone hub”

A major modern framework for Ssb2 function is that a substantial portion of cytosolic proteostasis is organized co-translationally at the ribosomal polypeptide exit tunnel, where nascent chains emerge and are immediately exposed to a local network of chaperones and biogenesis factors. Ssb2 belongs to an Hsp70 triad at the exit tunnel consisting of:
- RAC (ribosome-associated complex): Zuo1 (Hsp40/J-domain protein) + Ssz1 (atypical Hsp70)
- Ssb1/2 (Ssb): the canonical Hsp70(s) that directly bind nascent chains
This system is described as central to eukaryotic co-translational folding in yeast, with RAC both recruiting and activating Ssb near the tunnel exit. (chen2022structuralremodelingof pages 1-2, ziegelhoffer2024nacandzuotinhsp70 pages 1-2)

2) Molecular function, pathway placement, and mechanism

2.1 Primary biological role: RAC-dependent co-translational chaperoning of nascent chains

Ssb1/2 (including Ssb2) act as the direct nascent-chain binders during co-translational folding in yeast, functioning within the RAC–Ssb system: RAC is an obligate Zuo1–Ssz1 heterodimer attached to the ribosome (via Zuo1), and Zuo1’s J-domain stimulates Ssb ATP hydrolysis to stabilize nascent-chain binding. (chen2022structuralremodelingof pages 1-2)

Mechanistically, structural and crosslinking evidence supports a handover/relay: very early nascent chains are contacted by RAC components and, as the chain extends, Ssb becomes the predominant binder (a transition observed around ~50 amino acids in the cited crosslinking summary), consistent with “handoff” of the emerging peptide to Ssb for iterative binding–release cycles that promote productive folding and reduce off-pathway interactions. (chen2022structuralremodelingof pages 1-2)

2.2 Ribosome positioning and activation model (structural understanding; 2023 advance)

High-resolution cryo-EM work in 2023 provides a mechanistic picture of how RAC cooperates with Ssb at the tunnel exit. In the RAC-2 state, RAC is positioned such that Zuo1 contacts ribosomal protein uL29 near the tunnel exit, placing Ssb’s substrate-binding elements near the emerging chain; interactions extend beyond the canonical Hsp40–Hsp70 interface and center on the Zuo1 J-domain HPD motif in an activating arrangement with Ssb-ATP. A specific basic motif in Ssb (reported as KKR 429–431) is implicated in ribosome binding/engagement in this structural model. (kisonaite2023structuralinventoryof pages 21-23)

A structure-based working model (Figure 4 in Kišonaitė et al. 2023) proposes that RAC adopts distinct conformations and undergoes nascent-chain-triggered remodeling that exposes the Zuo1 HPD motif to enable productive Ssb activation and nascent chain capture adjacent to the tunnel exit. (kisonaite2023structuralinventoryof media 09f31c1f)

2.3 Interaction partners and connected processes

Key partners and connected processes supported by recent literature include:
- Zuo1 and Ssz1 (RAC): Zuo1 forms an extremely stable heterodimer with Ssz1, and Ssz1 transiently binds Ssb(ATP) in a recruitment/activation process; Zuo1 anchors the system near the exit tunnel to facilitate Ssb function. (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)
- NAC coexistence at the tunnel exit (2024 advance): In vivo crosslinking shows NAC and the Zuotin/Hsp70 system can coexist at the ribosome tunnel exit, rather than being strictly mutually exclusive, supporting an integrated tunnel-exit chaperone environment compatible with productive Ssb positioning. (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)
- Ribosome-associated quality control (RQC): Ssb/RAC is linked to recruitment of the ubiquitin ligase Ltn1, implicating Ssb in coupling co-translational chaperoning to quality-control ubiquitination of problematic nascent chains. (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26)

3) Cellular localization (where Ssb2 acts)

Ssb proteins (Ssb1/Ssb2) are cytosolic and ribosome-associated, positioned at the 60S tunnel exit where they can bind emerging nascent chains. Quantitatively, Ssb is described as binding ribosomes at approximately ~1:1 stoichiometry, while only about ~50% of total cellular Ssb is ribosome-associated at steady state (with the remainder cytosolic), consistent with a dynamic pool that can engage translating ribosomes and potentially other cytosolic substrates/aggregates. (black2023investigatingtherole pages 68-72)

4) Recent developments and latest research (prioritizing 2023–2024)

4.1 2023: Structural inventory of RAC and activation model for Ssb at the tunnel exit

Kišonaitė et al. (published 2023-06, URL https://doi.org/10.1038/s41594-023-00973-1) provide high-resolution structural snapshots of RAC bound to 80S ribosomes, supporting a mechanistic model for how RAC dynamics accommodate ribosome rotation while positioning and activating Ssb at the exit tunnel. This work strengthens a structure-based view of how the Zuo1 J-domain and RAC conformational remodeling coordinate Ssb activation and substrate capture during translation. (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f)

4.2 2024: Co-occupancy of NAC and Zuotin/Hsp70 at the tunnel exit in vivo

Ziegelhoffer et al. (published 2024-01, URL https://doi.org/10.1093/nar/gkae005) used in vivo site-specific crosslinking to show NAC and Zuotin/Hsp70 components can crosslink to one another at the ribosome and therefore can coexist near the tunnel exit. The inferred geometry supports that, even with NAC present, Hsp70 can adopt a productive orientation for nascent-chain engagement with the Zuo1 J-domain positioned to promote stable binding. (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)

4.3 2023: RAC/Ssb connects tunnel-exit proteostasis to TORC1-mediated translation control

Black et al. (published 2023-11, URL https://doi.org/10.15252/embj.2022113240) identified a role for RAC/Ssb in translational control during TORC1 inhibition. The study reports that zuo1Δ cells fail to appropriately reduce translation upon TORC1 inhibition and display proteostasis defects, with mechanistic connections to autophagy-mediated eIF4G degradation that is impaired in zuo1Δ. The requirement depends on a functional interaction between Zuo1 and Ssb. (black2023theribosome‐associatedchaperone pages 1-2, black2023theribosome‐associatedchaperone pages 9-11)

Quantitatively/experimentally, the authors report (i) a Zuo1 interactome remodeling upon rapamycin with 39 proteins increased and 11 decreased in association, (ii) rapamycin treatment conditions (e.g., 200 nM for ~1.5 h for some interactomics), and (iii) impaired eIF4G1/2 degradation in zuo1Δ, while eIF2α phosphorylation signaling remains intact (Sui2 phosphorylation not altered by Zuo1 loss). (black2023theribosome‐associatedchaperone pages 8-9, black2023theribosome‐associatedchaperone pages 9-11)

4.4 2023: Ssb suppresses multiple amyloid/prion-like heritable elements

Jay-Garcia et al. (published 2023-05, URL https://doi.org/10.3390/ijms24108660) synthesized prior knowledge and added data showing Ssb’s influence on several heritable protein-aggregate states, extending beyond the well-known [PSI+] system. Importantly, they report that after mild heat stress almost 20% of cells form a detectable prion in cultures lacking Ssb, supporting Ssb as a strong antagonist of stress-induced amyloid inheritance. (jaygarcia2023yeastchaperonehsp70ssb pages 24-26)

5) Current applications and real-world implementations

Although SSB2 itself is a yeast gene (not a direct clinical target), the RAC/Ssb system has practical “real-world” implementations in biotechnology and basic research:

Proteostasis engineering for recombinant expression in yeast: Understanding and manipulating co-translational folding (e.g., by modulating RAC/Ssb function) informs strategies to improve folding yields and reduce aggregation of recombinant proteins. The mechanistic model of tunnel-exit chaperoning provides a rational basis for tuning translation–folding coupling, especially for aggregation-prone proteins. (chen2022structuralremodelingof pages 1-2, kisonaite2023structuralinventoryof media 09f31c1f)
Models for translational stress responses: The connection of RAC/Ssb to TORC1 inhibition positions this system as an experimentally tractable module linking signaling, translation, and protein quality control; this is relevant for interpreting how translation reprogramming avoids proteotoxic stress during nutrient limitation or drug treatment (e.g., rapamycin). (black2023theribosome‐associatedchaperone pages 1-2, black2023theribosome‐associatedchaperone pages 9-11)
Aggregation and epigenetic-like inheritance studies (prions/mnemons): Ssb perturbation provides a tool to modulate amyloid formation and inheritance in yeast, enabling controlled studies of prion biology and proteostasis networks that can generalize to other organisms’ protein-aggregation problems. (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26)

6) Expert opinions and analysis (synthesis from authoritative sources)

A consensus emerging from recent high-quality mechanistic and in vivo work is that Ssb2’s “primary function” is best described not by narrow client specificity, but by architecting the earliest stages of proteome biogenesis through RAC-coupled Hsp70 cycling at the tunnel exit. Structural studies emphasize dynamic remodeling and precise geometry of RAC and Ssb positioning (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f), while in vivo crosslinking emphasizes that the tunnel exit is a shared platform where multiple factors can co-occupy and coordinate rather than simply compete (ziegelhoffer2024nacandzuotinhsp70 pages 1-2). Functional work extends this to systems-level physiology: when nutrient signaling requires translational downshift (TORC1 inhibition), RAC/Ssb is required to maintain viable proteostasis, suggesting the exit-tunnel chaperone platform is also a signal-responsive regulatory node in addition to a folding machine. (black2023theribosome‐associatedchaperone pages 1-2, black2023theribosome‐associatedchaperone pages 9-11)

7) Relevant statistics and quantitative data (recent/representative)

7.1 Stoichiometry and partitioning

Ssb:ribo ~1:1 stoichiometry, but only ~50% of cellular Ssb is ribosome-associated at steady state (remainder cytosolic). (black2023investigatingtherole pages 68-72)
Tunnel-exit factor abundance context: NAC:ribo ~1:1 and RAC:ribo ~0.3–0.5:1 reported. (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)

7.2 Proteome-wide substrate engagement

Ssb’s co-translational substrate coverage is broad:
- ~80% of cytosolic/nuclear proteins
- ~80% of (nascent) mitochondrial proteins
- ~46% (or “>40%”) of ER-targeted proteins
These values imply Ssb2 participates in folding/biogenesis decisions for a large fraction of the nascent proteome. (black2023investigatingtherole pages 68-72, chen2022structuralremodelingof pages 1-2)

7.3 Aggregation inheritance

After mild heat stress, ~20% of cells form a detectable prion in cultures lacking Ssb, supporting a strong suppressive role for Ssb in stress-linked amyloid inheritance. (jaygarcia2023yeastchaperonehsp70ssb pages 24-26)

8) Limitations and gene-specific caveats for SSB2

Most mechanistic and quantitative literature treats Ssb1 and Ssb2 together as “Ssb,” consistent with their near identity (4 amino acid differences) and functional redundancy; many studies therefore do not provide SSB2-only biochemical specificity or phenotypes. Nonetheless, statements about “Ssb (SSB1/SSB2)” map directly onto Ssb2’s annotated molecular role as the canonical ribosome-associated Hsp70 in yeast. (black2023investigatingtherole pages 68-72, chen2022structuralremodelingof pages 1-2)

Summary table

The following table consolidates functional annotation, partners, localization, pathway role, and 2023–2024 developments with URLs.

Section	SSB2-specific summary	Key evidence / details	Recent source(s) with date and URL
Identity / orthology / redundancy with SSB1	SSB2 encodes one of the two nearly identical ribosome-associated cytosolic Hsp70s in Saccharomyces cerevisiae; Ssb1 and Ssb2 differ by only 4 amino acids and are generally treated together as Ssb in the literature, with strong functional redundancy. Single-gene loss has little obvious phenotype, whereas combined ssb1/2Δ causes broad defects (black2023investigatingtherole pages 68-72, jaygarcia2023yeastchaperonehsp70ssb pages 2-3, black2023investigatingtherole pages 63-68, chen2022structuralremodelingof pages 1-2).	Confirms the target is the yeast ribosome-associated Ssb-type Hsp70 rather than unrelated “SSB2” genes from other organisms; literature usually does not distinguish unique biochemical activities of Ssb2 from Ssb1 (black2023investigatingtherole pages 68-72, ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Ziegelhoffer et al., 2024-01, Nucleic Acids Research, https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2); Jay-Garcia et al., 2023-05, Int. J. Mol. Sci., https://doi.org/10.3390/ijms24108660 (jaygarcia2023yeastchaperonehsp70ssb pages 2-3)
Molecular function	Ssb2 is a canonical Hsp70 chaperone with an N-terminal ATPase/nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD). In the ATP state, Ssb is poised for substrate capture; Zuo1 J-domain stimulates ATP hydrolysis, shifting Ssb to an ADP-bound high-affinity state that stabilizes nascent-chain binding (chen2022structuralremodelingof pages 1-2, zhang2026thecotranslationalcycle pages 1-2).	Substrate binding occurs near the ribosomal peptide exit tunnel; Ssb recognizes broad nascent-chain clients and engages them through repeated binding–release cycles typical of Hsp70s (chen2022structuralremodelingof pages 1-2, black2023investigatingtherole pages 68-72, zhang2026thecotranslationalcycle pages 1-2).	Chen et al., 2022-06, Nature Communications, https://doi.org/10.1038/s41467-022-31127-4 (chen2022structuralremodelingof pages 1-2); Zhang et al., 2026-01, Nature Communications, https://doi.org/10.1038/s41467-025-67685-6 (zhang2026thecotranslationalcycle pages 1-2)
Core pathway / biological process	SSB2 functions in the RAC–Ssb co-translational folding pathway at the ribosome exit tunnel. RAC is the ribosome-associated complex of Zuo1 (J-protein/Hsp40) plus Ssz1 (atypical Hsp70), which recruits and activates Ssb to receive emerging nascent chains and promote proper folding during translation (chen2022structuralremodelingof pages 1-2, kisonaite2023structuralinventoryof pages 21-23, ziegelhoffer2024nacandzuotinhsp70 pages 1-2, kisonaite2023structuralinventoryof media 09f31c1f).	Structural work supports a relay model: Zuo1/Ssz1 contact very short nascent chains first; as the chain extends, RAC rearranges to expose the Zuo1 HPD motif and position Ssb adjacent to the tunnel exit for efficient handoff and folding (chen2022structuralremodelingof pages 1-2, kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f).	Kišonaitė et al., 2023-06, Nature Structural & Molecular Biology, https://doi.org/10.1038/s41594-023-00973-1 (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f); Chen et al., 2022-06, https://doi.org/10.1038/s41467-022-31127-4 (chen2022structuralremodelingof pages 1-2)
Interaction partners	Major partners are Zuo1, Ssz1, the 80S ribosome near the peptide tunnel exit, and quality-control machinery including Ltn1; recent work also shows NAC can coexist with the Zuotin/Hsp70 system at the tunnel exit rather than being strictly mutually exclusive (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26, ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Structural/biochemical details include Zuo1 contact with ribosomal features near the exit tunnel and a conserved basic motif in Ssb implicated in ribosome engagement; RAC also coordinates Ssb activation. Ssb/RAC is linked to ribosome-associated quality control and ubiquitination of nascent chains through Ltn1 (kisonaite2023structuralinventoryof pages 21-23, jaygarcia2023yeastchaperonehsp70ssb pages 24-26, ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Ziegelhoffer et al., 2024-01, https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2); Kišonaitė et al., 2023-06, https://doi.org/10.1038/s41594-023-00973-1 (kisonaite2023structuralinventoryof pages 21-23); Jay-Garcia et al., 2023-05, https://doi.org/10.3390/ijms24108660 (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26)
Localization	Ssb2 is primarily ribosome-associated on the cytosolic face of translating 80S ribosomes, positioned near the 60S tunnel exit, but a substantial pool is also cytosolic. Ssb can shuttle, and RAC strongly promotes its association with translating ribosomes (black2023investigatingtherole pages 68-72, ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Direct ribosome interaction involves basic regions in Ssb and ribosomal proteins/rRNA near the exit tunnel; in vivo, RAC recruitment can compensate for loss of autonomous ribosome-binding determinants (black2023investigatingtherole pages 68-72).	Ziegelhoffer et al., 2024-01, https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2); Black et al., 2023-11, EMBO Journal, https://doi.org/10.15252/embj.2022113240 (functional RAC/Ssb context) (black2023investigatingtherole pages 63-68)
Quantitative stats	Reported quantitative values for Ssb/Ssb1/2 include: ~1:1 stoichiometry with ribosomes; only ~50% of total cellular Ssb is ribosome-bound, with the remainder cytosolic; substrate coverage includes ~80% of cytosolic/nuclear proteins, ~80% of mitochondrial proteins, and ~46% of ER-targeted proteins (black2023investigatingtherole pages 68-72, chen2022structuralremodelingof pages 1-2).	For contextual comparison, the RAC:ribo ratio is reported at ~0.3–0.5:1, while NAC:ribo is about ~1:1 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2). These values emphasize how broadly Ssb surveils the nascent proteome and how abundant the ribosome-tunnel chaperone environment is (black2023investigatingtherole pages 68-72, ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Ziegelhoffer et al., 2024-01, https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2); Chen et al., 2022-06, https://doi.org/10.1038/s41467-022-31127-4 (chen2022structuralremodelingof pages 1-2)
Recent development (structural mechanism)	Kišonaitė 2023 provided high-resolution cryo-EM views of RAC on the 80S ribosome and a model for how RAC dynamics accommodate ribosome rotation while positioning Ssb for activation at the tunnel exit (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f).	Key advance: RAC adopts at least two conformations; nascent-chain-triggered remodeling exposes the Zuo1 HPD motif and supports Ssb activation/substrate capture (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f).	Kišonaitė et al., 2023-06, https://doi.org/10.1038/s41594-023-00973-1 (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f)
Recent development (ribosome tunnel exit occupancy)	Ziegelhoffer 2024 showed that NAC and Zuotin/Hsp70 can coexist at the ribosome tunnel exit in vivo, revising a simplistic competition-only model of tunnel-exit factor occupancy (ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	This supports a more integrated chaperone platform at the exit tunnel, with productive positioning for Ssb-mediated nascent-chain capture even when NAC is present (ziegelhoffer2024nacandzuotinhsp70 pages 1-2).	Ziegelhoffer et al., 2024-01, https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)
Recent development (signaling / TORC1 response)	Black 2023 (EMBO J.) found that the RAC/Ssb system is required for proper translational downregulation and proteostasis during TORC1 inhibition, linking this ribosome-associated chaperone system to nutrient/stress signaling responses (black2023investigatingtherole pages 63-68).	In the absence of Zuo1, translation fails to decrease appropriately after TORC1 loss, and defects in autophagy/eIF4G turnover contribute to reduced survival; a functional interaction between Zuo1 and Ssb is required (black2023investigatingtherole pages 63-68).	Black et al., 2023-11, The EMBO Journal, https://doi.org/10.15252/embj.2022113240 (black2023investigatingtherole pages 63-68)
Recent development (proteostasis / prion control)	Jay-Garcia 2023 expanded the known proteostasis role of Ssb beyond general folding, showing that Ssb suppresses formation and/or inheritance of multiple amyloid/prion-like elements including [PSI+], [LSB+], [STE+], and influences [URE3] behavior (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26, jaygarcia2023yeastchaperonehsp70ssb pages 19-20).	Notably, loss of Ssb strongly enhances stress-associated aggregate inheritance; the paper reports that almost 20% of cells form a detectable prion after mild heat stress in strains lacking Ssb (jaygarcia2023yeastchaperonehsp70ssb pages 24-26).	Jay-Garcia et al., 2023-05, https://doi.org/10.3390/ijms24108660 (jaygarcia2023yeastchaperonehsp70ssb pages 1-2, jaygarcia2023yeastchaperonehsp70ssb pages 24-26, jaygarcia2023yeastchaperonehsp70ssb pages 19-20)

Table: This table summarizes validated functional annotation for yeast SSB2 (UniProt P40150/YNL209W), emphasizing its identity as the ribosome-associated Ssb-type Hsp70, core RAC-dependent co-translational folding role, localization, interaction partners, quantitative properties, and key 2023–2024 developments.

Key model figure (visual evidence)

A structure-based working model for RAC conformational states and Ssb activation at the ribosomal tunnel exit is shown in Kišonaitė et al. 2023 Figure 4. (kisonaite2023structuralinventoryof media 09f31c1f)

Selected recent references (with dates and URLs)

Kišonaitė M. et al. 2023-06. Nat Struct Mol Biol. “Structural inventory of cotranslational protein folding by the eukaryotic RAC complex.” https://doi.org/10.1038/s41594-023-00973-1 (kisonaite2023structuralinventoryof pages 21-23, kisonaite2023structuralinventoryof media 09f31c1f)
Ziegelhoffer T. et al. 2024-01. Nucleic Acids Research. “NAC and Zuotin/Hsp70 chaperone systems coexist at the ribosome tunnel exit in vivo.” https://doi.org/10.1093/nar/gkae005 (ziegelhoffer2024nacandzuotinhsp70 pages 1-2)
Black A. et al. 2023-11. The EMBO Journal. “The ribosome-associated chaperone Zuo1 controls translation upon TORC1 inhibition.” https://doi.org/10.15252/embj.2022113240 (black2023theribosome‐associatedchaperone pages 1-2, black2023theribosome‐associatedchaperone pages 9-11)
Jay-Garcia L.M. et al. 2023-05. Int J Mol Sci. “Yeast Chaperone Hsp70-Ssb Modulates a Variety of Protein-Based Heritable Elements.” https://doi.org/10.3390/ijms24108660 (jaygarcia2023yeastchaperonehsp70ssb pages 24-26)
Chen Y. et al. 2022-06. Nat Commun. “Structural remodeling of ribosome associated Hsp40-Hsp70 chaperones during co-translational folding.” https://doi.org/10.1038/s41467-022-31127-4 (chen2022structuralremodelingof pages 1-2)

References

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(black2023investigatingtherole pages 68-72): A Black. Investigating the role of the ribosome associated chaperone zuo1 under torc1 inhibition. Unknown journal, 2023.
(black2023theribosome‐associatedchaperone pages 1-2): Ailsa Black, Thomas D Williams, Flavie Soubigou, Ifeoluwapo M Joshua, Houjiang Zhou, Frederic Lamoliatte, and Adrien Rousseau. The ribosome‐associated chaperone zuo1 controls translation upon torc1 inhibition. The EMBO Journal, Nov 2023. URL: https://doi.org/10.15252/embj.2022113240, doi:10.15252/embj.2022113240. This article has 11 citations.
(black2023theribosome‐associatedchaperone pages 9-11): Ailsa Black, Thomas D Williams, Flavie Soubigou, Ifeoluwapo M Joshua, Houjiang Zhou, Frederic Lamoliatte, and Adrien Rousseau. The ribosome‐associated chaperone zuo1 controls translation upon torc1 inhibition. The EMBO Journal, Nov 2023. URL: https://doi.org/10.15252/embj.2022113240, doi:10.15252/embj.2022113240. This article has 11 citations.
(black2023theribosome‐associatedchaperone pages 8-9): Ailsa Black, Thomas D Williams, Flavie Soubigou, Ifeoluwapo M Joshua, Houjiang Zhou, Frederic Lamoliatte, and Adrien Rousseau. The ribosome‐associated chaperone zuo1 controls translation upon torc1 inhibition. The EMBO Journal, Nov 2023. URL: https://doi.org/10.15252/embj.2022113240, doi:10.15252/embj.2022113240. This article has 11 citations.
(black2023investigatingtherole pages 63-68): A Black. Investigating the role of the ribosome associated chaperone zuo1 under torc1 inhibition. Unknown journal, 2023.
(jaygarcia2023yeastchaperonehsp70ssb pages 19-20): Lina M. Jay-Garcia, Joseph L. Cornell, Rebecca L. Howie, Quincy L. Faber, Abigail Salas, Tatiana A. Chernova, and Yury O. Chernoff. Yeast chaperone hsp70-ssb modulates a variety of protein-based heritable elements. International Journal of Molecular Sciences, 24:8660, May 2023. URL: https://doi.org/10.3390/ijms24108660, doi:10.3390/ijms24108660. This article has 4 citations.

Artifacts

Edison artifact artifact-00

Citations

chen2022structuralremodelingof pages 1-2
kisonaite2023structuralinventoryof pages 21-23
black2023investigatingtherole pages 68-72
zhang2026thecotranslationalcycle pages 1-2
black2023investigatingtherole pages 63-68
PSI+
LSB+
STE+
URE3
https://doi.org/10.1038/s41594-023-00973-1
https://doi.org/10.1093/nar/gkae005
https://doi.org/10.15252/embj.2022113240
https://doi.org/10.3390/ijms24108660
https://doi.org/10.1038/s41467-022-31127-4
https://doi.org/10.1038/s41467-025-67685-6
https://doi.org/10.3390/ijms24108660,
https://doi.org/10.1093/nar/gkae005,
https://doi.org/10.1038/s41467-022-31127-4,
https://doi.org/10.1038/s41467-025-67685-6,
https://doi.org/10.1038/s41594-023-00973-1,
https://doi.org/10.15252/embj.2022113240,

📄 View Raw YAML

id: P40150
gene_symbol: SSB2
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:559292
  label: Saccharomyces cerevisiae
description: |-
  SSB2 (YNL209W, UniProt P40150) encodes one of the two nearly identical
  ribosome-associated cytosolic Hsp70 chaperones of Saccharomyces cerevisiae;
  its paralog SSB1 (P11484) differs by only ~4 residues, and the two are almost
  always studied together as "Ssb". GENE-IDENTITY NOTE: this is the genuine
  ribosome-associated Hsp70 paralog (Ssb-type Hsp70 subfamily), NOT to be
  confused with the unrelated sibling-symbol collision elsewhere in this dataset
  where "SSB1" resolved to Sbp1p/P10080, an RGG/RRM RNA-binding protein. Both
  the UniProt accession (P40150, "Ribosome-associated molecular chaperone SSB2",
  EC 3.6.4.10, heat shock protein 70 family / Ssb-type subfamily) and the falcon
  deep research report independently confirm the ribosome-associated Hsp70
  identity. Ssb2 is a canonical Hsp70 with an N-terminal nucleotide-binding/ATPase
  domain (NBD) and a C-terminal substrate-binding domain (SBD); it uses an
  ATP-driven conformational cycle to bind short hydrophobic segments of nascent
  polypeptides as they emerge from the ribosomal tunnel exit. Its core function
  is de novo cotranslational protein folding: Ssb directly binds nascent chains
  on translating 80S ribosomes and is activated by the ribosome-associated
  complex (RAC, the Zuo1 J-protein + Ssz1 atypical Hsp70 heterodimer), whose Zuo1
  J-domain stimulates Ssb ATP hydrolysis to drive the high-affinity substrate
  state. About 50% of cellular Ssb is ribosome-bound at any time (~1:1 with
  ribosomes), and Ssb engages a large fraction of the nascent proteome. Downstream
  Ssb biology includes maintenance of translational fidelity (especially
  termination and -1 programmed ribosomal frameshifting), suppression of
  protein aggregation and prion/amyloid inheritance, glucose sensing via the
  SNF1 network, and connections to ribosome-associated quality control (Ltn1).
existing_annotations:
- term:
    id: GO:0005634
    label: nucleus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 functions on cytosolic translating ribosomes; a nuclear pool is at
      most transient/peripheral (e.g., association with pre-ribosomes during
      ribosome biogenesis). Not a core localization.
    action: KEEP_AS_NON_CORE
    reason: |-
      Ssb is overwhelmingly cytosolic and ribosome-associated. The falcon report
      localizes Ssb to the cytosol and 60S tunnel exit with no evidence for an
      autonomous nuclear function; any nuclear signal is best treated as
      peripheral/context-dependent.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
      reference_section_type: OTHER
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 is a cytoplasmic/cytosolic chaperone. Cytoplasm is correct but
      generic; the more informative localization is cytosol/ribosome-associated.
    action: KEEP_AS_NON_CORE
    reason: |-
      Correct but non-specific. The functionally meaningful localization is the
      cytosolic translating ribosome (see cytosol annotation).
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
      reference_section_type: OTHER
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Plasma membrane is not a site of Ssb2 function. This likely reflects
      high-throughput proteome surveys detecting the abundant cytosolic Ssb pool.
    action: REMOVE
    reason: |-
      No experimental or literature support for plasma membrane function. The
      falcon report and UniProt localize Ssb to the cytosol and ribosome; a
      plasma membrane assignment for this abundant cytosolic Hsp70 is an
      over-annotation from high-throughput localization datasets.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
      reference_section_type: OTHER
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 is an Hsp70 ATPase (EC 3.6.4.10); ATP hydrolysis powers its chaperone
      cycle and is stimulated by the Zuo1 J-domain of RAC.
    action: ACCEPT
    reason: |-
      Directly supported. The Ssb ATPase activity is experimentally characterized
      and the ATP-driven conformational cycle is central to its function.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: J-domain protein **Zuo1** stimulates ATP hydrolysis of **Ssb1/2**, driving this high-affinity substrate engagement on nascent chains
      reference_section_type: OTHER
- term:
    id: GO:0031072
    label: heat shock protein binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 interacts with co-chaperone partners of the Hsp70 system, notably the
      RAC J-protein Zuo1/Ssz1 and the Hsp110 nucleotide exchange factor Sse1.
    action: ACCEPT
    reason: |-
      Consistent with the documented Ssb-RAC and Ssb-Sse1 (Hsp110) functional
      interactions that constitute the ribosome-associated Hsp70 chaperone system.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: RAC is an obligate Zuo1–Ssz1 heterodimer attached to the ribosome (via Zuo1)
      reference_section_type: OTHER
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 is a protein folding chaperone that assists cotranslational folding of
      nascent chains. The more precise MF for this ATP-driven Hsp70 is
      ATP-dependent protein folding chaperone (GO:0140662).
    action: ACCEPT
    reason: |-
      Core molecular function. Accepted; a more specific child term
      (ATP-dependent protein folding chaperone, GO:0140662) is captured in
      core_functions to reflect the ATP-dependent Hsp70 mechanism.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb1/2 (including Ssb2) act as the **direct nascent-chain binders** during co-translational folding in yeast
      reference_section_type: OTHER
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb2 is a cytosolic chaperone; ~50% is ribosome-associated and the
      remainder is free cytosolic Ssb. This is the core localization.
    action: ACCEPT
    reason: |-
      Strongly supported. The cytosol (and specifically cytosolic translating
      ribosomes) is where Ssb2 carries out its function.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: only about **~50% of total cellular Ssb** is ribosome-associated at steady state
      reference_section_type: OTHER
- term:
    id: GO:0042026
    label: protein refolding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      Ssb's primary, best-supported role is de novo cotranslational folding of
      nascent chains rather than refolding of pre-existing denatured proteins,
      though as an Hsp70 it can contribute to general proteostasis/aggregation
      prevention.
    action: KEEP_AS_NON_CORE
    reason: |-
      Plausible Hsp70 activity but not the core, distinguishing function of Ssb.
      The falcon report and primary literature emphasize cotranslational folding
      of nascent chains; refolding is a generic Hsp70 capability kept as non-core.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb suppresses formation and/or inheritance of multiple **amyloid/prion-like elements**
      reference_section_type: OTHER
- term:
    id: GO:0000054
    label: ribosomal subunit export from nucleus
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      Ssb/RAC participates in a ribosome-anchored chaperone network linked to
      ribosome biogenesis; a role in ribosomal subunit export was reported by
      genetic interaction. This is downstream/ancillary to the core folding role.
    action: KEEP_AS_NON_CORE
    reason: |-
      Supported by genetic interaction (PMID:20368619) but ancillary to the core
      cotranslational chaperone function. Kept as non-core.
- term:
    id: GO:0000166
    label: nucleotide binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: |-
      Generic parent of ATP binding. Ssb2 binds ATP via its NBD; the more
      specific ATP binding term is preferred.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Too generic. ATP binding (GO:0005524) captures the actual ligand more
      precisely; nucleotide binding is an uninformative parent.
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: |-
      Ssb2's N-terminal nucleotide-binding domain binds ATP, the basis of its
      ATP-driven Hsp70 chaperone cycle.
    action: ACCEPT
    reason: |-
      Directly supported by domain architecture and the ATP-driven conformational
      cycle of the chaperone.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: core biochemistry is an **ATP-driven conformational cycle**
      reference_section_type: OTHER
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: |-
      Cytoplasm is correct but generic; cytosol/ribosome-associated is the
      informative localization (duplicate of the IBA cytoplasm annotation).
    action: KEEP_AS_NON_CORE
    reason: |-
      Correct but non-specific localization, retained as non-core.
- term:
    id: GO:0006364
    label: rRNA processing
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      Ssb/RAC is part of a ribosome-anchored chaperone network implicated in
      ribosome biogenesis (rRNA processing reported by genetic interaction). This
      is ancillary to the core cotranslational folding role.
    action: KEEP_AS_NON_CORE
    reason: |-
      Indirect/ancillary role via the ribosome-anchored chaperone network
      (PMID:20368619); not the core function.
- term:
    id: GO:0006412
    label: translation
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: |-
      Generic parent. Ssb acts on translating ribosomes; the more specific
      cytoplasmic translation and de novo cotranslational protein folding terms
      better capture its role.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Too broad. Ssb is not a core translation factor; its role is cotranslational
      chaperoning. More specific terms (cytoplasmic translation,
      cotranslational protein folding) are preferred.
- term:
    id: GO:0006450
    label: regulation of translational fidelity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      Loss of Ssb1/2 (or RAC) impairs translational fidelity, primarily at
      translation termination. A genuine, experimentally supported downstream role.
    action: ACCEPT
    reason: |-
      Supported experimentally (PMID:15456889): RAC and Ssb1/2p are crucial for
      translational fidelity, with the principal defect in translation termination.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
- term:
    id: GO:0006452
    label: translational frameshifting
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      Deletion of Ssb1/2 (or RAC) specifically inhibits -1 programmed ribosomal
      frameshifting and impairs Killer virus maintenance.
    action: ACCEPT
    reason: |-
      Supported experimentally (PMID:16607023). Note the effect is specific to -1
      PRF (no effect on +1 PRF), a downstream consequence of Ssb's role at the
      translating ribosome.
- term:
    id: GO:0016787
    label: hydrolase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: |-
      Uninformative parent term. Ssb2's relevant activity is ATP hydrolysis
      (GO:0016887) as part of its Hsp70 ATPase cycle.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Too generic. ATP hydrolysis activity (GO:0016887) is the specific and
      accurate term.
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: |-
      Ssb2 hydrolyzes ATP as part of its Hsp70 chaperone cycle (EC 3.6.4.10),
      stimulated by the RAC J-protein Zuo1.
    action: ACCEPT
    reason: |-
      Directly supported; duplicate of the IBA/IDA ATP hydrolysis annotations.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      Ssb2 binds exposed hydrophobic segments of unfolded/nascent polypeptides.
      The chaperone activity is better captured by ATP-dependent protein folding
      chaperone (GO:0140662).
    action: MODIFY
    reason: |-
      The binding term is accurate but a holdase/binding-only term understates
      the ATP-driven Hsp70 mechanism. Modify to the ATP-dependent protein folding
      chaperone MF.
    proposed_replacement_terms:
    - id: GO:0140662
      label: ATP-dependent protein folding chaperone
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: J-domain protein **Zuo1** stimulates ATP hydrolysis of **Ssb1/2**, driving this high-affinity substrate engagement on nascent chains
      reference_section_type: OTHER
- term:
    id: GO:0051083
    label: '''de novo'' cotranslational protein folding'
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: |-
      This is the core biological process of Ssb2: direct binding to nascent
      chains at the ribosomal tunnel exit to promote de novo cotranslational
      folding, activated by RAC.
    action: ACCEPT
    reason: |-
      Core function, strongly supported by both the falcon report and primary
      literature (PMID:9670014, PMID:23332755).
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb1/2 (including Ssb2) act as the **direct nascent-chain binders** during co-translational folding in yeast
      reference_section_type: OTHER
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16429126
  review:
    summary: |-
      Generic protein-binding from a high-throughput proteome survey; provides no
      specific functional information.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative protein binding term per curation guidelines; the specific
      Ssb-RAC/Ssb-nascent chain interactions are captured by chaperone MF terms.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16554755
  review:
    summary: |-
      Generic protein-binding from a high-throughput complex landscape study; no
      specific functional information.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative protein binding term per curation guidelines.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19536198
  review:
    summary: |-
      Generic protein-binding from a chaperone-interaction atlas; no specific
      functional information beyond the chaperone network role.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative protein binding term per curation guidelines.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:23332755
  review:
    summary: |-
      This reference actually documents Ssb's cotranslational nascent-chain
      substrate binding and RAC modulation; the generic protein binding term
      understates it.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative as protein binding; the specific function from this paper
      (cotranslational chaperoning of nascent chains) is captured by the
      de novo cotranslational protein folding and chaperone MF annotations.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:37070168
  review:
    summary: |-
      Generic protein-binding from an RNA-dependent interactome study; no specific
      functional information.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative protein binding term per curation guidelines.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:37968396
  review:
    summary: |-
      Generic protein-binding from a global interactome architecture study; no
      specific functional information.
    action: MARK_AS_OVER_ANNOTATED
    reason: |-
      Uninformative protein binding term per curation guidelines.
- term:
    id: GO:0010494
    label: cytoplasmic stress granule
  evidence_type: HDA
  original_reference_id: PMID:26777405
  review:
    summary: |-
      As an abundant cytosolic Hsp70, Ssb2 is detected in stress granules; this
      is a stress-condition localization, not the core function.
    action: KEEP_AS_NON_CORE
    reason: |-
      Plausible stress-condition localization detected by high-throughput
      proteomics; peripheral to the core cotranslational folding function.
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: HDA
  original_reference_id: PMID:11914276
  review:
    summary: |-
      Cytoplasm localization confirmed by genome-wide GFP localization; correct
      but generic relative to cytosol/ribosome-associated.
    action: KEEP_AS_NON_CORE
    reason: |-
      Correct but non-specific localization, retained as non-core.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: HDA
  original_reference_id: PMID:16622836
  review:
    summary: |-
      Plasma membrane proteome detection of the abundant cytosolic Ssb; not a
      genuine functional localization.
    action: REMOVE
    reason: |-
      Over-annotation from a plasma-membrane proteome survey. Ssb is a cytosolic
      ribosome-associated Hsp70 with no functional role at the plasma membrane.
    additional_reference_ids:
    - file:yeast/SSB2/SSB2-deep-research-falcon.md
    supported_by:
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
      reference_section_type: OTHER
- term:
    id: GO:0006452
    label: translational frameshifting
  evidence_type: IMP
  original_reference_id: PMID:16607023
  review:
    summary: |-
      Deletion of Ssb1p/Ssb2p (or RAC) specifically inhibits -1 programmed
      ribosomal frameshifting and impairs Killer virus maintenance, with no effect
      on +1 PRF.
    action: ACCEPT
    reason: |-
      Strong direct IMP evidence (PMID:16607023). A genuine, specific downstream
      consequence of Ssb function at the translating ribosome.
    supported_by:
    - reference_id: PMID:16607023
      supporting_text: deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of -1
      reference_section_type: ABSTRACT
- term:
    id: GO:0000054
    label: ribosomal subunit export from nucleus
  evidence_type: IGI
  original_reference_id: PMID:20368619
  review:
    summary: |-
      Genetic interaction evidence places Ssb/RAC in a ribosome-anchored
      chaperone network facilitating ribosome biogenesis, including subunit
      export. Ancillary to the core cotranslational folding role.
    action: KEEP_AS_NON_CORE
    reason: |-
      Supported by genetic interaction (PMID:20368619) but ancillary; kept as
      non-core.
- term:
    id: GO:0002181
    label: cytoplasmic translation
  evidence_type: IMP
  original_reference_id: PMID:1394434
  review:
    summary: |-
      Ssb1/2 are associated with translating ribosomes; ssb1 ssb2 mutants grow
      slowly, have fewer translating ribosomes, and are hypersensitive to protein
      synthesis inhibitors, linking Ssb to cytoplasmic translation.
    action: ACCEPT
    reason: |-
      Supported by IMP (PMID:1394434). Ssb participates in cytoplasmic translation
      as a ribosome-associated cotranslational chaperone.
    supported_by:
    - reference_id: PMID:1394434
      supporting_text: Mutant ssb1 ssb2
      reference_section_type: ABSTRACT
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Single-gene loss has little obvious phenotype, whereas combined **ssb1/2Δ** causes broad defects
      reference_section_type: OTHER
- term:
    id: GO:0002181
    label: cytoplasmic translation
  evidence_type: IPI
  original_reference_id: PMID:1394434
  review:
    summary: |-
      Ssb1/2p associate with translating ribosomes and the association is
      disrupted by puromycin, suggesting direct binding to the nascent
      polypeptide during cytoplasmic translation.
    action: ACCEPT
    reason: |-
      Supported (PMID:1394434). Consistent IPI/IMP support for Ssb's role at the
      translating cytoplasmic ribosome (same action as the paired IMP annotation).
    supported_by:
    - reference_id: PMID:1394434
      supporting_text: The SSB hsp70s (Ssb1/2p) are associated with
      reference_section_type: ABSTRACT
- term:
    id: GO:0006364
    label: rRNA processing
  evidence_type: IGI
  original_reference_id: PMID:20368619
  review:
    summary: |-
      Genetic interaction places Ssb/RAC in a ribosome-anchored chaperone network
      facilitating ribosome biogenesis (rRNA processing). Ancillary to the core
      cotranslational folding role.
    action: KEEP_AS_NON_CORE
    reason: |-
      Indirect/ancillary role via the ribosome-anchored chaperone network
      (PMID:20368619); not the core function.
- term:
    id: GO:0006450
    label: regulation of translational fidelity
  evidence_type: IMP
  original_reference_id: PMID:15456889
  review:
    summary: |-
      Absence of RAC or Ssb1/2p impairs translational fidelity in vivo and in
      vitro, primarily through a defect in translation termination, enhanced by
      paromomycin.
    action: ACCEPT
    reason: |-
      Strong direct IMP evidence (PMID:15456889) that Ssb1/2p are crucial for
      translational fidelity beyond their chaperone role for nascent chains.
    supported_by:
    - reference_id: PMID:15456889
      supporting_text: Translational fidelity was impaired in the absence of functional RAC or Ssb1/2p
      reference_section_type: ABSTRACT
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IDA
  original_reference_id: PMID:9860955
  review:
    summary: |-
      Ssb has direct, biochemically characterized ATPase activity with unusual
      kinetics (low steady-state affinity for ATP, higher Vmax, K+-independent)
      governed by its C-terminal domains.
    action: ACCEPT
    reason: |-
      Strong direct IDA biochemical evidence (PMID:9860955) for the Ssb ATPase
      activity underlying its Hsp70 chaperone cycle.
    supported_by:
    - reference_id: PMID:9860955
      supporting_text: Ssb, however, has an unusually low steady-state affinity for ATP but a
      reference_section_type: ABSTRACT
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: core biochemistry is an **ATP-driven conformational cycle**
      reference_section_type: OTHER
- term:
    id: GO:0042149
    label: cellular response to glucose starvation
  evidence_type: IGI
  original_reference_id: PMID:19723765
  review:
    summary: |-
      Ssb is required for glucose sensing via the SNF1 kinase network: the
      chaperone keeps SNF1 in the nonphosphorylated state in the presence of
      glucose, and Deltassb1 Deltassb2 cells resemble glucose-repression mutants.
    action: ACCEPT
    reason: |-
      Supported by genetic interaction (PMID:19723765). A genuine downstream
      physiological role connecting Ssb chaperone function to glucose/SNF1
      signaling.
    supported_by:
    - reference_id: PMID:19723765
      supporting_text: the chaperone Ssb is required to keep SNF1 in the
      reference_section_type: ABSTRACT
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IDA
  original_reference_id: PMID:9670014
  review:
    summary: |-
      Ssb can be cross-linked to nascent chains and is released with nascent
      chains upon puromycin treatment, demonstrating direct binding to
      unfolded/nascent polypeptides. The ATP-driven chaperone mechanism is better
      captured by ATP-dependent protein folding chaperone (GO:0140662).
    action: MODIFY
    reason: |-
      The binding term is directly supported (PMID:9670014) but a binding-only
      term understates the ATP-dependent Hsp70 mechanism. Modify to the
      ATP-dependent protein folding chaperone MF.
    proposed_replacement_terms:
    - id: GO:0140662
      label: ATP-dependent protein folding chaperone
    supported_by:
    - reference_id: PMID:9670014
      supporting_text: Ssb could be cross-linked to nascent chains
      reference_section_type: ABSTRACT
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: J-domain protein **Zuo1** stimulates ATP hydrolysis of **Ssb1/2**, driving this high-affinity substrate engagement on nascent chains
      reference_section_type: OTHER
- term:
    id: GO:0051083
    label: '''de novo'' cotranslational protein folding'
  evidence_type: IDA
  original_reference_id: PMID:9670014
  review:
    summary: |-
      Ssb is a core component of the translating ribosome that interacts with both
      the nascent polypeptide and the ribosome, functioning as a chaperone to
      prevent misfolding of newly synthesized proteins. This is the core process.
    action: ACCEPT
    reason: |-
      Core function with direct IDA evidence (PMID:9670014). The defining
      biological role of Ssb2.
    supported_by:
    - reference_id: PMID:9670014
      supporting_text: Ssb to function as a chaperone on the ribosome, preventing the misfolding of
      reference_section_type: ABSTRACT
    - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
      supporting_text: Ssb2 belongs to an **Hsp70 triad at the exit tunnel**
      reference_section_type: OTHER
core_functions:
- description: |-
    ATP-dependent Hsp70 molecular chaperone that binds short, largely hydrophobic
    segments of nascent polypeptides emerging from the ribosomal tunnel exit,
    using an ATP-driven NBD/SBD conformational cycle (stimulated by the RAC
    J-protein Zuo1) to promote de novo cotranslational protein folding.
  molecular_function:
    id: GO:0140662
    label: ATP-dependent protein folding chaperone
  supported_by:
  - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
    supporting_text: Ssb1/2 (including Ssb2) act as the **direct nascent-chain binders** during co-translational folding in yeast
    reference_section_type: OTHER
  - reference_id: PMID:9670014
    supporting_text: Ssb to function as a chaperone on the ribosome, preventing the misfolding of
    reference_section_type: ABSTRACT
- description: |-
    Cotranslational chaperone acting on cytosolic translating 80S ribosomes near
    the 60S tunnel exit, directly engaging nascent chains as part of the
    RAC-Ssb system to support de novo folding of a large fraction of the nascent
    proteome.
  molecular_function:
    id: GO:0140662
    label: ATP-dependent protein folding chaperone
  supported_by:
  - reference_id: file:yeast/SSB2/SSB2-deep-research-falcon.md
    supporting_text: Ssb proteins (Ssb1/Ssb2) are **cytosolic** and **ribosome-associated**, positioned at the **60S tunnel exit**
    reference_section_type: OTHER
  - reference_id: PMID:23332755
    supporting_text: define the cotranslational substrate
    reference_section_type: ABSTRACT
references:
- 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:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
  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:11914276
  title: Subcellular localization of the yeast proteome.
  findings: []
- id: PMID:1394434
  title: The translation machinery and 70 kd heat shock protein cooperate in protein synthesis.
  findings:
  - statement: |-
      The SSB Hsp70s (Ssb1/2p) are associated with translating ribosomes, an
      association disrupted by puromycin, suggesting Ssb binds directly to the
      nascent polypeptide chain.
    supporting_text: The SSB hsp70s (Ssb1/2p) are associated with
      translating ribosomes. This association is disrupted by puromycin, suggesting
      that Ssb1/2p may bind directly to the nascent polypeptide.
    reference_section_type: ABSTRACT
  - statement: |-
      Mutant ssb1 ssb2 strains grow slowly, contain fewer translating ribosomes,
      and are hypersensitive to protein synthesis inhibitors.
    supporting_text: Mutant ssb1 ssb2
      strains grow slowly, contain a low number of translating ribosomes, and are
      hypersensitive to several inhibitors of protein synthesis.
    reference_section_type: ABSTRACT
- id: PMID:15456889
  title: The ribosome-bound chaperones RAC and Ssb1/2p are required for accurate translation in Saccharomyces cerevisiae.
  findings:
  - statement: |-
      Translational fidelity is impaired in the absence of functional RAC or
      Ssb1/2p, with the principal defect in translation termination.
    supporting_text: Translational fidelity was impaired in the absence of functional RAC or
      Ssb1/2p, and the effect was further enhanced by paromomycin. The mutant strains suffered
      primarily from a defect in translation termination
    reference_section_type: ABSTRACT
- id: PMID:16429126
  title: Proteome survey reveals modularity of the yeast cell machinery.
  findings: []
- id: PMID:16554755
  title: Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.
  findings: []
- id: PMID:16607023
  title: Specific effects of ribosome-tethered molecular chaperones on programmed -1 ribosomal frameshifting.
  findings:
  - statement: |-
      Deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p (RAC) specifically inhibits -1
      programmed ribosomal frameshifting and impairs Killer virus maintenance,
      with no effect on +1 PRF.
    supporting_text: deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of -1
      PRF and defects in Killer virus maintenance, while no effects were observed on
      +1 PRF.
    reference_section_type: ABSTRACT
- id: PMID:16622836
  title: The plasma membrane proteome of Saccharomyces cerevisiae and its response to the antifungal calcofluor.
  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:19723765
  title: The Hsp70 homolog Ssb is essential for glucose sensing via the SNF1 kinase network.
  findings:
  - statement: |-
      The chaperone Ssb is required to keep SNF1 in the nonphosphorylated state in
      the presence of glucose; Deltassb1 Deltassb2 cells display features
      reminiscent of glucose-repression mutants.
    supporting_text: the chaperone Ssb is required to keep SNF1 in the
      nonphosphorylated state in the presence of glucose.
    reference_section_type: ABSTRACT
- id: PMID:20368619
  title: A ribosome-anchored chaperone network that facilitates eukaryotic ribosome biogenesis.
  findings: []
- id: PMID:23332755
  title: The cotranslational function of ribosome-associated Hsp70 in eukaryotic protein homeostasis.
  findings:
  - statement: |-
      The yeast Hsp70 SSB binds a subset of nascent polypeptides whose intrinsic
      properties and slow translation rates hinder cotranslational folding; the
      SSB-ribosome cycle and substrate recognition are modulated by RAC.
    supporting_text: SSB binds to a subset of
      nascent polypeptides whose intrinsic properties and slow translation rates
      hinder efficient cotranslational folding. The SSB-ribosome cycle and substrate
      recognition is modulated by its ribosome-bound cochaperone, RAC.
    reference_section_type: ABSTRACT
  - statement: |-
      Deletion of SSB leads to widespread aggregation of newly synthesized
      polypeptides, demonstrating its proteome-wide cotranslational folding role.
    supporting_text: Deletion of SSB
      leads to widespread aggregation of newly synthesized polypeptides.
    reference_section_type: ABSTRACT
- id: PMID:26777405
  title: ATPase-Modulated Stress Granules Contain a Diverse Proteome and Substructure.
  findings: []
- id: PMID:37070168
  title: RNA-dependent interactome allows network-based assignment of RNA-binding protein function.
  findings: []
- id: PMID:37968396
  title: The social and structural architecture of the yeast protein interactome.
  findings: []
- id: PMID:9670014
  title: The molecular chaperone Ssb from Saccharomyces cerevisiae is a component of the ribosome-nascent chain complex.
  findings:
  - statement: |-
      Ssb can be cross-linked to nascent chains and is released together with
      nascent chains upon puromycin treatment, demonstrating direct interaction
      with the nascent polypeptide.
    supporting_text: Ssb could be cross-linked to nascent chains
      containing a modified lysine residue with a photoactivatable cross-linker.
    reference_section_type: ABSTRACT
  - statement: |-
      Ssb is a core component of the translating ribosome that interacts with both
      the nascent chain and the ribosome, functioning as a chaperone that prevents
      misfolding of newly synthesized proteins.
    supporting_text: Ssb is a core component of the translating ribosome which interacts with
      both the nascent polypeptide chain and the ribosome. These interactions allow
      Ssb to function as a chaperone on the ribosome, preventing the misfolding of
      newly synthesized proteins.
    reference_section_type: ABSTRACT
- id: PMID:9860955
  title: The biochemical properties of the ATPase activity of a 70-kDa heat shock protein (Hsp70) are governed by the C-terminal domains.
  findings:
  - statement: |-
      Ssb has an unusually low steady-state affinity for ATP but a higher maximal
      velocity, and its ATPase (unlike Ssa) is K+-independent; the peptide-binding
      domain shapes these properties.
    supporting_text: Ssb, however, has an unusually low steady-state affinity for ATP but a
      higher maximal velocity. In addition, the ATPase activity of Hsp70s, like that
      of Ssa1, depends on the addition of K+ whereas Ssb activity does not.
    reference_section_type: ABSTRACT
- id: file:yeast/SSB2/SSB2-deep-research-falcon.md
  title: Falcon deep research report on SSB2 (yeast, UniProt P40150 / YNL209W)
  findings:
  - statement: |-
      SSB2 (P40150/YNL209W) is one of two nearly identical ribosome-associated
      cytosolic Hsp70s in S. cerevisiae; SSB1 and SSB2 differ by ~4 residues and
      are generally studied together as "Ssb", confirming this is the genuine
      Ssb-type Hsp70 (not an unrelated same-symbol gene).
    supporting_text: Ssb is encoded by **two paralogous genes, SSB1 and SSB2**
    reference_section_type: OTHER
  - statement: |-
      Ssb2 is a canonical Hsp70 with an N-terminal ATPase/nucleotide-binding
      domain (NBD) and a C-terminal substrate-binding domain (SBD), driven by an
      ATP-dependent conformational cycle.
    supporting_text: core biochemistry is an **ATP-driven conformational cycle**
    reference_section_type: OTHER
  - statement: |-
      The RAC J-domain protein Zuo1 stimulates Ssb1/2 ATP hydrolysis, driving the
      high-affinity substrate state that stabilizes nascent-chain binding.
    supporting_text: J-domain protein **Zuo1** stimulates ATP hydrolysis of **Ssb1/2**, driving this high-affinity substrate engagement on nascent chains
    reference_section_type: OTHER
  - statement: |-
      Ssb belongs to an Hsp70 triad at the ribosomal tunnel exit (RAC = Zuo1 +
      Ssz1, plus Ssb), with RAC recruiting and activating Ssb to directly bind
      nascent chains for cotranslational folding.
    supporting_text: Ssb2 belongs to an **Hsp70 triad at the exit tunnel**
    reference_section_type: OTHER
  - statement: |-
      Ssb1/2 are the direct nascent-chain binders during cotranslational folding
      in yeast; RAC is an obligate Zuo1-Ssz1 heterodimer anchored to the ribosome
      via Zuo1.
    supporting_text: RAC is an obligate Zuo1–Ssz1 heterodimer attached to the ribosome (via Zuo1)
    reference_section_type: OTHER
  - statement: |-
      Ssb is cytosolic and ribosome-associated at the 60S tunnel exit; about 50%
      of total cellular Ssb is ribosome-bound at steady state (~1:1 with
      ribosomes), with the remainder free cytosolic.
    supporting_text: only about **~50% of total cellular Ssb** is ribosome-associated at steady state
    reference_section_type: OTHER
  - statement: |-
      Ssb's cotranslational substrate coverage is broad, engaging ~80% of
      cytosolic/nuclear nascent proteins, supporting a proteome-wide
      cotranslational folding role.
    supporting_text: Ssb’s co-translational substrate coverage is broad
    reference_section_type: OTHER
  - statement: |-
      Single SSB1 or SSB2 deletion gives little phenotype owing to redundancy,
      whereas combined ssb1/2 deletion causes broad defects.
    supporting_text: Single-gene loss has little obvious phenotype, whereas combined **ssb1/2Δ** causes broad defects
    reference_section_type: OTHER
  - statement: |-
      Ssb suppresses formation and/or inheritance of multiple amyloid/prion-like
      heritable elements, linking it to proteostasis and aggregation control.
    supporting_text: Ssb suppresses formation and/or inheritance of multiple **amyloid/prion-like elements**
    reference_section_type: OTHER
  - statement: |-
      The RAC/Ssb system is required for proper translational downregulation and
      proteostasis during TORC1 inhibition, connecting tunnel-exit chaperoning to
      nutrient/stress signaling.
    supporting_text: RAC/Ssb system is required for proper translational downregulation and proteostasis during TORC1 inhibition
    reference_section_type: OTHER
  - statement: |-
      Ssb/RAC is linked to recruitment of the ubiquitin ligase Ltn1, connecting
      cotranslational chaperoning to ribosome-associated quality control.
    supporting_text: Ssb/RAC is linked to recruitment of the ubiquitin ligase **Ltn1**
    reference_section_type: OTHER

Gene Description

Existing Annotations Review

Core Functions

ATP-dependent Hsp70 molecular chaperone that binds short, largely hydrophobic segments of nascent polypeptides emerging from the ribosomal tunnel exit, using an ATP-driven NBD/SBD conformational cycle (stimulated by the RAC J-protein Zuo1) to promote de novo cotranslational protein folding.

Cotranslational chaperone acting on cytosolic translating 80S ribosomes near the 60S tunnel exit, directly engaging nascent chains as part of the RAC-Ssb system to support de novo folding of a large fraction of the nascent proteome.

References

Deep Research

Falcon

Research report: Saccharomyces cerevisiae SSB2 (UniProt P40150; YNL209W) — ribosome-associated Hsp70 chaperone

0) Identity verification (critical disambiguation)

1) Key concepts and definitions (current understanding)

1.1 Hsp70 chaperone cycle (what Ssb2 does)

1.2 Co-translational protein folding and the “tunnel-exit chaperone hub”

2) Molecular function, pathway placement, and mechanism

2.1 Primary biological role: RAC-dependent co-translational chaperoning of nascent chains

2.2 Ribosome positioning and activation model (structural understanding; 2023 advance)

2.3 Interaction partners and connected processes

3) Cellular localization (where Ssb2 acts)

4) Recent developments and latest research (prioritizing 2023–2024)

4.1 2023: Structural inventory of RAC and activation model for Ssb at the tunnel exit

4.2 2024: Co-occupancy of NAC and Zuotin/Hsp70 at the tunnel exit in vivo

4.3 2023: RAC/Ssb connects tunnel-exit proteostasis to TORC1-mediated translation control

4.4 2023: Ssb suppresses multiple amyloid/prion-like heritable elements

5) Current applications and real-world implementations

6) Expert opinions and analysis (synthesis from authoritative sources)

7) Relevant statistics and quantitative data (recent/representative)

7.1 Stoichiometry and partitioning

7.2 Proteome-wide substrate engagement

7.3 Aggregation inheritance

8) Limitations and gene-specific caveats for SSB2

Summary table

Key model figure (visual evidence)

Selected recent references (with dates and URLs)

Artifacts

Citations

📄 View Raw YAML