BAG1 (BCL2-associated athanogene 1) is a co-chaperone and nucleotide exchange factor (NEF) for Hsp70/Hsc70. Its core function is to promote ADP release from the Hsp70 ATPase domain, facilitating the chaperone cycle. BAG1 contains a C-terminal BAG domain that binds the Hsp70 nucleotide-binding domain (NBD) and an N-terminal ubiquitin-like (UBL) domain that binds the 26S proteasome subunit Rpn1, enabling direct coupling of Hsp70-bound clients to proteasomal degradation. Multiple isoforms exist due to alternative translation initiation: BAG-1L (nuclear, with NLS), BAG-1M, and BAG-1S (predominantly cytosolic). While BAG1 was originally named for its interaction with BCL2, its primary evolved function is as an Hsp70/Hsc70 co-chaperone involved in proteostasis, not apoptosis regulation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: BAG1 isoforms BAG-1M and BAG-1S are predominantly cytosolic, where they function as Hsp70/Hsc70 co-chaperones. Phylogenetic inference supports cytoplasmic localization across BAG family members. Reason: Well-supported by phylogenetic inference and consistent with UniProt annotations indicating shorter isoforms localize predominantly to cytoplasm. The cytosolic BAG-1 immunostaining was also clearly associated with organelles in some cases [PMID:9679980]. Core localization for the chaperoning function. Supporting Evidence: PMID:9679980 cytosolic BAG-1 immunostaining was clearly associated with organelles resembling mitochondria |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: BAG-1L isoform contains an N-terminal NLS and is predominantly nuclear, where it modulates steroid receptor (ER/AR) transcriptional activity. Reason: Phylogenetically supported and consistent with isoform-specific localization. BAG-1L is nuclear and functions in receptor regulation [PMID:9679980]. Supporting Evidence: PMID:9679980 BAG-1L often resides in the nucleus, consistent with the presence of a nuclear localization sequence in the NH2-terminal unique domain of this protein |
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: Cytosolic localization is consistent with BAG1's role as a co-chaperone for cytosolic Hsp70/Hsc70 and its function in coupling clients to the proteasome. Reason: Phylogenetically supported. BAG1 functions in the cytosol as an Hsp70 NEF, promoting substrate release and proteasomal routing. Supporting Evidence: GO_REF:0000033 Phylogenetic annotation inference |
| GO:0016020 membrane | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Some evidence for membrane association, particularly in presence of BCL2 where BAG1 localizes to intracellular membranes including nuclear envelope. Reason: Membrane association appears secondary and dependent on BCL2 interaction. Not the primary location for core chaperone function [PMID:9679980]. Supporting Evidence: PMID:9679980 overexpression of Bcl-2 in cultured cells can cause intracellular redistribution of GFP-BAG-1, producing a membranous pattern typical of Bcl-2 family proteins |
| GO:0000774 adenyl-nucleotide exchange factor activity | IBA GO_REF:0000033 | ACCEPT | Summary: BAG1 functions as a nucleotide exchange factor (NEF) for Hsp70/Hsc70, promoting ADP release from the chaperone to reset it for another substrate cycle. This is the core molecular function of BAG1. Reason: This is the primary molecular function of BAG1. The BAG domain binds the Hsp70 nucleotide-binding domain to accelerate ADP release [PMID:24318877, PMID:9305631]. Well-supported by phylogenetic inference and direct experimental evidence. Supporting Evidence: PMID:24318877 Proteins with Bcl2-associated anthanogene (BAG) domains act as nucleotide exchange factors (NEFs) for the molecular chaperone heat shock protein 70 PMID:9305631 BAG-1 binds to the ATPase domain of Hsp70 and Hsc70 |
| GO:0050821 protein stabilization | IBA GO_REF:0000033 | ACCEPT | Summary: BAG1 participates in protein stabilization through its role in the Hsp70 chaperone system, helping maintain client protein conformations. Reason: Phylogenetically supported and consistent with BAG1's role in the Hsp70 chaperone machinery. However, BAG1 actually tends to bias clients toward degradation via its UBL domain-proteasome coupling, whereas BAG3 promotes stabilization. This annotation may be somewhat imprecise. Supporting Evidence: GO_REF:0000033 Phylogenetic annotation inference |
| GO:0051087 protein-folding chaperone binding | IBA GO_REF:0000033 | ACCEPT | Summary: BAG1 directly binds Hsp70/Hsc70 through its conserved BAG domain. This interaction is central to its function as a co-chaperone and NEF. Reason: Core molecular function. BAG1 binds the ATPase domain of Hsp70/Hsc70 with high affinity (KD ~22 nM for subdomain) [PMID:11741305, PMID:24318877]. Supporting Evidence: PMID:11741305 The molecular co-chaperone BAG1 and other members of the BAG family bind to Hsp70/Hsc70 heat shock proteins through a conserved BAG domain PMID:9305631 BAG-1 binds to the ATPase domain of Hsp70 and Hsc70 file:human/BAG1/BAG1-deep-research-falcon.md BAG-1 is a co-chaperone/NEF for Hsp70/Hsc70 mediated by its C-terminal BAG domain |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation based on UniProt subcellular location. Consistent with IBA annotation and experimental evidence for BAG-1L nuclear localization. Reason: Redundant with IBA annotation but correctly reflects nuclear localization of BAG-1L isoform based on UniProt mapping. Supporting Evidence: GO_REF:0000044 UniProt subcellular location mapping |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic annotation based on combined automated methods. Consistent with cytoplasmic localization of BAG-1M and BAG-1S isoforms. Reason: Redundant with IBA annotation but correctly reflects cytoplasmic localization. Supporting Evidence: GO_REF:0000120 Combined automated annotation |
| GO:0006915 apoptotic process | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: This annotation derives from the UniProt keyword "Apoptosis" based on historical literature describing BAG1's interaction with BCL2. However, BAG1's core function is as an Hsp70/Hsc70 co-chaperone involved in proteostasis, not apoptosis. Reason: BAG1 is a clear OVER-ANNOTATION for apoptotic process. Its core evolved function is as an Hsp70/Hsc70 nucleotide exchange factor involved in proteostasis. While BAG1 was named for "BCL2-associated athanogene" and can interact with BCL2, this is a secondary interaction, not the primary function. Any anti-apoptotic effects are indirect consequences of its chaperoning role [PMID:9305631, deep research]. The UniProt keyword mapping is misleading about the primary function. Supporting Evidence: PMID:9305631 The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed between BAG-1 and several other proteins, including Raf-1, steroid hormone receptors |
| GO:0043066 negative regulation of apoptotic process | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: ARBA machine learning annotation suggesting anti-apoptotic function. While BAG1 overexpression can protect cells from apoptosis, this is secondary to its core chaperone function. Reason: Over-annotation. BAG1's primary function is proteostasis via Hsp70 co-chaperoning. Anti-apoptotic effects are indirect and secondary to chaperoning function. The name "BCL2-associated athanogene" is historical but misleading about the core molecular function [deep research, PMID:9305631]. Supporting Evidence: PMID:9305631 The inhibitory effects of BAG-1 on Hsp/Hsc70 chaperone activity suggest that BAG-1 represents a novel type of chaperone regulatory proteins |
| GO:0051087 protein-folding chaperone binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation from BAG domain. Correctly reflects the core function of binding Hsp70/Hsc70 chaperones. Reason: Correct annotation based on domain architecture. BAG domain mediates Hsp70/Hsc70 binding, which is the core molecular function. Supporting Evidence: GO_REF:0000002 InterPro BAG domain annotation |
| GO:0005515 protein binding | IPI PMID:11741305 The carboxyl-terminal lobe of Hsc70 ATPase domain is suffici... | MODIFY | Summary: Paper demonstrates BAG1 binding to Hsc70 ATPase domain. The C-terminal lobe of Hsc70 ATPase domain is sufficient for binding. Reason: Generic "protein binding" is uninformative. This paper specifically demonstrates Hsp70/Hsc70 binding through the BAG domain. Should be annotated with more specific term GO:0051087 (protein-folding chaperone binding). Proposed replacements: protein-folding chaperone binding Supporting Evidence: PMID:11741305 The molecular co-chaperone BAG1 and other members of the BAG family bind to Hsp70/Hsc70 heat shock proteins through a conserved BAG domain |
| GO:0005515 protein binding | IPI PMID:19060904 An empirical framework for binary interactome mapping | REMOVE | Summary: High-throughput interactome mapping study providing empirical framework for binary interactome mapping. Reason: Generic "protein binding" from high-throughput study is uninformative and does not tell us about BAG1's actual function. Should be removed or replaced with more specific functional terms if the interaction is biologically relevant. Supporting Evidence: PMID:19060904 an empirically-based approach to rigorously dissect quality parameters of currently available human interactome maps |
| GO:0005515 protein binding | IPI PMID:19800331 Short peptides derived from the BAG-1 C-terminus inhibit the... | MODIFY | Summary: Paper on BAG-1/HSC70 interaction inhibiting peptides. Demonstrates BAG1-HSC70 binding relevant to breast cancer cell growth. Reason: Generic protein binding is uninformative. The specific interaction is with HSC70 (HSPA8), which should be annotated as protein-folding chaperone binding. Proposed replacements: protein-folding chaperone binding Supporting Evidence: PMID:19800331 the interaction with HSC70 and HSP70, is considered vital |
| GO:0005515 protein binding | IPI PMID:25036637 A quantitative chaperone interaction network reveals the arc... | KEEP AS NON CORE | Summary: Quantitative chaperone interaction network study. Multiple interactions detected including with proteasome subunit PSMD2. Reason: While generic, this high-throughput study reveals BAG1's broader interaction network in proteostasis. The study provides a framework for deciphering the proteostasis network. Supporting Evidence: PMID:25036637 We have combined mass spectrometry and quantitative high-throughput LUMIER assays to systematically characterize the chaperone/co-chaperone/client interaction network in human cells |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network | REMOVE | Summary: Proteome-scale human interactome network study. High-throughput data. Reason: Generic protein binding from high-throughput interactome study. Does not provide functional insight beyond what is captured by more specific annotations. Supporting Evidence: PMID:25416956 Just as reference genome sequences revolutionized human genetics, reference maps of interactome networks will be critical to fully understand genotype-phenotype relationships |
| GO:0005515 protein binding | IPI PMID:26871637 Widespread Expansion of Protein Interaction Capabilities by ... | REMOVE | Summary: Study on alternative splicing effects on protein interactions. Reason: Generic protein binding annotation. Uninformative without more specific functional context. Supporting Evidence: PMID:26871637 alternative splicing is known to diversify the functional characteristics of some genes |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | REMOVE | Summary: Study on genetic variants disrupting protein interactions. Reason: Generic protein binding from high-throughput study. Does not inform about BAG1's core function. Supporting Evidence: PMID:31515488 the impact of 2009 missense single nucleotide variants (SNVs) across 2185 protein-protein interactions |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome | REMOVE | Summary: Reference map of human binary protein interactome. Reason: Generic protein binding from interactome mapping. Uninformative. Supporting Evidence: PMID:32296183 A reference map of the human binary protein interactome |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | REMOVE | Summary: Dual proteome-scale networks study. Reason: Generic protein binding from high-throughput study. Supporting Evidence: PMID:33961781 we have created two proteome-scale, cell-line-specific interaction networks |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | REMOVE | Summary: Multimodal cell maps study. Reason: Generic protein binding from high-throughput study. Supporting Evidence: PMID:40205054 Multimodal cell maps as a foundation for structural and functional genomics |
| GO:0005515 protein binding | IPI PMID:8692945 Bcl-2 interacting protein, BAG-1, binds to and activates the... | KEEP AS NON CORE | Summary: Original paper showing BAG1 binds and activates Raf-1 kinase. This was part of early characterization suggesting BAG1 links BCL2 to signaling. Reason: The Raf-1 interaction is documented but represents a secondary function. The primary function of BAG1 is Hsp70 co-chaperoning. The Raf-1 interaction may be mediated by Hsp70 chaperone complexes [PMID:9305631]. Supporting Evidence: PMID:8692945 Raf-1 and BAG-1 specifically interact in vitro and in yeast two-hybrid assays PMID:9305631 The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed between BAG-1 and several other proteins, including Raf-1 |
| GO:0005654 nucleoplasm | IDA GO_REF:0000052 | ACCEPT | Summary: HPA immunofluorescence data showing nucleoplasm localization. Consistent with BAG-1L nuclear localization. Reason: Direct experimental evidence for nucleoplasm localization, consistent with BAG-1L isoform nuclear function in steroid receptor regulation. Supporting Evidence: GO_REF:0000052 HPA immunofluorescence curation |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: HPA immunofluorescence data showing cytosol localization. Reason: Direct experimental evidence. Consistent with BAG-1M and BAG-1S cytosolic function as Hsp70 co-chaperones. Supporting Evidence: GO_REF:0000052 HPA immunofluorescence curation |
| GO:0034393 positive regulation of smooth muscle cell apoptotic process | ISS GO_REF:0000024 | UNDECIDED | Summary: ISS annotation from mouse ortholog. This is a highly specific annotation suggesting BAG1 promotes smooth muscle cell apoptosis, which contradicts the general anti-apoptotic characterization. Reason: This annotation seems contradictory to the general characterization of BAG1 as anti-apoptotic. The ISS transfer from mouse (UniProtKB:B0K019) suggests context-specific pro-apoptotic effects. This may relate to BAG1's role in STUB1-mediated ESR1 degradation under specific conditions. Requires further evaluation of the mouse literature. Supporting Evidence: GO_REF:0000024 Manual transfer from mouse ortholog |
| GO:0031625 ubiquitin protein ligase binding | IPI PMID:16207813 BAG-2 acts as an inhibitor of the chaperone-associated ubiqu... | ACCEPT | Summary: Paper demonstrates BAG1 interacts with CHIP (STUB1), the chaperone-associated ubiquitin ligase. BAG1 stimulates CHIP-mediated degradation of clients. Reason: Important functional interaction. BAG1's cooperation with CHIP links the Hsp70 chaperone system to proteasomal degradation. BAG1 can bind simultaneously with CHIP to Hsc70 and recruit complexes to the proteasome via its UBL domain [PMID:16207813]. Supporting Evidence: PMID:16207813 The cochaperone BAG-1, for example, was shown to stimulate the CHIP-mediated degradation of the glucocorticoid hormone receptor |
| GO:0000774 adenyl-nucleotide exchange factor activity | IDA PMID:24318877 Binding of human nucleotide exchange factors to heat shock p... | ACCEPT | Summary: Direct assay demonstrating BAG1 NEF activity. This paper measured binding of BAG1 to Hsp72 and showed hierarchy of affinities and potency in nucleotide release assays. Reason: Core molecular function with direct experimental evidence. BAG1 is a bona fide NEF for Hsp70, promoting ADP release to reset the chaperone cycle [PMID:24318877]. Supporting Evidence: PMID:24318877 Proteins with Bcl2-associated anthanogene (BAG) domains act as nucleotide exchange factors (NEFs) for the molecular chaperone heat shock protein 70 |
| GO:0005515 protein binding | IPI PMID:24318877 Binding of human nucleotide exchange factors to heat shock p... | MODIFY | Summary: Same paper as NEF activity annotation. Shows BAG1 binding to HSPA1A, HSPA1B, and HSPA8 (Hsc70). Reason: Generic protein binding is uninformative. The specific binding is to Hsp70 family chaperones. Should be annotated with GO:0051087 (protein-folding chaperone binding). Proposed replacements: protein-folding chaperone binding Supporting Evidence: PMID:24318877 we measured the binding of human Hsp72 (HSPA1A) to BAG1 |
| GO:0005634 nucleus | HDA PMID:21630459 Proteomic characterization of the human sperm nucleus | ACCEPT | Summary: High-throughput data from sperm nucleus proteomics. Reason: Consistent with nuclear localization of BAG-1L isoform. Supports nuclear presence in specialized cell type. Supporting Evidence: PMID:21630459 403 different proteins have been identified from the isolated sperm nuclei |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5252079 | ACCEPT | Summary: Reactome pathway annotation for HSP110-mediated nucleotide exchange on HSP70. BAG1 is part of the cytosolic chaperone machinery. Reason: Consistent with BAG1's cytosolic function in Hsp70 chaperone regulation. Reactome pathway places BAG1 appropriately in the chaperone cycle. Supporting Evidence: Reactome:R-HSA-5252079 HSP110s exchange ATP for ADP on HSP70s |
| GO:0005634 nucleus | IDA GO_REF:0000054 | ACCEPT | Summary: LIFEdb annotation based on GFP fusion protein localization. Reason: Consistent with BAG-1L nuclear localization demonstrated by other methods. Supporting Evidence: GO_REF:0000054 GFP fusion protein localization |
| GO:0006457 protein folding | IDA PMID:12853476 Cofactor Tpr2 combines two TPR domains and a J domain to reg... | ACCEPT | Summary: Paper on Tpr2 regulation of Hsp70/Hsp90 system. BAG1 is mentioned as part of the chaperone folding machinery context. Reason: BAG1 participates in protein folding through its role as an Hsp70 NEF. The paper discusses co-chaperone regulation of the Hsp70/Hsp90 system in the context of glucocorticoid receptor folding [PMID:12853476]. Supporting Evidence: PMID:12853476 In the eukaryotic cytosol, Hsp70 and Hsp90 cooperate with various co-chaperone proteins in the folding of a growing set of substrates |
| GO:0005515 protein binding | IPI PMID:9305631 BAG-1 modulates the chaperone activity of Hsp70/Hsc70 | MODIFY | Summary: Key paper demonstrating BAG1 binds Hsp70/Hsc70 and BCL2. This established BAG1 as a chaperone modulator. Reason: This paper demonstrates specific interactions with Hsp70/Hsc70 and BCL2. The Hsp70 interaction should be annotated as GO:0051087 (protein-folding chaperone binding). The BCL2 interaction could be annotated separately but is secondary to the core chaperone function. Proposed replacements: protein-folding chaperone binding Supporting Evidence: PMID:9305631 BAG-1 binds to the ATPase domain of Hsp70 and Hsc70 |
| GO:0005634 nucleus | IDA PMID:9679980 Expression and location of Hsp70/Hsc-binding anti-apoptotic ... | ACCEPT | Summary: Original characterization of BAG1 isoform localization. Shows BAG-1L localizes predominantly to nucleus. Reason: Primary literature establishing isoform-specific localization. BAG-1L is nuclear due to N-terminal NLS [PMID:9679980]. Supporting Evidence: PMID:9679980 BAG-1L often resides in the nucleus, consistent with the presence of a nuclear localization sequence in the NH2-terminal unique domain of this protein |
| GO:0005737 cytoplasm | TAS PMID:8947043 HGF receptor associates with the anti-apoptotic protein BAG-... | ACCEPT | Summary: Early paper on HGF receptor association with BAG1. Describes cytoplasmic localization in context of receptor signaling. Reason: Consistent with cytoplasmic localization of BAG1 isoforms. Supporting Evidence: PMID:8947043 Association of the receptor with BAG-1 occurs in intact cells |
| GO:0007166 cell surface receptor signaling pathway | TAS PMID:8947043 HGF receptor associates with the anti-apoptotic protein BAG-... | KEEP AS NON CORE | Summary: Based on BAG1 association with HGF and PDGF receptors. Paper suggests BAG1 links growth factor receptors to anti-apoptotic machinery. Reason: This represents a secondary function. The receptor associations may be mediated through Hsp70 chaperone complexes. Not the core function of BAG1 [PMID:9305631, PMID:8947043]. Supporting Evidence: PMID:8947043 BAG-1 also enhances platelet-derived growth factor (PDGF)-mediated protection from apoptosis and associates with the PDGF receptor PMID:9305631 The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed between BAG-1 and several other proteins |
| GO:0043066 negative regulation of apoptotic process | TAS PMID:8947043 HGF receptor associates with the anti-apoptotic protein BAG-... | MARK AS OVER ANNOTATED | Summary: Based on early characterization showing BAG1 enhances HGF-mediated protection from apoptosis. Reason: Over-annotation. While BAG1 can enhance protection from apoptosis when overexpressed, this is secondary to its core function as an Hsp70 co-chaperone. The "anti-apoptotic" characterization is historical and does not reflect the primary evolved function. The paper itself notes BAG1 acts through "unknown mechanisms" - we now know this involves Hsp70 chaperoning [PMID:8947043, PMID:9305631]. Supporting Evidence: PMID:8947043 Overexpression of BAG-1 in liver progenitor cells enhances protection from apoptosis by HGF PMID:9305631 The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed |
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Download this section (compressed HTML)Q: What is the relative contribution of BAG1 vs BAG3 to Hsp70 client fate determination (UPS degradation vs autophagy)? BAG1 biases toward UPS degradation via its UBL domain, while BAG3 promotes autophagy. The BAG1:BAG3 ratio may determine client fate.
Q: How does the ubiquitin-independent proteasomal degradation pathway mediated by BAG1-Hsp70-Rpn1 ternary complex function physiologically? Recent cryo-EM structures suggest BAG1 can deliver Hsp70 clients to the proteasome without ubiquitination. The scope of this pathway is unclear.
Experiment: Compare the protein folding vs degradation outcomes for specific Hsp70 clients in cells with BAG1 knockout, BAG1 UBL domain deletion, or BAG1 BAG domain mutations. This would distinguish BAG1's NEF function from its proteasome-coupling function and clarify which is more important for different client fates.
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