BTT1 encodes the low-abundance beta-2 paralog of the yeast nascent-polypeptide-associated complex (NAC). Btt1 pairs with the alpha NAC subunit Egd2 to form an alternative NAC heterodimer that associates with cytosolic translating ribosomes near the nascent-chain exit tunnel. The best supported core functions are participation in the NAC complex, ribosome-linked cotranslational nascent-chain handling/folding, and regulation of cotranslational protein targeting to membranes. Btt1 also has reproducible physical and genetic links to Caf130/CCR4-NOT, but that appears to be a specialized or context-dependent regulatory role rather than the general NAC core function.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005854
nascent polypeptide-associated complex
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: The IBA annotation is consistent with Btt1's conserved NAC-beta family identity and with direct yeast experiments showing that BTT1 encodes one of the beta subunits of the nascent polypeptide-associated complex.
Reason: Part_of NAC is a core cellular-component annotation for Btt1. It is supported by direct yeast IDA evidence and conserved NAC-family context.
Supporting Evidence:
PMID:10219998
The three subunits of the nascent polypeptide-associated complex (alpha, beta1, beta3) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1).
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon identifies Btt1 as the low-abundance beta-2 paralog of yeast NAC.
|
|
GO:0005829
cytosol
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: The cytosol IBA annotation is consistent with the cytosolic, ribosome-associated site of NAC action.
Reason: Btt1 acts on cytosolic ribosome-nascent-chain complexes; cytosol is the appropriate core localization.
Supporting Evidence:
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon places Btt1's primary site of action at cytosolic ribosomes near the polypeptide exit tunnel.
|
|
GO:0005634
nucleus
|
IEA
GO_REF:0000044 |
KEEP AS NON CORE |
Summary: UniProt records possible transient nuclear localization by similarity, but the curated Btt1 evidence reviewed here primarily supports a cytosolic ribosome-associated NAC role.
Reason: The nucleus annotation should not be elevated to core function for Btt1. It may reflect a minor or context-dependent localization of NAC-family proteins, while Btt1's best-supported activity is cytosolic and ribosome-proximal.
|
|
GO:0005737
cytoplasm
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: Cytoplasm is consistent with Btt1's placement in the cytosolic ribosome-associated NAC complex.
Reason: Although cytosol is more precise, cytoplasm is not misleading for this protein and is consistent with UniProt localization and NAC biology.
Supporting Evidence:
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon describes Btt1 as acting primarily on cytosolic ribosomes.
|
|
GO:0005854
nascent polypeptide-associated complex
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: The ARBA electronic annotation to NAC is consistent with direct IDA and IBA evidence for Btt1 as a NAC beta paralog.
Reason: This electronic annotation is redundant with stronger evidence but correct and should be retained.
Supporting Evidence:
PMID:10219998
The three subunits of the nascent polypeptide-associated complex (alpha, beta1, beta3) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1).
|
|
GO:0006613
cotranslational protein targeting to membrane
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: The ARBA annotation is consistent with direct yeast NAC evidence showing that NAC controls inappropriate ribosome-nascent-chain association with ER membranes.
Reason: Cotranslational membrane targeting regulation is a legitimate NAC function. For Btt1 specifically, it should be interpreted as part of the NAC beta subunit's ribosome-associated role rather than as a standalone membrane transporter function.
Supporting Evidence:
PMID:10518932
We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes.
|
|
GO:0015031
protein transport
|
IEA
GO_REF:0000043 |
MODIFY |
Summary: The UniProt keyword-derived protein transport annotation is too broad for Btt1. The supported transport-related role is cotranslational control of ribosome-nascent-chain targeting to membranes.
Reason: Replace broad protein transport with GO:0006613 cotranslational protein targeting to membrane, which is already experimentally supported for yeast NAC and better captures the process represented by the annotation.
Proposed replacements:
cotranslational protein targeting to membrane
Supporting Evidence:
PMID:10518932
In the absence of NAC, signal-less RNCs are able to bind to ER membranes.
|
|
GO:0005515
protein binding
|
IPI
PMID:11283351 A comprehensive two-hybrid analysis to explore the yeast pro... |
MARK AS OVER ANNOTATED |
Summary: This protein binding annotation comes from a large two-hybrid interactome screen and reports a generic Caf130 interaction. The interaction may be real, but GO:0005515 is not informative for Btt1 function.
Reason: Generic protein binding should not be retained as a functional endpoint. The physiologically meaningful context is better represented by Btt1's NAC complex membership and by the non-core CCR4-NOT/Caf130 annotation.
Supporting Evidence:
PMID:11283351
A comprehensive two-hybrid analysis to explore the yeast protein interactome.
|
|
GO:0005515
protein binding
|
IPI
PMID:16554755 Global landscape of protein complexes in the yeast Saccharom... |
MARK AS OVER ANNOTATED |
Summary: This interaction-derived GO:0005515 annotation is from global protein complex mapping. It is not wrong that Btt1 physically associates with proteins, but the term is too vague to describe Btt1's NAC biology.
Reason: The supported interactions with CCR4, Caf130, and YJR011C should not be curated as generic protein binding. More informative annotations capture NAC complex membership and the contextual CCR4-NOT association.
Supporting Evidence:
PMID:16554755
Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.
|
|
GO:0005515
protein binding
|
IPI
PMID:37968396 The social and structural architecture of the yeast protein ... |
MARK AS OVER ANNOTATED |
Summary: The 2023 proteome-scale interactome evidence supports physical association context but not a specific molecular function captured by GO:0005515.
Reason: Protein binding is over-annotated because it does not identify what Btt1 does. The interaction information is better interpreted through NAC and CCR4-NOT context rather than as a standalone MF term.
Supporting Evidence:
PMID:37968396
The social and structural architecture of the yeast protein interactome.
|
|
GO:0051083
'de novo' cotranslational protein folding
|
NAS
PMID:26618777 Functional Dissection of the Nascent Polypeptide-Associated ... |
ACCEPT |
Summary: The NAS annotation is consistent with the NAC complex functioning with other ribosome-associated chaperone systems to assist early folding of nascent polypeptides.
Reason: Btt1-containing NAC is a ribosome-associated cotranslational proteostasis factor. Falcon and the 2015 NAC dissection support the view that Btt1 has weaker, more specialized activity than the major Egd1-containing NAC, but the process annotation remains appropriate.
Supporting Evidence:
PMID:26618777
assist cotranslational processes such as folding of nascent polypeptides.
PMID:26618777
aggregation of newly synthesized proteins
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon identifies Btt1 as a ribosome-associated NAC beta paralog involved in cotranslational nascent-chain handling.
|
|
GO:0006613
cotranslational protein targeting to membrane
|
IGI
PMID:10518932 The nascent polypeptide-associated complex (NAC) of yeast fu... |
ACCEPT |
Summary: The IGI annotation captures the experimentally supported NAC role in preventing signal-less ribosome-nascent-chain complexes from binding ER membranes.
Reason: This is a core NAC process and is supported by yeast-derived in vitro targeting experiments. For Btt1, it should be understood as a beta-subunit contribution to the NAC complex rather than a direct membrane-targeting receptor activity.
Supporting Evidence:
PMID:10518932
We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes.
PMID:10518932
In the absence of NAC, signal-less RNCs are able to bind to ER membranes.
|
|
GO:0030015
CCR4-NOT core complex
|
IPI
PMID:18214544 Genome wide expression analysis of the CCR4-NOT complex indi... |
KEEP AS NON CORE |
Summary: Btt1 reproducibly interacts with Caf130 and was described as a tenth member of the CCR4-NOT complex. This is biologically meaningful, but the main evolved function of Btt1 remains NAC/ribosome-proximal cotranslational regulation.
Reason: Retain the annotation as a non-core contextual complex association. The GO term may overstate Btt1 as part of the CCR4-NOT core relative to its low-abundance NAC beta-subunit identity, but the Caf130 interaction and CCR4-NOT regulatory context are well supported.
Supporting Evidence:
PMID:18214544
BTT1, a member of the nascent polypeptide association complex that binds the ribosome, was shown to be a tenth member of the CCR4-NOT complex
PMID:18214544
interacting through CAF130
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon highlights Btt1-Caf130/CCR4-NOT coupling as a specialized, context-dependent regulatory role.
|
|
GO:0005854
nascent polypeptide-associated complex
|
IDA
PMID:10219998 Initial characterization of the nascent polypeptide-associat... |
ACCEPT |
Summary: The original characterization directly supports BTT1 as encoding one of the yeast NAC beta subunits and places the complex on ribosomes close to nascent chains.
Reason: This is direct experimental evidence for the central cellular-component annotation of Btt1.
Supporting Evidence:
PMID:10219998
BTT1). We found the complex bound to ribosomes via the beta-subunits
PMID:10219998
in close proximity to nascent polypeptides
|
|
GO:0051082
unfolded protein binding
|
IMP
PMID:10219998 Initial characterization of the nascent polypeptide-associat... |
MODIFY |
Summary: The original NAC study supports proximity to nascent polypeptides and a role in preventing mistargeting, but GO:0051082 is too vague for Btt1. The better curation is ribosome-associated cotranslational nascent-chain handling, represented by ribosome binding plus cotranslational folding and membrane-targeting process annotations.
Reason: Replace generic unfolded protein binding with the more directly supported MF GO:0043022 ribosome binding. Btt1 is a NAC beta subunit that anchors NAC at translating ribosomes, while the biological-process annotations capture the nascent-chain folding and targeting outcomes.
Proposed replacements:
ribosome binding
Supporting Evidence:
PMID:10219998
BTT1). We found the complex bound to ribosomes via the beta-subunits
PMID:10219998
in close proximity to nascent polypeptides
file:yeast/BTT1/BTT1-deep-research-falcon.md
Falcon supports interpreting Btt1 as a ribosome-associated NAC beta subunit rather than a generic unfolded-protein binder.
|
Q: Should Btt1 receive a distinct curated annotation for Caf130/CCR4-NOT-linked cotranslational mRNA regulation of ribosomal protein production, separate from its core NAC complex annotations?
Suggested experts: Craig EA, Deuerling E, Collart MA
Experiment: Compare wild-type BTT1, ribosome-binding-defective BTT1 mutants, and Caf130-interface BTT1 mutants for NAC ribosome occupancy, nascent-chain aggregation, ER membrane mistargeting, and Caf130-dependent RPL4 mRNA decay.
Hypothesis: Btt1's low-abundance NAC beta-2 functions separate its general ribosome-associated NAC activity from its specialized Caf130/CCR4-NOT regulatory role.
Type: ribosome profiling and nascent-chain quality-control assay
provider: falcon
model: Edison Scientific Literature
cached: false
start_time: '2026-05-04T10:45:43.308716'
end_time: '2026-05-04T11:03:27.648545'
duration_seconds: 1064.34
template_file: templates/gene_research_go_focused.md
template_variables:
organism: yeast
gene_id: BTT1
gene_symbol: BTT1
uniprot_accession: P40314
protein_description: 'RecName: Full=Nascent polypeptide-associated complex subunit
beta-2; Short=NAC-beta-2; AltName: Full=BTF3 homolog BTT1; AltName: Full=Beta-2-NAC;'
gene_info: Name=BTT1; OrderedLocusNames=YDR252W; ORFNames=YD9320A.02;
organism_full: Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
protein_family: Belongs to the NAC-beta family. .
protein_domains: BTF3. (IPR039370); NAC_A/B_dom_sf. (IPR038187); Nas_poly-pep-assoc_cplx_dom.
(IPR002715); NAC (PF01849)
provider_config:
timeout: 600
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parameters:
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temperature: 0.1
citation_count: 36
BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'BTT1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene BTT1 (gene ID: BTT1, UniProt: P40314) in yeast.
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.
Question: You are an expert researcher providing comprehensive, well-cited information.
Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies
Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.
BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'BTT1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene BTT1 (gene ID: BTT1, UniProt: P40314) in yeast.
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.
BTT1 encodes the minor β subunit paralog of the nascent polypeptide‑associated complex (NAC) in budding yeast (often denoted β′‑NAC or Nacβ2). NAC is a ribosome‑associated, cotranslational protein‑biogenesis factor positioned at the polypeptide exit tunnel, where it binds ribosomes and nascent chains and modulates access of other factors (e.g., targeting and processing enzymes). In yeast, Btt1 forms an alternative heterodimer with the α‑NAC subunit Egd2 (αβ′‑NAC), which is far less abundant than the major αβ‑NAC (Egd2–Egd1) complex and is functionally distinct. The clearest BTT1‑specific advance in 2024 is that Btt1/Nacβ2 acquired a specialized interface with the CCR4–Not subunit Caf130, enabling cotranslational downregulation of RPL4 mRNA when the dedicated Rpl4 chaperone Acl4 is limiting, thereby protecting proteostasis. In contrast, for mitophagy under respiratory growth, Btt1 has only a minor effect relative to Egd1. (schilke2024functionalsimilaritiesand pages 1-3, schilke2024functionalsimilaritiesand pages 9-10, tian2024thenascentpolypeptideassociated pages 2-3)
The literature examined explicitly matches the user‑specified target:
* Gene symbol: BTT1 is described as encoding the minor NAC β paralog (Nacβ2/β′‑NAC) in S. cerevisiae. (ott2015functionaldissectionof pages 1-2, schilke2024functionalsimilaritiesand pages 1-3)
* Organism: All key primary studies cited here are in Saccharomyces cerevisiae (budding yeast), consistent with UniProt P40314 context. (ott2015functionaldissectionof pages 1-2, tian2024thenascentpolypeptideassociated pages 1-2)
* Family/domain alignment: The protein is treated as a NAC β‑type subunit; NAC β subunits provide ribosome‑binding through an N‑terminal basic motif and dimerize via the NAC domain, consistent with the UniProt “NAC‑beta family/BTF3 domain” description. (panasenko2009ribosomeassociationand pages 1-2, ott2015functionaldissectionof pages 2-4)
NAC is a conserved, ribosome‑associated complex that binds close to the nascent chain exit site and interacts with translating ribosome–nascent chain complexes (RNCs). A core functional principle is that NAC’s interactions with nascent chains are ribosome‑dependent; purified NAC does not bind free nascent chains efficiently without ribosome association. (alamo2011definingthespecificity pages 17-18)
In budding yeast, NAC is built from:
* One α subunit (Egd2) and
* Two alternative β paralogs: Egd1 (major β; Nacβ1) and Btt1 (minor β′; Nacβ2), producing at least two main heterodimers: αβ‑NAC and αβ′‑NAC. (tian2024thenascentpolypeptideassociated pages 1-2, ott2015functionaldissectionof pages 2-4)
BTT1 is not an enzyme and does not catalyze a chemical reaction. Its primary function is as a ribosome‑associated regulatory/chaperone subunit that helps NAC:
* bind at/near the ribosome exit tunnel,
* interact with specific classes of nascent chains, and
* in yeast, enable certain cotranslational mRNA‑regulatory pathways (notably via Caf130/CCR4–Not in specific contexts). (schilke2024functionalsimilaritiesand pages 1-3)
A major 2024 advance is the demonstration that Nacβ2/Btt1 (but not Nacβ1/Egd1) harbors determinants enabling interaction with Caf130, a CCR4–Not component, and that this connection mediates a protective negative‑feedback mechanism on RPL4 mRNA when Acl4, the dedicated Rpl4 chaperone, is absent. (schilke2024functionalsimilaritiesand pages 1-3)
Mechanistic elements highlighted in 2024 include:
* Btt1/Nacβ2 is the minor NAC β paralog (20–100× less abundant than Nacα/Nacβ1 in the authors’ framing). (schilke2024functionalsimilaritiesand pages 1-3)
* Residues just C‑terminal to the Nacβ2 globular domain are required for Caf130 interaction and for the Nacβ2‑dependent growth phenotype in ∆acl4 cells; swapping this segment into Nacβ1 can confer Caf130 binding and the ∆acl4 growth effect. (schilke2024functionalsimilaritiesand pages 1-3)
* N‑terminal ribosome‑association determinants modulate the ∆acl4 phenotype, consistent with a model that correct NAC positioning on the ribosome is required for productive recruitment of CCR4–Not to the relevant RNC/mRNA. (schilke2024functionalsimilaritiesand pages 1-3)
Visual evidence: Schilke et al. include growth spot assays and a mechanistic model figure (Figures 2–4) connecting Nacα/Nacβ2, Caf130/CCR4–Not, ribosome binding, and RPL4 mRNA degradation in the ∆acl4 context. (schilke2024functionalsimilaritiesand media cc836246, schilke2024functionalsimilaritiesand media 4ede7f43)
Tian & Okamoto (2024) examined NAC subunits in respiration‑induced mitophagy (glycerol medium; stationary phase). They report that Egd1 loss strongly suppresses mitophagy, whereas loss of Btt1 has only a minor effect (supplemental figure referenced, not present in the main PDF). Quantitatively, using free mCherry release from a mito‑DHFR‑mCherry reporter at 72 h, they report free mCherry levels relative to wild type (set to 100%):
* egd1Δ: 36%
* egd2Δ: 72%
* egd1Δ egd2Δ: 38%
These results frame Btt1 as comparatively dispensable for this mitophagy phenotype. (tian2024thenascentpolypeptideassociated pages 2-3, tian2024thenascentpolypeptideassociated pages 3-5)
Visual evidence: the quantitative bar plot and corresponding western blot for free mCherry (Figure 1b) support these values. (tian2024thenascentpolypeptideassociated media e16695b8)
Although not BTT1‑specific, recent high‑impact work emphasizes NAC as an organizer of ribosome‑proximal protein biogenesis. For example, structural/biochemical studies (in other eukaryotes) show NAC can scaffold multienzyme processing at the ribosomal exit site (e.g., coordinating N‑terminal processing enzymes). This strengthens the conceptual framework that yeast Btt1’s roles are executed on ribosomes during translation, by modulating factor recruitment and nascent‑chain handling. (ott2015functionaldissectionof pages 1-2)
BTT1 itself is not an “application target” in the clinical or industrial sense, but BTT1/NAC biology is actively used in real-world research workflows:
Proteostasis and cotranslational quality control models. Yeast NAC subunits, including Btt1, are used to dissect how nascent chain handling prevents aggregation and mistargeting, especially in sensitized genetic backgrounds (e.g., ∆ssb or ribosomal protein chaperone mutants). (ott2015functionaldissectionof pages 1-2, ott2015functionaldissectionof pages 2-4)
Cotranslational mRNA surveillance and gene-expression coupling to assembly. The Btt1–Caf130 link places BTT1 within a practical framework for studying how cells prevent toxic accumulation of “orphan” ribosomal proteins by degrading their mRNAs when chaperone buffering is insufficient. This is a central theme for ribosome biogenesis/assembly homeostasis. (schilke2024functionalsimilaritiesand pages 1-3, pillet2022dedicatedchaperonescoordinate pages 4-7)
Mitochondria-associated translation studies. Prior work shows NAC affects ribosome association with mitochondria and that Btt1 is enriched with mitochondrial-protein mRNAs, making BTT1 relevant in experimental designs probing localized translation near mitochondria and coordination with import. (lesnik2015localizedtranslationnear pages 11-15, alamo2011definingthespecificity pages 17-18)
Two independent yeast studies converge that Btt1/Nacβ2 is much less abundant than Egd1/Nacβ1 and that low abundance helps explain its limited ability to replace major NAC functions under stress.
* Ott et al. report β′‑NAC (BTT1) expression is ~100‑fold lower than β‑NAC (EGD1). (ott2015functionaldissectionof pages 2-4)
* Schilke et al. discuss Nacβ2 as ~20–100‑fold less abundant than Nacα/Nacβ1. (schilke2024functionalsimilaritiesand pages 1-3)
Functionally, Ott et al. show that in severe proteostasis stress backgrounds (nacΔ ssbΔ), the abundant αβ‑NAC (Egd2–Egd1) can suppress defects broadly, whereas αβ′‑NAC does not fully restore growth/translation phenotypes; Btt1 can reduce aggregation somewhat but is not equivalently protective. (ott2015functionaldissectionof pages 7-9, ott2015functionaldissectionof pages 11-14)
Schilke et al. further argue the β paralogs can be functionally interchangeable for some NAC activities when expression is forced high (ADH1 promoter), suggesting quantitative expression is a key constraint in vivo. (schilke2024functionalsimilaritiesand pages 9-10)
The most Btt1‑specific mechanistic story in the current evidence set is Btt1’s acquired ability to engage Caf130 and thereby connect ribosome‑proximal sensing to the CCR4–Not complex to regulate RPL4 mRNA under conditions where the dedicated chaperone Acl4 is limiting. (schilke2024functionalsimilaritiesand pages 1-3)
Tian & Okamoto quantified mitophagy using free mCherry generation from a mitochondrial reporter (wild type at 72 h set to 100%). At 72 h in glycerol medium:
* egd1Δ: 36% of wild-type free mCherry
* egd2Δ: 72%
* egd1Δ egd2Δ: 38%
They also report btt1Δ has only a minor effect (supplement referenced). (tian2024thenascentpolypeptideassociated pages 2-3, tian2024thenascentpolypeptideassociated pages 3-5)
These values are supported visually by Figure 1b (bar graph + western blot). (tian2024thenascentpolypeptideassociated media e16695b8)
In adriamycin sensitivity assays, btt1Δ behaved like wild type, whereas egd1Δ/egd2Δ showed enhanced sensitivity, implying Btt1 is not the main determinant of NAC-dependent adriamycin resistance. (takahashi2009dysfunctionalnascentpolypeptideassociated pages 2-6, takahashi2009dysfunctionalnascentpolypeptideassociated pages 1-2)
A core mechanistic concept in yeast is that NAC’s ribosome binding is mediated by the β subunit N‑terminus via a conserved basic motif, placing NAC adjacent to the exit tunnel and allowing nascent chain interaction. This general mechanism is used to interpret Btt1 function as a β‑type subunit. (panasenko2009ribosomeassociationand pages 1-2, ott2015functionaldissectionof pages 2-4)
Multiple studies indicate that αβ′‑NAC (Egd2–Btt1) is biased toward ribosomes translating mitochondrial or ribosomal proteins, consistent with an interpretation that Btt1 contributes to a specialized cotranslational program (rather than being a generic essential chaperone). (ott2015functionaldissectionof pages 11-14, alamo2011definingthespecificity pages 17-18)
Primary site of action: cytosolic ribosomes at/near the polypeptide exit tunnel. (schilke2024functionalsimilaritiesand pages 1-3, ott2015functionaldissectionof pages 2-4)
Mitochondria-associated translation context: Btt1-containing NAC is implicated in localized translation near the mitochondrial outer membrane, including enrichment with mitochondrial-protein mRNAs and roles in RNC association with mitochondria (as discussed in authoritative reviews and primary substrate-association studies). (lesnik2015localizedtranslationnear pages 11-15, alamo2011definingthespecificity pages 17-18)
Nuclear/transcriptional roles: Within the retrieved evidence set, nuclear functions are discussed mainly for the Egd1/Egd2 system rather than directly demonstrated for Btt1. The adriamycin-toxicity letter discusses NAC subunits as having reported nuclear transcriptional roles historically, but their experiments emphasize ribosome-associated NAC function in resistance. (takahashi2009dysfunctionalnascentpolypeptideassociated pages 2-6, takahashi2009dysfunctionalnascentpolypeptideassociated pages 1-2)
NAC operates alongside the RAC–Ssb Hsp70/Hsp40 system. In yeast, deletion of NAC alone can be phenotypically mild, but combined deletion with Ssb leads to severe defects (growth sensitivity, aggregation, translation/ribosome biogenesis defects), demonstrating NAC’s buffering role in proteostasis. (ott2015functionaldissectionof pages 2-4)
Btt1/Nacβ2 physically and genetically connects to Caf130 and thus to CCR4–Not, enabling mRNA downregulation of ribosomal proteins (notably RPL4) when dedicated chaperone protection is absent, preventing proteotoxic outcomes. (schilke2024functionalsimilaritiesand pages 1-3, pillet2022dedicatedchaperonescoordinate pages 4-7)
NAC subunits can influence mitochondrial protein biogenesis and mitochondrial quality control. In the respiration‑induced mitophagy paradigm, however, Egd1 is dominant, Egd2 is intermediate, and Btt1 is minor. (tian2024thenascentpolypeptideassociated pages 2-3, tian2024thenascentpolypeptideassociated pages 1-2)
The following table maps the main claims to the most relevant sources.
| Topic | Key finding | Organism/strain | Evidence type | Year | Source (first author) | Publication venue | URL | Citation ID |
|---|---|---|---|---|---|---|---|---|
| Identity | BTT1 matches the requested Saccharomyces cerevisiae YDR252W gene encoding the minor NAC beta paralog, termed β′-NAC/Nacβ2, which partners with Egd2 (α-NAC) in an alternative NAC heterodimer distinct from the abundant Egd1-containing complex. | S. cerevisiae (budding yeast; S288c-context literature) | Primary | 2015 | Ott | PLOS ONE | https://doi.org/10.1371/journal.pone.0143457 | (ott2015functionaldissectionof pages 1-2, ott2015functionaldissectionof pages 2-4) |
| Function | Btt1 is part of the ribosome-associated nascent polypeptide-associated complex (NAC), an early cotranslational factor at the peptide exit tunnel that contacts nascent chains and helps regulate folding/targeting; Btt1-containing αβ′-NAC is functionally distinct from the major αβ-NAC. | S. cerevisiae | Primary | 2015 | Ott | PLOS ONE | https://doi.org/10.1371/journal.pone.0143457 | (ott2015functionaldissectionof pages 1-2, ott2015functionaldissectionof pages 11-14) |
| Ribosome binding/localization | Eukaryotic NAC ribosome attachment is mediated by the β-subunit N-terminus through a conserved basic motif; NAC crosslinking places the complex near the ribosomal exit site (e.g., Rpl25/uL23 and Rpl31/eL31). This mechanistic framework applies to yeast β paralogs, including Btt1 as a β-type NAC subunit. | S. cerevisiae | Primary | 2006 | Wegrzyn | Journal of Biological Chemistry | https://doi.org/10.1074/jbc.m511420200 | (ott2015functionaldissectionof pages 1-2, panasenko2009ribosomeassociationand pages 1-2) |
| Expression level / stoichiometry | BTT1 is much less abundant than EGD1: Ott reports β′-NAC/Btt1 expression at about 100-fold lower than β-NAC/Egd1, while Schilke summarizes Nacβ2 as roughly 20–100-fold less abundant than Nacα/Nacβ1. Low expression is a major explanation for weaker complementation by Btt1. | S. cerevisiae | Primary | 2015, 2024 | Ott; Schilke | PLOS ONE; Cell Stress and Chaperones | https://doi.org/10.1371/journal.pone.0143457; https://doi.org/10.1016/j.cstres.2024.10.004 | (ott2015functionaldissectionof pages 1-2, schilke2024functionalsimilaritiesand pages 1-3, ott2015functionaldissectionof pages 2-4) |
| Phenotype: single mutant stress response | In adriamycin-toxicity assays, btt1Δ showed the same sensitivity as wild type, unlike egd1Δ or egd2Δ, indicating Btt1 is not the NAC subunit primarily responsible for adriamycin resistance. The paper also notes intracellular Btt1 is only ~1% of Egd1. | S. cerevisiae BY4742 and deletion strains | Primary | 2009 | Takahashi | Journal of Toxicological Sciences | https://doi.org/10.2131/jts.34.703 | (takahashi2009dysfunctionalnascentpolypeptideassociated pages 2-6, takahashi2009dysfunctionalnascentpolypeptideassociated pages 1-2) |
| Phenotype: mitophagy | Tian & Okamoto 2024 show mitophagy defects are dominated by Egd1, not Btt1. At 72 h in glycerol medium, free mCherry from the mitophagy reporter was 36% of wild type in egd1Δ, 72% in egd2Δ, and 38% in egd1Δ egd2Δ; loss of Btt1 caused only a minor effect (reported in Fig. S1a,b, no exact value in main text). | S. cerevisiae | Primary | 2024 | Tian | Scientific Reports | https://doi.org/10.1038/s41598-023-50245-7 | (tian2024thenascentpolypeptideassociated pages 2-3, tian2024thenascentpolypeptideassociated pages 3-5, tian2024thenascentpolypeptideassociated pages 1-2) |
| Pathway context: mitochondrial protein biogenesis | Earlier substrate-specificity work found Btt1-associated NAC shows preference for nascent chains encoded by mRNAs for mitochondrial proteins, supporting a specialized role in cotranslational mitochondrial protein biogenesis/localized translation near mitochondria. | S. cerevisiae | Primary | 2011 | del Alamo | PLOS Biology | https://doi.org/10.1371/journal.pbio.1001100 | (alamo2011definingthespecificity pages 17-18, ott2015functionaldissectionof pages 11-14) |
| Functional complementation | In nacΔ ssbΔ backgrounds, the abundant αβ-NAC fully suppresses aggregation/translation/growth defects, whereas αβ′-NAC does not. Btt1/β′-NAC alone can reduce aggregation somewhat but fails to restore growth defects; weak rescue is attributed to lower abundance and functional divergence. | S. cerevisiae nacΔ ssbΔ strains | Primary | 2015 | Ott | PLOS ONE | https://doi.org/10.1371/journal.pone.0143457 | (ott2015functionaldissectionof pages 1-2, ott2015functionaldissectionof pages 7-9, ott2015functionaldissectionof pages 2-4, ott2015functionaldissectionof pages 11-14) |
| Quantitative/conditional rescue | Schilke et al. show that when Nacβ2/Btt1 is expressed from the ADH1 promoter to approximately the same level as Nacβ1, it can rescue the temperature-sensitive growth defect of strains lacking Nacα and Nacβ1. Thus, at least part of the difference between β paralogs is quantitative (expression level), not solely qualitative. | S. cerevisiae mutant strains lacking major NAC subunits | Primary | 2024 | Schilke | Cell Stress and Chaperones | https://doi.org/10.1016/j.cstres.2024.10.004 | (schilke2024functionalsimilaritiesand pages 9-10, schilke2024functionalsimilaritiesand pages 1-3) |
| Specific interaction / regulatory neofunctionalization | A recent Btt1-specific advance is that Nacβ2/Btt1 interacts with Caf130 of the CCR4–Not complex; residues just C-terminal to the globular domain are required. This interaction promotes Rpl4 mRNA degradation when the dedicated Rpl4 chaperone Acl4 is absent, indicating a specialized surveillance/regulatory role acquired by Btt1. | S. cerevisiae | Primary | 2024 | Schilke | Cell Stress and Chaperones | https://doi.org/10.1016/j.cstres.2024.10.004 | (schilke2024functionalsimilaritiesand pages 1-3, schilke2024functionalsimilaritiesand pages 9-10) |
| Ubiquitination / complex regulation | NAC regulation in yeast also involves ubiquitination of Egd1/Egd2 by Not4 and stress-induced ubiquitination/degradation of Egd2 by Rsp5. These studies mainly concern Egd1/Egd2 rather than Btt1, but they provide pathway context showing NAC composition and localization are dynamically regulated by ubiquitin systems. | S. cerevisiae | Primary | 2006, 2009 | Panasenko; Hiraishi | Journal of Biological Chemistry; FEBS Journal | https://doi.org/10.1074/jbc.M604986200; https://doi.org/10.1111/j.1742-4658.2009.07226.x | (panasenko2006theyeastccr4not pages 1-2, hiraishi2009theyeastubiquitin pages 1-2, hiraishi2009theyeastubiquitin pages 2-4) |
Table: This table summarizes the main published evidence for BTT1/YDR252W (UniProt P40314) in budding yeast, emphasizing experimentally supported function, localization, phenotypes, interactions, and the most informative quantitative findings. It is useful as a source-mapped overview separating Btt1-specific evidence from broader NAC-complex context.
BTT1/YDR252W (UniProt P40314) encodes Nacβ2/β′‑NAC, a low-abundance, ribosome‑associated NAC β paralog whose core activity is executed at the ribosomal exit tunnel via β‑subunit N‑terminal ribosome binding and dimerization with Egd2 (α‑NAC). Compared with the major NAC β paralog Egd1, Btt1 is generally less critical for global proteostasis or mitophagy phenotypes, but it has a well-supported specialized regulatory role: it provides a Caf130‑binding interface that links translating ribosomes to CCR4–Not to regulate specific mRNAs (notably RPL4) when dedicated chaperone buffering is compromised, thereby preventing toxic protein surplus/aggregation. This specialization, together with enrichment on RNCs translating mitochondrial proteins, positions Btt1 as a cotranslational selector/regulator rather than a broadly essential chaperone in standard growth conditions. (schilke2024functionalsimilaritiesand pages 1-3, tian2024thenascentpolypeptideassociated pages 2-3, ott2015functionaldissectionof pages 11-14)
References
(schilke2024functionalsimilaritiesand pages 1-3): Brenda A. Schilke, Thomas Ziegelhoffer, Przemyslaw Domanski, Jaroslaw Marszalek, Bartlomiej Tomiczek, and Elizabeth A. Craig. Functional similarities and differences among subunits of the nascent polypeptide-associated complex (nac) of saccharomyces cerevisiae. Cell Stress and Chaperones, 29:721-734, Dec 2024. URL: https://doi.org/10.1016/j.cstres.2024.10.004, doi:10.1016/j.cstres.2024.10.004. This article has 1 citations and is from a peer-reviewed journal.
(schilke2024functionalsimilaritiesand pages 9-10): Brenda A. Schilke, Thomas Ziegelhoffer, Przemyslaw Domanski, Jaroslaw Marszalek, Bartlomiej Tomiczek, and Elizabeth A. Craig. Functional similarities and differences among subunits of the nascent polypeptide-associated complex (nac) of saccharomyces cerevisiae. Cell Stress and Chaperones, 29:721-734, Dec 2024. URL: https://doi.org/10.1016/j.cstres.2024.10.004, doi:10.1016/j.cstres.2024.10.004. This article has 1 citations and is from a peer-reviewed journal.
(tian2024thenascentpolypeptideassociated pages 2-3): Yuan Tian and Koji Okamoto. The nascent polypeptide-associated complex subunit egd1 is required for efficient selective mitochondrial degradation in budding yeast. Scientific Reports, Jan 2024. URL: https://doi.org/10.1038/s41598-023-50245-7, doi:10.1038/s41598-023-50245-7. This article has 1 citations and is from a peer-reviewed journal.
(ott2015functionaldissectionof pages 1-2): Ann-Kathrin Ott, Lisa Locher, Miriam Koch, and Elke Deuerling. Functional dissection of the nascent polypeptide-associated complex in saccharomyces cerevisiae. PLoS ONE, 10:e0143457, Nov 2015. URL: https://doi.org/10.1371/journal.pone.0143457, doi:10.1371/journal.pone.0143457. This article has 41 citations and is from a peer-reviewed journal.
(tian2024thenascentpolypeptideassociated pages 1-2): Yuan Tian and Koji Okamoto. The nascent polypeptide-associated complex subunit egd1 is required for efficient selective mitochondrial degradation in budding yeast. Scientific Reports, Jan 2024. URL: https://doi.org/10.1038/s41598-023-50245-7, doi:10.1038/s41598-023-50245-7. This article has 1 citations and is from a peer-reviewed journal.
(panasenko2009ribosomeassociationand pages 1-2): Olesya O Panasenko, Fabrice P A David, and Martine A Collart. Ribosome association and stability of the nascent polypeptide-associated complex is dependent upon its own ubiquitination. Genetics, 181:447-460, Feb 2009. URL: https://doi.org/10.1534/genetics.108.095422, doi:10.1534/genetics.108.095422. This article has 48 citations and is from a domain leading peer-reviewed journal.
(ott2015functionaldissectionof pages 2-4): Ann-Kathrin Ott, Lisa Locher, Miriam Koch, and Elke Deuerling. Functional dissection of the nascent polypeptide-associated complex in saccharomyces cerevisiae. PLoS ONE, 10:e0143457, Nov 2015. URL: https://doi.org/10.1371/journal.pone.0143457, doi:10.1371/journal.pone.0143457. This article has 41 citations and is from a peer-reviewed journal.
(alamo2011definingthespecificity pages 17-18): Marta del Alamo, Daniel J. Hogan, Sebastian Pechmann, Veronique Albanese, Patrick O. Brown, and Judith Frydman. Defining the specificity of cotranslationally acting chaperones by systematic analysis of mrnas associated with ribosome-nascent chain complexes. PLoS Biology, 9:e1001100, Jul 2011. URL: https://doi.org/10.1371/journal.pbio.1001100, doi:10.1371/journal.pbio.1001100. This article has 201 citations and is from a highest quality peer-reviewed journal.
(schilke2024functionalsimilaritiesand media cc836246): Brenda A. Schilke, Thomas Ziegelhoffer, Przemyslaw Domanski, Jaroslaw Marszalek, Bartlomiej Tomiczek, and Elizabeth A. Craig. Functional similarities and differences among subunits of the nascent polypeptide-associated complex (nac) of saccharomyces cerevisiae. Cell Stress and Chaperones, 29:721-734, Dec 2024. URL: https://doi.org/10.1016/j.cstres.2024.10.004, doi:10.1016/j.cstres.2024.10.004. This article has 1 citations and is from a peer-reviewed journal.
(schilke2024functionalsimilaritiesand media 4ede7f43): Brenda A. Schilke, Thomas Ziegelhoffer, Przemyslaw Domanski, Jaroslaw Marszalek, Bartlomiej Tomiczek, and Elizabeth A. Craig. Functional similarities and differences among subunits of the nascent polypeptide-associated complex (nac) of saccharomyces cerevisiae. Cell Stress and Chaperones, 29:721-734, Dec 2024. URL: https://doi.org/10.1016/j.cstres.2024.10.004, doi:10.1016/j.cstres.2024.10.004. This article has 1 citations and is from a peer-reviewed journal.
(tian2024thenascentpolypeptideassociated pages 3-5): Yuan Tian and Koji Okamoto. The nascent polypeptide-associated complex subunit egd1 is required for efficient selective mitochondrial degradation in budding yeast. Scientific Reports, Jan 2024. URL: https://doi.org/10.1038/s41598-023-50245-7, doi:10.1038/s41598-023-50245-7. This article has 1 citations and is from a peer-reviewed journal.
(tian2024thenascentpolypeptideassociated media e16695b8): Yuan Tian and Koji Okamoto. The nascent polypeptide-associated complex subunit egd1 is required for efficient selective mitochondrial degradation in budding yeast. Scientific Reports, Jan 2024. URL: https://doi.org/10.1038/s41598-023-50245-7, doi:10.1038/s41598-023-50245-7. This article has 1 citations and is from a peer-reviewed journal.
(pillet2022dedicatedchaperonescoordinate pages 4-7): Benjamin Pillet, Alfonso Méndez-Godoy, Guillaume Murat, Sébastien Favre, Michael Stumpe, Laurent Falquet, and Dieter Kressler. Dedicated chaperones coordinate co-translational regulation of ribosomal protein production with ribosome assembly to preserve proteostasis. BioRxiv, Oct 2022. URL: https://doi.org/10.1101/2021.10.05.463164, doi:10.1101/2021.10.05.463164. This article has 30 citations.
(lesnik2015localizedtranslationnear pages 11-15): Chen Lesnik, Adi Golani-Armon, and Yoav Arava. Localized translation near the mitochondrial outer membrane: an update. RNA Biology, 12:801-809, Jul 2015. URL: https://doi.org/10.1080/15476286.2015.1058686, doi:10.1080/15476286.2015.1058686. This article has 142 citations and is from a peer-reviewed journal.
(ott2015functionaldissectionof pages 7-9): Ann-Kathrin Ott, Lisa Locher, Miriam Koch, and Elke Deuerling. Functional dissection of the nascent polypeptide-associated complex in saccharomyces cerevisiae. PLoS ONE, 10:e0143457, Nov 2015. URL: https://doi.org/10.1371/journal.pone.0143457, doi:10.1371/journal.pone.0143457. This article has 41 citations and is from a peer-reviewed journal.
(ott2015functionaldissectionof pages 11-14): Ann-Kathrin Ott, Lisa Locher, Miriam Koch, and Elke Deuerling. Functional dissection of the nascent polypeptide-associated complex in saccharomyces cerevisiae. PLoS ONE, 10:e0143457, Nov 2015. URL: https://doi.org/10.1371/journal.pone.0143457, doi:10.1371/journal.pone.0143457. This article has 41 citations and is from a peer-reviewed journal.
(takahashi2009dysfunctionalnascentpolypeptideassociated pages 2-6): Tsutomu Takahashi, Ken-ichiro Hirose, Emi Mizutani, and Akira Naganuma. Dysfunctional nascent polypeptide-associated complex (nac) activity in ribosomes enhances adriamycin toxicity in budding yeast. The Journal of toxicological sciences, 34 6:703-8, Dec 2009. URL: https://doi.org/10.2131/jts.34.703, doi:10.2131/jts.34.703. This article has 9 citations.
(takahashi2009dysfunctionalnascentpolypeptideassociated pages 1-2): Tsutomu Takahashi, Ken-ichiro Hirose, Emi Mizutani, and Akira Naganuma. Dysfunctional nascent polypeptide-associated complex (nac) activity in ribosomes enhances adriamycin toxicity in budding yeast. The Journal of toxicological sciences, 34 6:703-8, Dec 2009. URL: https://doi.org/10.2131/jts.34.703, doi:10.2131/jts.34.703. This article has 9 citations.
(tian2024thenascentpolypeptideassociated media 8ed09aa4): Yuan Tian and Koji Okamoto. The nascent polypeptide-associated complex subunit egd1 is required for efficient selective mitochondrial degradation in budding yeast. Scientific Reports, Jan 2024. URL: https://doi.org/10.1038/s41598-023-50245-7, doi:10.1038/s41598-023-50245-7. This article has 1 citations and is from a peer-reviewed journal.
(panasenko2006theyeastccr4not pages 1-2): Olesya Panasenko, Emilie Landrieux, Marc Feuermann, Andrija Finka, Nicole Paquet, and Martine A. Collart. The yeast ccr4-not complex controls ubiquitination of the nascent-associated polypeptide (nac-egd) complex*. Journal of Biological Chemistry, 281:31389-31398, Oct 2006. URL: https://doi.org/10.1074/jbc.m604986200, doi:10.1074/jbc.m604986200. This article has 125 citations and is from a domain leading peer-reviewed journal.
(hiraishi2009theyeastubiquitin pages 1-2): Hiroyuki Hiraishi, Takashi Shimada, Iwao Ohtsu, Taka‐Aki Sato, and Hiroshi Takagi. The yeast ubiquitin ligase rsp5 downregulates the alpha subunit of nascent polypeptide‐associated complex egd2 under stress conditions. The FEBS Journal, 276:5287-5297, Sep 2009. URL: https://doi.org/10.1111/j.1742-4658.2009.07226.x, doi:10.1111/j.1742-4658.2009.07226.x. This article has 21 citations.
(hiraishi2009theyeastubiquitin pages 2-4): Hiroyuki Hiraishi, Takashi Shimada, Iwao Ohtsu, Taka‐Aki Sato, and Hiroshi Takagi. The yeast ubiquitin ligase rsp5 downregulates the alpha subunit of nascent polypeptide‐associated complex egd2 under stress conditions. The FEBS Journal, 276:5287-5297, Sep 2009. URL: https://doi.org/10.1111/j.1742-4658.2009.07226.x, doi:10.1111/j.1742-4658.2009.07226.x. This article has 21 citations.
id: P40314
gene_symbol: BTT1
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:559292
label: Saccharomyces cerevisiae
description: >-
BTT1 encodes the low-abundance beta-2 paralog of the yeast
nascent-polypeptide-associated complex (NAC). Btt1 pairs with the alpha NAC
subunit Egd2 to form an alternative NAC heterodimer that associates with
cytosolic translating ribosomes near the nascent-chain exit tunnel. The best
supported core functions are participation in the NAC complex, ribosome-linked
cotranslational nascent-chain handling/folding, and regulation of
cotranslational protein targeting to membranes. Btt1 also has reproducible
physical and genetic links to Caf130/CCR4-NOT, but that appears to be a
specialized or context-dependent regulatory role rather than the general NAC
core function.
existing_annotations:
- term:
id: GO:0005854
label: nascent polypeptide-associated complex
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
The IBA annotation is consistent with Btt1's conserved NAC-beta family
identity and with direct yeast experiments showing that BTT1 encodes one
of the beta subunits of the nascent polypeptide-associated complex.
action: ACCEPT
reason: >-
Part_of NAC is a core cellular-component annotation for Btt1. It is
supported by direct yeast IDA evidence and conserved NAC-family context.
supported_by:
- reference_id: PMID:10219998
supporting_text: "The three subunits of the nascent polypeptide-associated complex (alpha, beta1, beta3) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1)."
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon identifies Btt1 as the low-abundance beta-2 paralog of yeast NAC.
- term:
id: GO:0005829
label: cytosol
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
The cytosol IBA annotation is consistent with the cytosolic,
ribosome-associated site of NAC action.
action: ACCEPT
reason: >-
Btt1 acts on cytosolic ribosome-nascent-chain complexes; cytosol is the
appropriate core localization.
supported_by:
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon places Btt1's primary site of action at cytosolic ribosomes near the polypeptide exit tunnel.
- term:
id: GO:0005634
label: nucleus
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
UniProt records possible transient nuclear localization by similarity, but
the curated Btt1 evidence reviewed here primarily supports a cytosolic
ribosome-associated NAC role.
action: KEEP_AS_NON_CORE
reason: >-
The nucleus annotation should not be elevated to core function for Btt1.
It may reflect a minor or context-dependent localization of NAC-family
proteins, while Btt1's best-supported activity is cytosolic and
ribosome-proximal.
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
Cytoplasm is consistent with Btt1's placement in the cytosolic
ribosome-associated NAC complex.
action: ACCEPT
reason: >-
Although cytosol is more precise, cytoplasm is not misleading for this
protein and is consistent with UniProt localization and NAC biology.
supported_by:
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon describes Btt1 as acting primarily on cytosolic ribosomes.
- term:
id: GO:0005854
label: nascent polypeptide-associated complex
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: >-
The ARBA electronic annotation to NAC is consistent with direct IDA and IBA
evidence for Btt1 as a NAC beta paralog.
action: ACCEPT
reason: >-
This electronic annotation is redundant with stronger evidence but correct
and should be retained.
supported_by:
- reference_id: PMID:10219998
supporting_text: "The three subunits of the nascent polypeptide-associated complex (alpha, beta1, beta3) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1)."
- term:
id: GO:0006613
label: cotranslational protein targeting to membrane
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: >-
The ARBA annotation is consistent with direct yeast NAC evidence showing
that NAC controls inappropriate ribosome-nascent-chain association with ER
membranes.
action: ACCEPT
reason: >-
Cotranslational membrane targeting regulation is a legitimate NAC function.
For Btt1 specifically, it should be interpreted as part of the NAC beta
subunit's ribosome-associated role rather than as a standalone membrane
transporter function.
supported_by:
- reference_id: PMID:10518932
supporting_text: "We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes."
- term:
id: GO:0015031
label: protein transport
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
The UniProt keyword-derived protein transport annotation is too broad for
Btt1. The supported transport-related role is cotranslational control of
ribosome-nascent-chain targeting to membranes.
action: MODIFY
reason: >-
Replace broad protein transport with GO:0006613 cotranslational protein
targeting to membrane, which is already experimentally supported for yeast
NAC and better captures the process represented by the annotation.
proposed_replacement_terms:
- id: GO:0006613
label: cotranslational protein targeting to membrane
supported_by:
- reference_id: PMID:10518932
supporting_text: "In the absence of NAC, signal-less RNCs are able to bind to ER membranes."
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:11283351
review:
summary: >-
This protein binding annotation comes from a large two-hybrid interactome
screen and reports a generic Caf130 interaction. The interaction may be
real, but GO:0005515 is not informative for Btt1 function.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Generic protein binding should not be retained as a functional endpoint.
The physiologically meaningful context is better represented by Btt1's NAC
complex membership and by the non-core CCR4-NOT/Caf130 annotation.
supported_by:
- reference_id: PMID:11283351
supporting_text: "A comprehensive two-hybrid analysis to explore the yeast protein interactome."
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:16554755
review:
summary: >-
This interaction-derived GO:0005515 annotation is from global protein
complex mapping. It is not wrong that Btt1 physically associates with
proteins, but the term is too vague to describe Btt1's NAC biology.
action: MARK_AS_OVER_ANNOTATED
reason: >-
The supported interactions with CCR4, Caf130, and YJR011C should not be
curated as generic protein binding. More informative annotations capture
NAC complex membership and the contextual CCR4-NOT association.
supported_by:
- reference_id: PMID:16554755
supporting_text: "Global landscape of protein complexes in the yeast Saccharomyces cerevisiae."
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:37968396
review:
summary: >-
The 2023 proteome-scale interactome evidence supports physical association
context but not a specific molecular function captured by GO:0005515.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Protein binding is over-annotated because it does not identify what Btt1
does. The interaction information is better interpreted through NAC and
CCR4-NOT context rather than as a standalone MF term.
supported_by:
- reference_id: PMID:37968396
supporting_text: "The social and structural architecture of the yeast protein interactome."
- term:
id: GO:0051083
label: "'de novo' cotranslational protein folding"
evidence_type: NAS
original_reference_id: PMID:26618777
review:
summary: >-
The NAS annotation is consistent with the NAC complex functioning with
other ribosome-associated chaperone systems to assist early folding of
nascent polypeptides.
action: ACCEPT
reason: >-
Btt1-containing NAC is a ribosome-associated cotranslational proteostasis
factor. Falcon and the 2015 NAC dissection support the view that Btt1 has
weaker, more specialized activity than the major Egd1-containing NAC, but
the process annotation remains appropriate.
supported_by:
- reference_id: PMID:26618777
supporting_text: "assist cotranslational processes such as folding of nascent polypeptides."
- reference_id: PMID:26618777
supporting_text: "aggregation of newly synthesized proteins"
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon identifies Btt1 as a ribosome-associated NAC beta paralog involved in cotranslational nascent-chain handling.
- term:
id: GO:0006613
label: cotranslational protein targeting to membrane
evidence_type: IGI
original_reference_id: PMID:10518932
review:
summary: >-
The IGI annotation captures the experimentally supported NAC role in
preventing signal-less ribosome-nascent-chain complexes from binding ER
membranes.
action: ACCEPT
reason: >-
This is a core NAC process and is supported by yeast-derived in vitro
targeting experiments. For Btt1, it should be understood as a beta-subunit
contribution to the NAC complex rather than a direct membrane-targeting
receptor activity.
supported_by:
- reference_id: PMID:10518932
supporting_text: "We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes."
- reference_id: PMID:10518932
supporting_text: "In the absence of NAC, signal-less RNCs are able to bind to ER membranes."
- term:
id: GO:0030015
label: CCR4-NOT core complex
evidence_type: IPI
original_reference_id: PMID:18214544
review:
summary: >-
Btt1 reproducibly interacts with Caf130 and was described as a tenth member
of the CCR4-NOT complex. This is biologically meaningful, but the main
evolved function of Btt1 remains NAC/ribosome-proximal cotranslational
regulation.
action: KEEP_AS_NON_CORE
reason: >-
Retain the annotation as a non-core contextual complex association. The
GO term may overstate Btt1 as part of the CCR4-NOT core relative to its
low-abundance NAC beta-subunit identity, but the Caf130 interaction and
CCR4-NOT regulatory context are well supported.
supported_by:
- reference_id: PMID:18214544
supporting_text: "BTT1, a member of the nascent polypeptide association complex that binds the ribosome, was shown to be a tenth member of the CCR4-NOT complex"
- reference_id: PMID:18214544
supporting_text: "interacting through CAF130"
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon highlights Btt1-Caf130/CCR4-NOT coupling as a specialized, context-dependent regulatory role.
- term:
id: GO:0005854
label: nascent polypeptide-associated complex
evidence_type: IDA
original_reference_id: PMID:10219998
review:
summary: >-
The original characterization directly supports BTT1 as encoding one of
the yeast NAC beta subunits and places the complex on ribosomes close to
nascent chains.
action: ACCEPT
reason: >-
This is direct experimental evidence for the central cellular-component
annotation of Btt1.
supported_by:
- reference_id: PMID:10219998
supporting_text: "BTT1). We found the complex bound to ribosomes via the beta-subunits"
- reference_id: PMID:10219998
supporting_text: "in close proximity to nascent polypeptides"
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IMP
original_reference_id: PMID:10219998
review:
summary: >-
The original NAC study supports proximity to nascent polypeptides and a
role in preventing mistargeting, but GO:0051082 is too vague for Btt1.
The better curation is ribosome-associated cotranslational nascent-chain
handling, represented by ribosome binding plus cotranslational folding and
membrane-targeting process annotations.
action: MODIFY
reason: >-
Replace generic unfolded protein binding with the more directly supported
MF GO:0043022 ribosome binding. Btt1 is a NAC beta subunit that anchors
NAC at translating ribosomes, while the biological-process annotations
capture the nascent-chain folding and targeting outcomes.
proposed_replacement_terms:
- id: GO:0043022
label: ribosome binding
supported_by:
- reference_id: PMID:10219998
supporting_text: "BTT1). We found the complex bound to ribosomes via the beta-subunits"
- reference_id: PMID:10219998
supporting_text: "in close proximity to nascent polypeptides"
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon supports interpreting Btt1 as a ribosome-associated NAC beta subunit rather than a generic unfolded-protein binder.
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: PMID:10219998
title: Initial characterization of the nascent polypeptide-associated complex in yeast.
findings:
- statement: BTT1 encodes one of the yeast NAC beta subunits.
supporting_text: "The three subunits of the nascent polypeptide-associated complex (alpha, beta1, beta3) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1)."
- statement: Yeast NAC binds ribosomes through its beta subunits near nascent polypeptides.
supporting_text: "BTT1). We found the complex bound to ribosomes via the beta-subunits"
- statement: NAC is positioned near nascent polypeptides.
supporting_text: "in close proximity to nascent polypeptides"
- id: PMID:10518932
title: The nascent polypeptide-associated complex (NAC) of yeast functions in the targeting process of ribosomes to the ER membrane.
findings:
- statement: NAC prevents inappropriate binding of signal-less ribosome-nascent-chain complexes to yeast membranes.
supporting_text: "We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes."
- statement: Loss of NAC allows signal-less ribosome-nascent-chain complexes to bind ER membranes.
supporting_text: "In the absence of NAC, signal-less RNCs are able to bind to ER membranes."
- id: PMID:11283351
title: A comprehensive two-hybrid analysis to explore the yeast protein interactome.
findings:
- statement: The cited source is a large two-hybrid interactome screen, so its Btt1 protein-binding annotation is low-specificity.
supporting_text: "A comprehensive two-hybrid analysis to explore the yeast protein interactome."
- id: PMID:16554755
title: Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.
findings:
- statement: The cited source is a global protein complex map, supporting interaction context but not a specific Btt1 molecular activity.
supporting_text: "Global landscape of protein complexes in the yeast Saccharomyces cerevisiae."
- id: PMID:18214544
title: Genome wide expression analysis of the CCR4-NOT complex indicates that it consists of three modules with the NOT module controlling SAGA-responsive genes.
findings:
- statement: Btt1 was reported as a CCR4-NOT-associated protein interacting through Caf130.
supporting_text: "BTT1, a member of the nascent polypeptide association complex that binds the ribosome, was shown to be a tenth member of the CCR4-NOT complex"
- statement: Caf130 mediates the Btt1-CCR4-NOT interaction.
supporting_text: "interacting through CAF130"
- id: PMID:26618777
title: Functional Dissection of the Nascent Polypeptide-Associated Complex in Saccharomyces cerevisiae.
findings:
- statement: Yeast NAC and RAC-Ssb are ribosome-tethered systems assisting cotranslational folding.
supporting_text: "assist cotranslational processes such as folding of nascent polypeptides."
- statement: NAC loss in sensitized cells contributes to aggregation of newly synthesized proteins.
supporting_text: "aggregation of newly synthesized proteins"
- id: PMID:37968396
title: The social and structural architecture of the yeast protein interactome.
findings:
- statement: The cited source is a proteome-scale interactome study, supporting interaction context but not a specific Btt1 molecular activity.
supporting_text: "The social and structural architecture of the yeast protein interactome."
- id: file:yeast/BTT1/BTT1-deep-research-falcon.md
title: Falcon deep research report on BTT1
findings:
- statement: Falcon synthesis identifies Btt1 as a low-abundance NAC beta paralog acting at cytosolic ribosomes with specialized Caf130/CCR4-NOT regulatory context.
core_functions:
- description: >-
Btt1 is the beta-2 subunit of an alternative yeast nascent-polypeptide-
associated complex. As part of NAC, it associates with cytosolic translating
ribosomes near nascent polypeptides and contributes to cotranslational
protein folding and control of ribosome-nascent-chain targeting to
membranes. Its Caf130/CCR4-NOT interactions are retained as important
non-core context.
molecular_function:
id: GO:0043022
label: ribosome binding
directly_involved_in:
- id: GO:0051083
label: "'de novo' cotranslational protein folding"
- id: GO:0006613
label: cotranslational protein targeting to membrane
locations:
- id: GO:0005829
label: cytosol
- id: GO:0005737
label: cytoplasm
in_complex:
id: GO:0005854
label: nascent polypeptide-associated complex
supported_by:
- reference_id: PMID:10219998
supporting_text: "BTT1). We found the complex bound to ribosomes via the beta-subunits"
- reference_id: PMID:10518932
supporting_text: "We provide evidence that NAC prevents binding of RNCs without a signal sequence to yeast membranes."
- reference_id: PMID:26618777
supporting_text: "assist cotranslational processes such as folding of nascent polypeptides."
- reference_id: file:yeast/BTT1/BTT1-deep-research-falcon.md
supporting_text: Falcon synthesis supports Btt1 as a ribosome-associated NAC beta paralog with cotranslational proteostasis and targeting roles.
proposed_new_terms: []
suggested_questions:
- question: >-
Should Btt1 receive a distinct curated annotation for Caf130/CCR4-NOT-linked
cotranslational mRNA regulation of ribosomal protein production, separate
from its core NAC complex annotations?
experts:
- Craig EA
- Deuerling E
- Collart MA
suggested_experiments:
- hypothesis: >-
Btt1's low-abundance NAC beta-2 functions separate its general
ribosome-associated NAC activity from its specialized Caf130/CCR4-NOT
regulatory role.
description: >-
Compare wild-type BTT1, ribosome-binding-defective BTT1 mutants, and
Caf130-interface BTT1 mutants for NAC ribosome occupancy, nascent-chain
aggregation, ER membrane mistargeting, and Caf130-dependent RPL4 mRNA decay.
experiment_type: ribosome profiling and nascent-chain quality-control assay