SSB1

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005737 cytoplasm	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Ssb1 is cytoplasmic, but cytosol and ribosome association are more informative localizations. Reason: Correct broad localization for a cytosolic ribosome-associated Hsp70; retained as non-core because GO:0005829 is more precise. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN002321897 · PAINT Hsp70 family node SUPPORTS TRANSFER The PAINT node contains SSB1 among its experimentally grounded descendants and correctly transfers a broad cytoplasmic localization; cytosol is more informative for this target.
GO:0016887 ATP hydrolysis activity	IBA GO_REF:0000033	ACCEPT	Summary: Ssb1 is a directly characterized Hsp70 ATPase. Reason: ATP hydrolysis powers the Hsp70 substrate-binding cycle and is directly supported by PMID:9860955. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000452648 · PAINT Hsp70 family node SUPPORTS TRANSFER SSB1 is among the experimentally grounded descendants used for this conserved Hsp70-family ATPase inference.
GO:0044183 protein folding chaperone	IBA GO_REF:0000033	MODIFY	Summary: Ssb1 is an ATP-dependent protein folding chaperone acting on nascent chains. Reason: The annotation is correct but GO:0140662 specifies the ATP-dependent Hsp70 mechanism more precisely. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000452648 · PAINT Hsp70 family node SUPPORTS TRANSFER The family-level inference correctly identifies folding-chaperone activity, but the ATP-dependent child term is more informative for this directly characterized Hsp70. Proposed replacements: ATP-dependent protein folding chaperone Supporting Evidence: PMID:28771464 The RAC heterodimer stimulates the ATPase activity of the ribosome-bound Hsp70 homolog Ssb, which interacts with nascent polypeptide chains to facilitate de novo protein folding.
GO:0005634 nucleus	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: RAC-Ssb contributes to nuclear steps of ribosome biogenesis, although Ssb1 is cytosolic at steady state and actively exported. Reason: PMID:20368619 supports a nuclear RAC-Ssb role in ribosome biogenesis, while PMID:10347213 shows that Ssb1 is cytosolic at steady state; nucleus is therefore a specialized/non-core site rather than the principal localization. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN002500132 · PAINT Hsp70 family node SUPPORTS TRANSFER The family inference is compatible with Ssb1 nuclear shuttling and RAC-Ssb participation in nuclear ribosome biogenesis, but this is not its predominant site of action.
GO:0031072 heat shock protein binding	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Ssb1 engages other heat-shock proteins and cochaperones in the cytosolic chaperone network. Reason: Interactions with RAC and the Hsp110 nucleotide-exchange factor Sse1 are real, but this binding term is ancillary to Ssb1's direct folding-chaperone activity. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN000452648 · PAINT Hsp70 family node SUPPORTS TRANSFER Conserved Hsp70-network interactions support the transfer, while the binding term remains ancillary to Ssb1's ATP-dependent chaperone activity.
GO:0005829 cytosol	IBA GO_REF:0000033	ACCEPT	Summary: Cytosol is the principal compartment for Ssb1's ribosome-associated chaperone activity. Reason: Direct localization and biochemical studies place Ssb in the cytosol and on cytosolic translating ribosomes. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002500132 · PAINT Hsp70 family node SUPPORTS TRANSFER The ancestral localization inference agrees with direct localization and Ssb1's experimentally established function on cytosolic translating ribosomes. Supporting Evidence: file:yeast/SSB1/SSB1-deep-research-openscientist.md encodes Ssb1, a cytosolic, ATP-dependent molecular chaperone of the heat shock protein 70 (Hsp70) family
GO:0042026 protein refolding	IBA GO_REF:0000033	MODIFY	Summary: Ssb1 promotes folding of newly synthesized nascent chains, not a demonstrated general refolding program for pre-existing denatured proteins. Reason: The family-level refolding claim is unsupported for the specialized Ssb subfamily. GO:0051083 is already directly annotated by IDA from PMID:9670014, so this MODIFY effectively removes the propagated general-refolding claim in favor of the independently established de novo cotranslational process. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000452648 · PAINT Hsp70 family node SUPPORTS SOURCE BUT NOT TARGET General refolding is defensible elsewhere in the Hsp70 family, but the ribosome-specialized Ssb subfamily is directly established as a de novo cotranslational chaperone and lacks target-specific evidence for refolding pre-existing denatured clients. Proposed replacements: 'de novo' cotranslational protein folding Supporting Evidence: PMID:9670014 These interactions allow Ssb to function as a chaperone on the ribosome, preventing the misfolding of newly synthesized proteins.
GO:0000054 ribosomal subunit export from nucleus	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Genetic evidence links the RAC-Ssb network to ribosomal subunit maturation and export. Reason: PMID:20368619 supports an ancillary ribosome-biogenesis role; export is downstream of the core cotranslational folding activity.
GO:0005524 ATP binding	IEA GO_REF:0000002	ACCEPT	Summary: Ssb1's nucleotide-binding domain binds ATP to drive its Hsp70 cycle. Reason: ATP binding is intrinsic to the directly demonstrated ATPase-dependent chaperone mechanism.
GO:0005737 cytoplasm	IEA GO_REF:0000120	KEEP AS NON CORE	Summary: Ssb1 is cytoplasmic, but this broad localization is less informative than cytosol. Reason: Correct electronic localization; retained as non-core because cytosol and ribosome association better describe the functional pool.
GO:0006364 rRNA processing	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: RAC-Ssb contributes to rRNA maturation during ribosome biogenesis. Reason: PMID:20368619 supports this genetically, but it is ancillary to Ssb1's core nascent-chain folding function.
GO:0006450 regulation of translational fidelity	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Ssb1 and RAC are required for accurate translation, especially termination. Reason: PMID:15456889 and PMID:31114879 experimentally support a genuine role in translational fidelity, but this phenotype is downstream of the core cotranslational chaperone cycle.
GO:0006452 translational frameshifting	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: Loss of Ssb1/2 specifically alters programmed -1 ribosomal frameshifting. Reason: PMID:16607023 directly measured inhibition of -1, but not +1, programmed frameshifting in ssb1 ssb2 mutants; this is a genuine secondary translation phenotype rather than Ssb1's core molecular role.
GO:0016887 ATP hydrolysis activity	IEA GO_REF:0000120	ACCEPT	Summary: Ssb1 is a directly characterized Hsp70 ATPase. Reason: This electronic assertion is independently confirmed by the biochemical measurements in PMID:9860955.
GO:0051083 'de novo' cotranslational protein folding	IEA GO_REF:0000117	ACCEPT	Summary: De novo cotranslational protein folding is Ssb1's defining biological process. Reason: Ssb directly binds nascent chains at translating ribosomes, and SSB loss causes aggregation of newly synthesized proteins (PMID:9670014; PMID:23332755).
GO:0005515 protein binding	IPI PMID:11805837 Systematic identification of protein complexes in Saccharomy...	MARK AS OVER ANNOTATED	Summary: Generic protein binding from a large-scale affinity-purification study is not functionally informative. Reason: The interaction evidence may be valid, but GO:0005515 does not identify Ssb1's chaperone mechanism or a specific functional partner.
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 proteome-wide complex survey is not functionally informative. Reason: The high-throughput interaction does not warrant an unqualified protein-binding molecular function for Ssb1.
GO:0005515 protein binding	IPI PMID:16688211 Chaperone network in the yeast cytosol: Hsp110 is revealed a...	MARK AS OVER ANNOTATED	Summary: Sse1 binding is mechanistically relevant, but generic protein binding understates the nucleotide-exchange interaction. Reason: PMID:16688211 identifies Sse1 as an Ssb1 nucleotide-exchange factor; GO:0005515 adds no useful specificity.
GO:0005515 protein binding	IPI PMID:17559233 Proteomic analysis of in vivo 14-3-3 interactions in the yea...	MARK AS OVER ANNOTATED	Summary: A high-throughput 14-3-3 interaction does not justify generic protein binding as a useful function term. Reason: PMID:27001512 mechanistically supports cooperation of Ssb and Bmh in glucose repression, but GO:0005515 is uninformative and does not capture that regulatory context. Supporting Evidence: PMID:27001512 Here we show that the defect in glucose-repression in the absence of Ssb is due to the ability of the chaperone to bridge between the SNF1 and Glc7 complexes.
GO:0005515 protein binding	IPI PMID:17612295 Yeast split-ubiquitin-based cytosolic screening system to de...	MARK AS OVER ANNOTATED	Summary: Generic protein binding from a split-ubiquitin interaction screen is not functionally informative. Reason: The assay identified an interaction in a screening context but does not define a specific molecular activity of Ssb1.
GO:0005515 protein binding	IPI PMID:19387492 Yeast Uri1p promotes translation initiation and may provide ...	MARK AS OVER ANNOTATED	Summary: Association with the Uri1 cotranslational quality-control network is real, but generic protein binding is too broad. Reason: PMID:19387492 places Ssb1 in a chaperone network; GO:0005515 does not capture the relevant cotranslational chaperone function.
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 map is not functionally informative. Reason: The high-throughput interaction evidence does not define Ssb1's molecular function or a specific mechanistic binding activity.
GO:0005737 cytoplasm	EXP PMID:10347213 A nuclear export signal prevents Saccharomyces cerevisiae Hs...	KEEP AS NON CORE	Summary: GFP-Ssb1 was cytosolic at steady state because an active nuclear export signal limits nuclear accumulation. Reason: PMID:10347213 directly supports cytoplasmic/cytosolic localization while also showing that Ssb1 can transiently shuttle through the nucleus.
GO:0005737 cytoplasm	EXP PMID:23332755 The cotranslational function of ribosome-associated Hsp70 in...	KEEP AS NON CORE	Summary: Global cotranslational profiling places Ssb on cytosolic translating ribosomes and nascent chains. Reason: PMID:23332755 directly studies the cytosolic Ssb-ribosome cycle and supports this broad cellular localization.
GO:0005886 plasma membrane	HDA PMID:16622836 The plasma membrane proteome of Saccharomyces cerevisiae and...	MARK AS OVER ANNOTATED	Summary: A bulk plasma-membrane-fraction hit is weak localization evidence for the abundant soluble cytosolic Hsp70 Ssb1. Reason: PMID:16622836 is a high-throughput survey of a stripped plasma-membrane fraction, whereas direct studies place Ssb1 in the cytosol and on cytosolic translating ribosomes. The fraction-detection annotation is retained as an experimental observation but should not be interpreted as evidence that the plasma membrane is a functional Ssb1 compartment. Supporting Evidence: PMID:16622836 Proteins from a stripped plasma membrane fraction were solubilized with the neutral and non-denaturing detergent, the n-dodecyl beta-D-maltoside.
GO:0006452 translational frameshifting	IMP PMID:16607023 Specific effects of ribosome-tethered molecular chaperones o...	KEEP AS NON CORE	Summary: Deletion of SSB1 and SSB2 specifically inhibits programmed -1 ribosomal frameshifting. Reason: PMID:16607023 directly measured a selective effect on -1 PRF, with no effect on +1 PRF; this is a secondary translational consequence of losing the ribosome-associated chaperone system.
GO:0000054 ribosomal subunit export from nucleus	IGI PMID:20368619 A ribosome-anchored chaperone network that facilitates eukar...	KEEP AS NON CORE	Summary: Genetic evidence links the Zuo1-Ssb chaperone network to ribosomal subunit maturation and export. Reason: PMID:20368619 supports this ribosome-biogenesis role, which is downstream of Ssb1's core cotranslational folding activity.
GO:0002181 cytoplasmic translation	IMP PMID:1394434 The translation machinery and 70 kd heat shock protein coope...	KEEP AS NON CORE	Summary: Ssb1/2 associate with translating ribosomes and are required for normal protein synthesis. Reason: PMID:1394434 shows slow growth, fewer translating ribosomes, and sensitivity to translation inhibitors in ssb1 ssb2 mutants. Cytoplasmic translation is the functional context for Ssb1's core nascent-chain chaperone activity, not a separate primary activity.
GO:0002181 cytoplasmic translation	IPI PMID:1394434 The translation machinery and 70 kd heat shock protein coope...	KEEP AS NON CORE	Summary: Puromycin-sensitive Ssb-ribosome association supports direct engagement of nascent chains during cytoplasmic translation. Reason: PMID:1394434 directly links Ssb1/2 to translating ribosomes and nascent-polypeptide handling, supporting translation as the context for the core folding mechanism rather than a distinct core process.
GO:0005516 calmodulin binding	IDA PMID:17146552 Identification of the divergent calmodulin binding motif in ...	KEEP AS NON CORE	Summary: Calmodulin-affinity purification and mass spectrometry identified Ssb1 as a calmodulin-binding protein. Reason: The biochemical binding evidence is direct, but PMID:17146552 did not establish a physiological consequence or a role central to Ssb1 function.
GO:0005737 cytoplasm	IDA PMID:19502427 Widespread reorganization of metabolic enzymes into reversib...	KEEP AS NON CORE	Summary: Ssb1 forms reversible cytoplasmic assemblies during nutrient starvation. Reason: PMID:19502427 supports stress-condition cytoplasmic localization, but the punctate assembly is not the core ribosome-associated state.
GO:0006364 rRNA processing	IGI PMID:20368619 A ribosome-anchored chaperone network that facilitates eukar...	KEEP AS NON CORE	Summary: The Zuo1-Ssb chaperone network contributes to maturation of 35S rRNA. Reason: PMID:20368619 supports this genetically as an ancillary ribosome-biogenesis role.
GO:0006415 translational termination	IMP PMID:17483428 Fine-tuning of translation termination efficiency in Sacchar...	KEEP AS NON CORE	Summary: SSB1 overexpression increases translation termination efficiency. Reason: PMID:17483428 identified SSB1 as one of the strongest antisuppressors in a chromosomal stop-codon readthrough screen, a genuine but downstream role relative to cotranslational folding. Supporting Evidence: PMID:17483428 Among them, SSB1 and snR18, two factors close to the exit tunnel of the ribosome, directed the strongest antisuppression effects when overexpressed, showing that they may be involved in fine-tuning of the translation termination level.
GO:0006450 regulation of translational fidelity	IMP PMID:15456889 The ribosome-bound chaperones RAC and Ssb1/2p are required f...	KEEP AS NON CORE	Summary: RAC and Ssb1/2 are required for accurate translation, with the strongest defect at termination. Reason: PMID:15456889 directly measured impaired fidelity in vivo and in vitro; PMID:31114879 later established complementary nascent-chain and ribosome-biogenesis mechanisms. This is a supported secondary consequence of RAC-Ssb function.
GO:0016887 ATP hydrolysis activity	IDA PMID:9860955 The biochemical properties of the ATPase activity of a 70-kD...	ACCEPT	Summary: Purified Ssb has directly measured ATPase activity with distinctive kinetics. Reason: PMID:9860955 reports Ssb ATP hydrolysis, including low steady-state ATP affinity, high maximal velocity, and C-terminal control of ATPase behavior.
GO:0051083 'de novo' cotranslational protein folding	IDA PMID:9670014 The molecular chaperone Ssb from Saccharomyces cerevisiae is...	ACCEPT	Summary: Ssb directly contacts nascent chains on translating ribosomes and prevents misfolding of newly synthesized proteins. Reason: PMID:9670014 provides direct puromycin-release and cross-linking evidence for Ssb as a ribosome-nascent-chain chaperone.
GO:0043022 ribosome binding	IDA PMID:9670014 The molecular chaperone Ssb from Saccharomyces cerevisiae is...	NEW	Summary: Proposed new annotation for Ssb1's directly characterized physical association with translating ribosomes. Reason: PMID:9670014 used puromycin release, salt resistance, and nascent-chain cross-linking to characterize Ssb-ribosome interaction; the current GOA set lacks a ribosome-binding molecular-function annotation. Supporting Evidence: PMID:9670014 We propose that Ssb is a core component of the translating ribosome which interacts with both the nascent polypeptide chain and the ribosome.
GO:0022626 cytosolic ribosome	IDA PMID:9670014 The molecular chaperone Ssb from Saccharomyces cerevisiae is...	NEW	Summary: Proposed new annotation for the cytosolic translating ribosome where Ssb1 performs its cotranslational chaperone cycle. Reason: PMID:9670014 directly demonstrates stable Ssb association with translating ribosomes, and PMID:1394434 identifies the SSB proteins as cytosolic Hsp70s associated with translating ribosomes. Supporting Evidence: PMID:9670014 The Ssbs of Saccharomyces cerevisiae are an abundant type of Hsp70 found associated with translating ribosomes. PMID:1394434 We suggest that cytosolic hsp70 aids in the passage of the nascent polypeptide chain through the ribosome in a manner analogous to the role played by organelle-localized hsp70 in the transport of proteins across membranes.

Gene Description

Existing Annotations Review

Core Functions

Ssb1 is an ATP-dependent Hsp70 folding chaperone on cytosolic translating ribosomes. RAC positions and activates Ssb1 near the 60S exit tunnel, where repeated ATP-driven binding and release cycles capture emerging nascent chains, limit their aggregation, and promote de novo cotranslational folding.

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