bag-1 encodes the Caenorhabditis elegans member of the BAG (Bcl-2-associated athanogene) family of Hsp70/Hsc70 co-chaperones. The 210-residue protein has an N-terminal ubiquitin-like domain and a C-terminal BAG domain; the BAG domain binds the nucleotide-binding (ATPase) domain of Hsp70-family chaperones and acts as a nucleotide-exchange factor, promoting ADP release and ATP-dependent discharge of client substrates from the chaperone. By accelerating the Hsp70/Hsc70 nucleotide cycle, BAG-1 modulates chaperone-assisted protein folding, and at high co-chaperone levels this activity can antagonize productive folding. In the worm, BAG-1 stimulates the ATPase activity of the constitutive Hsc70 (HSP-1) and, together with the J-domain protein DNJ-13 (Hsp40) and the paralogous BAG-domain protein UNC-23 (BAG2 ortholog), belongs to the cofactor network that tunes the Hsc70 chaperone cycle. The BAG domain adopts a helical bundle fold, and the worm protein can oligomerize into dimers and tetramers. It is a soluble, predominantly cytosolic protein.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: BAG-1 is a soluble co-chaperone that acts on cytoplasmic Hsp70/Hsc70; cytoplasmic localization is expected and consistent with the absence of any transmembrane or signal-peptide features. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Nuclear localization in the BAG family is a property of the mammalian BAG-1L isoform, which carries an N-terminal extension bearing a nuclear-localization region. The single 210-residue worm protein lacks that extension and there is no experimental evidence for nuclear BAG-1 in C. elegans, so this phylogenetically propagated term likely over-annotates an isoform-specific mammalian feature. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000269156 · BAG family node (nucleus IBA source) SUPPORTS SOURCE BUT NOT TARGET Nuclear localization derives from the N-terminally extended mammalian BAG-1L isoform; the single short worm protein lacks that nuclear-targeting extension, so the source annotation is sound but does not transfer. |
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: Cytosol is a more precise and appropriate location for a soluble BAG-domain nucleotide-exchange factor acting on cytosolic Hsc70; consistent with the lack of membrane or organelle-targeting features. |
| GO:0016020 membrane | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Worm BAG-1 has no transmembrane segment, no signal peptide, and no lipid-anchor features; it is a soluble cytosolic co-chaperone. The membrane term is a weak phylogenetic propagation (partly from a BAG3-type ortholog) and is not expected to apply to this protein. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN000269156 · BAG family node (membrane IBA source) SUPPORTS SOURCE BUT NOT TARGET Membrane association is not expected for a soluble BAG-domain NEF with no transmembrane segment or lipid anchor; the term is partly propagated from a BAG3-type ortholog and does not transfer to this protein. |
| GO:0000774 adenyl-nucleotide exchange factor activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the defining molecular function of the BAG family: the BAG domain binds the Hsp70/Hsc70 ATPase domain and promotes ADP release / ATP-dependent substrate discharge, i.e. nucleotide exchange. Well supported by family biology and the worm structure, and consistent with the experimental Hsc70 ATPase stimulation measured for worm BAG-1. Supporting Evidence: PMID:15333932 Binding of the BAG domain to the eukaryotic chaperone heat-shock protein (Hsp70) promotes ATP-dependent release of the protein substrate from Hsp70. |
| GO:0050821 protein stabilization | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Plausible downstream consequence of BAG-1's modulation of the Hsp70/Hsc70 cycle, propagated from mammalian orthologs. No worm-specific evidence that BAG-1 stabilizes particular clients, so retained as a non-core biological process. |
| GO:0051087 protein-folding chaperone binding | IBA GO_REF:0000033 | ACCEPT | Summary: Binding to the Hsp70/Hsc70 chaperone via the BAG domain is central to BAG-1 function and is the enabling interaction underlying its nucleotide-exchange activity. Well supported for a BAG-domain protein. Supporting Evidence: PMID:9873016 BAG-1 binds the ATPase domains of Hsp70 and Hsc70, modulating their chaperone activity and functioning as a competitive antagonist of the co-chaperone Hip |
| GO:0051087 protein-folding chaperone binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic annotation from the BAG-domain signatures. Correctly captures Hsp70/Hsc70 chaperone binding and corroborates the phylogenetic and experimental support for this interaction (an independent evidence line for the same accepted chaperone-binding function). |
| GO:0005515 protein binding | IPI PMID:14704431 A map of the interactome network of the metazoan C. elegans. | KEEP AS NON CORE | Summary: High-throughput yeast-two-hybrid interaction with the DUF727 protein Y43F8B.2 (Q9XWX7), a partner of unknown function that is not an Hsp70. The generic "protein binding" term is uninformative about BAG-1's molecular function; retained as a real but non-core experimental interaction. Supporting Evidence: PMID:14704431 more than 4000 interactions were identified from high-throughput, yeast two-hybrid (HT=Y2H) screens |
| GO:0005515 protein binding | IPI PMID:19123269 Empirically controlled mapping of the Caenorhabditis elegans... | KEEP AS NON CORE | Summary: Second high-throughput yeast-two-hybrid report of the same Y43F8B.2 (Q9XWX7) interaction. As above, the generic "protein binding" term is uninformative about function; retained as a non-core experimental interaction rather than a core molecular function. Supporting Evidence: PMID:19123269 We present an expanded C. elegans protein-protein interaction network, or 'interactome' map, derived from testing a matrix of approximately 10,000 x approximately 10,000 proteins using a highly specific, high-throughput yeast two-hybrid system. |
| GO:0001671 ATPase activator activity | IDA PMID:25053410 The balanced regulation of Hsc70 by DNJ-13 and UNC-23 is req... | ACCEPT | Summary: Experimentally supported for worm BAG-1: as a nucleotide-exchange factor it accelerates the Hsc70 ATPase cycle. Papsdorf et al. measured ATPase stimulation and folding regulation for the BAG cofactors and report that BAG-1 acts with higher affinity than the paralog UNC-23. This mechanistically complements the nucleotide-exchange-factor annotation and is a core function. Supporting Evidence: PMID:25053410 C-terminal fragments of UNC-23 instead perform all Hsc70-related functions, like ATPase stimulation and regulation of folding activity, albeit with lower affinity than BAG-1 |
| GO:0006457 protein folding | ISS PMID:9873016 An evolutionarily conserved family of Hsp70/Hsc70 molecular ... | ACCEPT | Summary: As an Hsp70/Hsc70 co-chaperone that regulates the ATP-driven folding cycle, BAG-1 is legitimately involved in the protein-folding process; the involved_in qualifier does not imply it is itself a foldase (indeed excess BAG can inhibit chaperone activity). The specific regulatory mechanism is captured by the nucleotide-exchange-factor and ATPase-activator annotations. Supporting Evidence: PMID:9873016 The human BAG-1, BAG-2, and BAG-3 proteins bind with high affinity (KD congruent with 1-10 nM) to the ATPase domain of Hsc70 and inhibit its chaperone activity in a Hip-repressible manner |
| GO:0051087 protein-folding chaperone binding | ISS PMID:15333932 Structural genomics of Caenorhabditis elegans: structure of ... | ACCEPT | Summary: Sequence/structure-based inference of Hsp70 chaperone binding, supported by the crystal structure of the worm BAG domain and its documented role in Hsp70-dependent substrate release. An independent evidence line for the same accepted chaperone-binding function. Supporting Evidence: PMID:15333932 Binding of the BAG domain to the eukaryotic chaperone heat-shock protein (Hsp70) promotes ATP-dependent release of the protein substrate from Hsp70. |
Loading supporting content…
Download this section (compressed HTML)Q: Does C. elegans BAG-1 form a physiological complex with the constitutive Hsc70 HSP-1, and is this interaction functionally distinct from that of the paralog UNC-23?
Q: Does BAG-1 direct Hsc70 clients toward refolding or toward proteasomal degradation, and does its ubiquitin-like domain mediate a proteasome link as in mammalian BAG-1?
Experiment: Generate and phenotype a bag-1 loss-of-function allele (CRISPR deletion or RNAi), alone and combined with unc-23 loss, assaying viability, motility, thermotolerance, and aggregation/proteostasis reporters to test whether bag-1 has an independent role or is redundant with unc-23.
Experiment: Affinity-purify tagged BAG-1 from worm lysates followed by mass spectrometry to define the native BAG-1 interactome (HSP-1/Hsc70 and clients), and combine with in-vitro Hsc70 ATPase and luciferase refolding assays to quantify BAG-1's effect on the chaperone cycle.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: No in-vivo client or substrate of C. elegans BAG-1 has been identified, and it is untested whether worm BAG-1 biases Hsc70-bound clients toward productive refolding or, via its ubiquitin-like domain, toward proteasomal degradation (the folding-versus-degradation decision that mammalian BAG-1 helps make).
OPEN BIOLOGY BP_DARK
What is known: It is firmly established that BAG-1 is a BAG-domain nucleotide-exchange factor that binds the Hsp70/Hsc70 ATPase domain, stimulates the Hsc70 ATPase, and promotes ATP-dependent substrate release, and that it carries an N-terminal ubiquitin-like domain. What clients pass through this activity in the worm, and what fate BAG-1 directs them to, are unknown.
Significance: The functional output of an Hsp70 NEF depends entirely on which clients it engages and whether it promotes their folding or clearance; without this, the worm-specific biological role of BAG-1 cannot be assigned, and its relationship to proteostasis-network aging phenotypes remains inferential.
What would resolve it: Affinity-proteomics / co-immunoprecipitation of BAG-1 (and BAG-1-vs-UNC-23) from C. elegans to define native clients, combined with degradation/refolding reporter assays and tests of the ubiquitin-like-domain proteasome link, would close the gap.
Provenance (the field's own admissions):
Gap: No loss-of-function phenotype has been reported for the bag-1 gene itself in C. elegans, so it is unknown whether bag-1 is essential, is functionally redundant with its paralog unc-23, or has a distinct tissue-restricted role.
OPEN BIOLOGY BP_DARK
What is known: In this cofactor system the characterized in-vivo phenotype (severe motility dysfunction, muscle-attachment-site localization) belongs to the paralog UNC-23/BAG2, whereas BAG-1 has only been characterized biochemically (higher Hsc70 affinity than UNC-23). A bag-1 mutant or RNAi phenotype is not described in the cached literature.
Significance: Whether bag-1 is dispensable, buffered by unc-23, or independently required in a particular tissue determines its true biological role and its weight in the worm proteostasis network.
What would resolve it: Characterization of a bag-1 deletion/RNAi (alone and in a unc-23 background) for viability, motility, stress resistance, and proteostasis reporters would define the biological role and test redundancy with unc-23.
Provenance (the field's own admissions):
Gap: A direct physical BAG-1-to-HSP-1/Hsc70 complex in C. elegans has not been demonstrated, and the subcellular site of BAG-1 action in the worm is untested; the only experimentally reported worm partner is the DUF727 protein Y43F8B.2, whose functional relationship to BAG-1 is unexplained.
OPEN BIOLOGYCURATION CC_DARK
What is known: Family biology and the worm BAG-domain crystal structure predict binding to the Hsp70/Hsc70 ATPase domain, and biochemistry shows worm BAG-1 stimulates Hsc70; but the demonstrated worm two-hybrid partner is Y43F8B.2, not HSP-1, and no localization data exist for endogenous BAG-1.
Significance: Establishing the physiological BAG-1/Hsc70 complex and where it forms would convert the inferred co-chaperone role into a directly evidenced one and clarify the meaning (if any) of the Y43F8B.2 interaction.
What would resolve it: Co-immunoprecipitation or proximity labeling of endogenous BAG-1 with HSP-1, plus a tagged-BAG-1 expression/localization reporter, would confirm the complex and its site of action.
Provenance (the field's own admissions):
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)