DNAJB8 is a class B J-domain protein (Hsp40/DNAJ family, subfamily B) that functions as a potent holdase chaperone suppressing polyglutamine and other aggregation-prone protein aggregation. Its domain architecture comprises an N-terminal J-domain (JD), a G/F-rich region, an S/T-rich region containing the key 147-AFSSFN-152 steric-zipper oligomerization motif, and a C-terminal domain (CTD). A key regulatory mechanism is an intramolecular JD-CTD interaction that can sequester the JD surface and control Hsp70 recruitment as a reversible switch (DOI:10.1038/s41467-021-21147-x). Unlike canonical DNAJ co-chaperones that assist Hsp70-mediated protein refolding, DNAJB8 acts primarily as a holdase, with anti-aggregation activity largely independent of the J-domain residing in the C-terminal serine-rich (SSF-SST) region and C-terminal tail (PMID:20159555). DNAJB8 self-assembles into oligomeric puncta via the AFSSFN steric-zipper motif (resolved at 0.75 angstrom by MicroED, class 6 steric zipper), but engineered monomeric variants retain substrate binding and anti-aggregation function, demonstrating that oligomerization is mechanistically separable from chaperone activity (DOI:10.1016/j.str.2024.02.015). DNAJB8 uses distinct substrate-binding modes: a serine-rich stretch for fibrillar/amyloid aggregation suppression and a separate CTD TTK-LKS motif for amorphous inclusion suppression via Hsp70-dependent proteasomal degradation (DOI:10.1242/jcs.255596). DNAJB8 is also a cancer-testis antigen preferentially expressed in renal and colorectal cancer stem-like cells, controlling tumor-initiating capacity (DOI:10.1158/0008-5472.CAN-11-3062), and promotes oxaliplatin chemoresistance in colon cancer via TP53 stabilization leading to MDR1/P-gp upregulation (DOI:10.1038/s41419-022-04599-x). DNAJB8 is enriched in testis but expressed in multiple tissues, and localizes to both cytosol and nucleus.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for cytoplasm localization is well supported. DNAJB8 is a cytoplasmic J-domain co-chaperone. The GOA record includes IDA evidence for the more specific cytosol term (GO:0005829) from PMID:21231916, and the cached abstract states that most human HSP70/HSPA and HSP40/DNAJ proteins are localized in the cytosol. The IBA phylogenetic inference is consistent with experimental data for both DNAJB8 and its close paralogue DNAJB6, which is also cytoplasmic. Note that cytoplasm (GO:0005737) is broader than cytosol (GO:0005829) but the IBA annotation at this level is appropriate given the phylogenetic evidence base. Reason: Cytoplasmic localization is consistent with the GOA IDA evidence from PMID:21231916 for the more specific term cytosol (GO:0005829), and with the function of DNAJB8 as a cytoplasmic holdase chaperone that suppresses polyQ aggregation (PMID:20159555). The IBA inference is phylogenetically sound given conservation across DNAJB6/DNAJB8 orthologues. Supporting Evidence: PMID:21231916 Humans contain many HSP (heat-shock protein) 70/HSPA- and HSP40/DNAJ-encoding genes and most of the corresponding proteins are localized in the cytosol. PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. |
| GO:0006457 protein folding | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for protein folding (GO:0006457) is broadly acceptable as DNAJB8 is a J-domain co-chaperone involved in proteostasis. However, DNAJB8 acts primarily as a holdase that suppresses aggregation of polyQ-expanded proteins rather than promoting productive protein folding (PMID:21231916, PMID:20159555). Hageman et al. (PMID:21231916) showed that chaperones suppressing polyQ aggregation (like DNAJB8) could not stimulate luciferase refolding, distinguishing holdase from foldase activities. The term protein folding is therefore somewhat imprecise for DNAJB8, but the broader proteostasis role is real. As an IBA annotation it represents a reasonable phylogenetic inference at an appropriate level of specificity. Reason: Although DNAJB8 is functionally a holdase rather than a canonical foldase, it participates in the broader protein folding quality control network as a J-domain co-chaperone. The IBA phylogenetic inference is sound for the DNAJB subfamily. The qualifier in GOA is 'involved_in' which is appropriate for a chaperone that participates in protein folding pathways even if its specific role is aggregation suppression rather than productive folding. Experimental support comes from PMID:23612975 (IDA for the same term) and PMID:20159555. Supporting Evidence: PMID:21231916 Overexpressed chaperones that suppressed polyQ aggregation were found not to be able to stimulate luciferase refolding. Inversely, chaperones that supported luciferase refolding were poor suppressors of polyQ aggregation. PMID:23612975 we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for nuclear localization is supported by HDA evidence from PMID:21630459, a sperm nuclear proteomics study, and by the GOA IDA record from PMID:21231916. The cached PMID:21231916 abstract available here does not include the localization figure, so the explicit text support below is the independent sperm nuclear proteomics evidence. The IBA phylogenetic inference is consistent with experimental data for DNAJB8 itself and with the known nuclear localization of its close paralogue DNAJB6a. Reason: Nuclear localization is supported by sperm nuclear proteomics (PMID:21630459) and is consistent with the GOA IDA localization record from PMID:21231916. The IBA phylogenetic inference is consistent with these observations and with the dual cytosol/nucleus localization pattern of the DNAJB6/B8 subfamily. Supporting Evidence: PMID:21630459 403 different proteins have been identified from the isolated sperm nuclei. |
| GO:0051087 protein-folding chaperone binding | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for protein-folding chaperone binding (GO:0051087) is well supported. DNAJB8 is a J-domain co-chaperone that physically interacts with Hsp70 family members (HSPA1A/P0DMV8, HSPA1B/P0DMV9, HSPA6/P17066) as shown by IPI evidence from PMID:21231916. The J-domain of DNAJB8 stimulates Hsp70 ATPase activity, which is the canonical mechanism of J-domain protein function. The IBA phylogenetic inference is consistent with this being a core function of J-domain proteins across the DNAJB subfamily. Reason: Chaperone binding is a core molecular function of J-domain proteins. DNAJB8 contains a canonical J-domain (residues 3-69, UniProt) that mediates interaction with Hsp70 chaperones. IPI evidence from PMID:21231916 directly demonstrates physical interaction with multiple Hsp70 family members. The IBA phylogenetic inference is phylogenetically sound and at the right level of specificity. Supporting Evidence: PMID:20159555 The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. |
| GO:0044183 protein folding chaperone | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for protein folding chaperone (GO:0044183) is well supported and represents a core molecular function of DNAJB8. This term describes the activity of binding to a protein to assist the protein folding process, which is the canonical function of J-domain co-chaperones. DNAJB8 specifically acts as a holdase chaperone that binds polyQ-expanded proteins and other aggregation-prone substrates to prevent their aggregation (PMID:20159555, PMID:23612975). IDA evidence for this same term exists from PMID:23612975. The IBA phylogenetic inference is sound and at the right level of specificity for the DNAJB subfamily. Reason: GO:0044183 (protein folding chaperone) is the best available GO molecular function term for DNAJB8's holdase/anti-aggregation chaperone activity. DNAJB8 binds aggregation-prone substrates to prevent their misfolding and aggregation, which falls within the scope of this term. IDA evidence from PMID:23612975 independently supports this annotation. The IBA phylogenetic inference is consistent with the conserved chaperone function across the DNAJB6/B8 clade. Supporting Evidence: PMID:23612975 we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract. PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. file:human/DNAJB8/DNAJB8-deep-research-falcon.md Multiple studies place DNAJB8 among potent anti-aggregation DNAJBs, particularly against polyglutamine aggregation, with holdase activity that can be partly independent of canonical J-domain/Hsp70 co-chaperone action. |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MODIFY | Summary: GO:0051082 (unfolded protein binding) is now formally obsolete (go-ontology#30962). While DNAJB8 does bind unfolded/aggregation-prone substrates, this term does not accurately capture its functional activity. DNAJB8 is a holdase chaperone that binds polyQ-expanded proteins and other aggregation-prone substrates to prevent their aggregation, largely independently of Hsp70 (PMID:20159555). The best available replacement term is GO:0044183 (protein folding chaperone), which is already annotated via IBA and IDA evidence for this gene. Note that DNAJB8 is functionally a holdase rather than a foldase, but no dedicated holdase term currently exists in GO. Reason: GO:0051082 is now formally obsolete. DNAJB8 does bind unfolded/misfolded proteins, but this binding is in the context of its holdase chaperone activity -- it prevents aggregation of polyQ substrates rather than merely binding unfolded proteins passively. The recommended interim replacement is GO:0044183 (protein folding chaperone), which better captures the functional activity, though it is imprecise for a holdase. DNAJB8 already has GO:0044183 annotations (IBA and IDA), so this modification aligns the annotation set. Hageman et al. (PMID:20159555) showed that the anti-aggregation activity of DNAJB8 is largely independent of the J-domain and resides in the C-terminal serine-rich region, demonstrating direct substrate binding in a holdase capacity. Gillis et al. (PMID:23612975) confirmed that DNAJB6 and DNAJB8 inhibit polyQ peptide aggregation directly by binding the polyQ tract. Proposed replacements: protein folding chaperone Supporting Evidence: PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. The SSF-SST region is involved in substrate binding, formation of polydisperse oligomeric complexes, and interaction with histone deacetylases (HDAC4, HDAC6, SIRT2). PMID:23612975 we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract. |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation for cytoplasm localization from ARBA machine learning models. This is consistent with the IBA annotation for cytoplasm and the IDA annotation for the more specific term cytosol (GO:0005829) from PMID:21231916. The IEA annotation is broader than the experimentally determined cytosol localization but is not incorrect. Reason: Cytoplasmic localization is confirmed by IDA evidence for cytosol (PMID:21231916) and by IBA phylogenetic inference. The IEA annotation is redundant with but consistent with higher-quality evidence. Acceptable as a broader computational annotation that aligns with experimental data. Supporting Evidence: PMID:21231916 Humans contain many HSP (heat-shock protein) 70/HSPA- and HSP40/DNAJ-encoding genes and most of the corresponding proteins are localized in the cytosol. |
| GO:0030544 Hsp70 protein binding | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation for Hsp70 protein binding (GO:0030544) inferred from the InterPro domain IPR043183 (DNJB2/6-like). DNAJB8 contains a canonical J-domain (residues 3-69) that mediates physical interaction with Hsp70 family chaperones. Hageman et al. (PMID:21231916) demonstrated by IPI that DNAJB8 physically interacts with HSPA1A (P0DMV8), HSPA1B (P0DMV9), and HSPA6 (P17066), all Hsp70 family members. While the IPI annotations are curated under the broader GO:0051087 (protein-folding chaperone binding), Hsp70 binding is a well-established core function of J-domain proteins. This IEA annotation is more specific than GO:0051087 regarding the binding partner identity and is well supported. Reason: Hsp70 binding is the canonical function of J-domain proteins. DNAJB8 has a well-characterized J-domain (UniProt domain annotation, residues 3-69) and IPI evidence from PMID:21231916 confirms physical interaction with three Hsp70 family members. The InterPro-based IEA inference from IPR043183 is correct and specific. Hageman et al. (PMID:20159555) further showed that while the antiaggregation activity of DNAJB8 is largely J-domain independent, the J-domain still mediates Hsp70 interaction. Supporting Evidence: PMID:20159555 The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. PMID:21231916 Overexpressed chaperones that suppressed polyQ aggregation were found not to be able to stimulate luciferase refolding. |
| GO:0051082 unfolded protein binding | IEA GO_REF:0000002 | MODIFY | Summary: GO:0051082 (unfolded protein binding) is now formally obsolete (go-ontology#30962). This IEA annotation was inferred from the InterPro domain IPR043183 (DNJB2/6-like), which is shared across DNAJB2, DNAJB6, and DNAJB8. While these proteins do interact with unfolded/misfolded substrates, the term fails to capture the functional nature of the interaction. DNAJB8 specifically acts as a holdase chaperone that suppresses aggregation of polyQ-expanded proteins by directly binding the polyQ tract (PMID:20159555, PMID:23612975). The best interim replacement is GO:0044183 (protein folding chaperone), noting that DNAJB8 is a holdase rather than a foldase and no dedicated holdase GO term currently exists. Reason: GO:0051082 is now formally obsolete. The InterPro-based IEA mapping from IPR043183 correctly identified that DNAJB8 interacts with unfolded substrates, but GO:0044183 (protein folding chaperone) is the recommended replacement that better captures the functional chaperone activity. DNAJB8 already has GO:0044183 annotations from both IBA and IDA evidence. Caveat: DNAJB8 functions as a holdase rather than a foldase -- it suppresses aggregation rather than promoting productive folding -- but GO currently lacks a dedicated holdase term. Proposed replacements: protein folding chaperone Supporting Evidence: PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. PMID:23612975 we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract. |
| GO:0051087 protein-folding chaperone binding | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation for protein-folding chaperone binding from ARBA machine learning models. This is consistent with the IBA annotation and the IPI evidence from PMID:21231916 showing DNAJB8 physically interacts with Hsp70 chaperones (HSPA1A, HSPA1B, HSPA6). The computational inference is correct and redundant with higher-quality evidence. Reason: Protein-folding chaperone binding is well established for DNAJB8 through IPI evidence (PMID:21231916) and IBA phylogenetic inference. The IEA annotation is redundant with but consistent with these higher-quality annotations. Supporting Evidence: PMID:20159555 The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. |
| GO:0090084 negative regulation of inclusion body assembly | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation for negative regulation of inclusion body assembly from ARBA machine learning models. DNAJB8 is a potent suppressor of polyQ protein aggregation and inclusion body formation (PMID:20159555, PMID:23612975). Hageman et al. (PMID:20159555) demonstrated that DNAJB8 suppresses aggregation and toxicity of disease-associated polyglutamine proteins. Gillis et al. (PMID:23612975) showed that DNAJB8 prevents intracellular aggregation of polyQ peptides. This is a core function of DNAJB8 and the IEA annotation is well supported by IDA evidence from PMID:21231916 for the same term. Reason: Negative regulation of inclusion body assembly is a core function of DNAJB8, supported by IDA evidence from PMID:21231916 and extensive experimental literature (PMID:20159555, PMID:23612975). The IEA annotation is redundant with but consistent with the higher-quality IDA annotation. Supporting Evidence: PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. PMID:23612975 we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. |
| GO:0051087 protein-folding chaperone binding | IPI PMID:21231916 The diverse members of the mammalian HSP70 machine show dist... | ACCEPT | Summary: IPI annotation for protein-folding chaperone binding based on physical interaction evidence from Hageman et al. (PMID:21231916). The GOA records three separate IPI entries for this annotation, each with a different Hsp70 interacting partner: HSPA1A (UniProtKB:P0DMV8), HSPA1B (UniProtKB:P0DMV9), and HSPA6 (UniProtKB:P17066). This study systematically tested the chaperone activities of HSP70 and DNAJ family members and demonstrated physical interactions between DNAJB8 and these Hsp70 proteins. DNAJB8 contains a canonical J-domain (residues 3-69) that mediates Hsp70 interaction. This is a core molecular function of J-domain co-chaperones. Reason: Physical interaction between DNAJB8 and Hsp70 chaperones is directly demonstrated by co-immunoprecipitation or equivalent assays in Hageman et al. (PMID:21231916). The J-domain of DNAJB8 is the canonical Hsp70 interaction module. This annotation is well supported and represents a core function. The term protein-folding chaperone binding (GO:0051087) is appropriate as it captures the interaction with Hsp70 chaperones, which are protein-folding chaperones. Supporting Evidence: PMID:20159555 The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. |
| GO:0005634 nucleus | IDA PMID:21231916 The diverse members of the mammalian HSP70 machine show dist... | ACCEPT | Summary: IDA annotation for nuclear localization from Hageman et al. (PMID:21231916). The cached PMID:21231916 abstract available here does not include the localization figure, so the explicit text support below uses the independent HDA annotation from PMID:21630459 (sperm nuclear proteomics). The close paralogue DNAJB6 also shows nuclear localization, with the DNAJB6a isoform being predominantly nuclear. DNAJB8 localizes to both cytosol and nucleus, consistent with the description in UniProt. Reason: Nuclear localization is represented in GOA by the IDA record from PMID:21231916 and independently supported by sperm nuclear proteomics (PMID:21630459). The dual cytosol/nucleus localization is consistent with the known biology of the DNAJB6/B8 subfamily and with a potential role in nuclear proteostasis. Supporting Evidence: PMID:21630459 403 different proteins have been identified from the isolated sperm nuclei. |
| GO:0005829 cytosol | IDA PMID:21231916 The diverse members of the mammalian HSP70 machine show dist... | ACCEPT | Summary: IDA annotation for cytosol localization from Hageman et al. (PMID:21231916). The cached abstract states that most human HSP70/HSPA and HSP40/DNAJ proteins are cytosolic. This is consistent with its function as a cytoplasmic holdase chaperone that suppresses polyQ protein aggregation (PMID:20159555, PMID:23612975). The cytosol is the primary site where DNAJB8 encounters and binds aggregation-prone substrates. This is also consistent with the IBA annotation for the broader term cytoplasm (GO:0005737). Reason: Cytosolic localization is recorded by GOA from Hageman et al. (PMID:21231916) and is consistent with the primary function of DNAJB8 as a cytoplasmic holdase chaperone. The cytosol is where polyQ-expanded proteins aggregate and where DNAJB8 exerts its anti-aggregation activity. Supporting Evidence: PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. PMID:21231916 Humans contain many HSP (heat-shock protein) 70/HSPA- and HSP40/DNAJ-encoding genes and most of the corresponding proteins are localized in the cytosol. |
| GO:0051082 unfolded protein binding | IDA PMID:21231916 The diverse members of the mammalian HSP70 machine show dist... | MODIFY | Summary: GO:0051082 (unfolded protein binding) is now formally obsolete (go-ontology#30962). This IDA annotation cites PMID:21231916 (Hageman et al. 2011), which demonstrated that different HSP70/DNAJ family members have distinct chaperone-like activities. Chaperones that suppressed polyQ aggregation (holdase activity) were found not to stimulate luciferase refolding (foldase activity), and vice versa (PMID:21231916). DNAJB8 falls in the holdase category. The earlier study by the same group (PMID:20159555, Hageman et al. 2010) specifically showed DNAJB8 is a superior suppressor of polyQ aggregation with activity largely independent of its J-domain. Gillis et al. (PMID:23612975) further confirmed that DNAJB8 suppresses polyQ protein aggregation by directly binding the polyQ tract. The best interim replacement is GO:0044183 (protein folding chaperone), with the caveat that DNAJB8 is a holdase, not a foldase, and no dedicated holdase GO term currently exists. Reason: GO:0051082 is now formally obsolete. The experimental evidence from Hageman et al. (PMID:21231916) demonstrated that DNAJB8 has chaperone-like holdase activity rather than mere unfolded protein binding. The study showed that chaperones suppressing polyQ aggregation could not stimulate luciferase refolding, establishing a functional distinction between holdase and foldase activities. GO:0044183 (protein folding chaperone) is the best available replacement term, though it is imprecise for a holdase. DNAJB8 already has GO:0044183 annotations from IBA and IDA (PMID:23612975) evidence. The IDA evidence supporting the original GO:0051082 annotation is consistent with chaperone activity and should be retained under the replacement term. Proposed replacements: protein folding chaperone Supporting Evidence: PMID:21231916 Overexpressed chaperones that suppressed polyQ aggregation were found not to be able to stimulate luciferase refolding. Inversely, chaperones that supported luciferase refolding were poor suppressors of polyQ aggregation. This was not related to client specificity itself, as the polyQ aggregation inhibitors often also suppressed heat-induced aggregation of luciferase. PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. Rather, a C-terminal serine-rich (SSF-SST) region and the C-terminal tail are essential. PMID:23612975 we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. In contrast, HSPA/Hsp70 and DNAJB1, also members of the DNAJ chaperone family, did not prevent peptide-initiated aggregation. |
| GO:0090084 negative regulation of inclusion body assembly | IDA PMID:21231916 The diverse members of the mammalian HSP70 machine show dist... | ACCEPT | Summary: IDA annotation for negative regulation of inclusion body assembly from Hageman et al. (PMID:21231916). This study demonstrated that DNAJB8 suppresses polyQ aggregation, which directly manifests as prevention of inclusion body formation. The earlier study by the same group (PMID:20159555) showed DNAJB8 is a superior suppressor of aggregation and toxicity of disease-associated polyglutamine proteins, with the antiaggregation activity residing in the C-terminal serine-rich region. Gillis et al. (PMID:23612975) independently confirmed that DNAJB8 prevents intracellular aggregation of polyQ peptides by directly binding the polyQ tract. This is a core function of DNAJB8 and the annotation is well supported by direct experimental evidence. Reason: Negative regulation of inclusion body assembly is directly demonstrated by multiple studies. Hageman et al. (PMID:21231916) showed DNAJB8 suppresses polyQ aggregation in cellular assays. The same group (PMID:20159555) established the mechanism involving C-terminal substrate binding and HDAC interactions. Gillis et al. (PMID:23612975) confirmed DNAJB8 prevents intracellular polyQ peptide aggregation. This is arguably the most distinctive and potent function of DNAJB8. Supporting Evidence: PMID:21231916 Overexpressed chaperones that suppressed polyQ aggregation were found not to be able to stimulate luciferase refolding. Inversely, chaperones that supported luciferase refolding were poor suppressors of polyQ aggregation. PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. PMID:23612975 we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. In contrast, HSPA/Hsp70 and DNAJB1, also members of the DNAJ chaperone family, did not prevent peptide-initiated aggregation. |
| GO:0005634 nucleus | HDA PMID:21630459 Proteomic characterization of the human sperm nucleus. | ACCEPT | Summary: HDA annotation for nuclear localization from de Mateo et al. (PMID:21630459), a proteomic characterization of the human sperm nucleus. DNAJB8 was identified among 403 proteins in isolated human sperm nuclei by LC-MS/MS. DNAJB8 is enriched in testis (HPA: tissue enriched in testis; Bgee: expressed in sperm), so its detection in the sperm nuclear proteome is biologically plausible. However, this is a high-throughput proteomics study and the nuclear detection could reflect a genuine nuclear pool or could be a consequence of the protein being highly abundant in testis. The nuclear localization is consistent with the GOA IDA record from PMID:21231916. DNAJB8 is known to localize to both cytosol and nucleus. Reason: The HDA evidence from sperm nuclear proteomics is consistent with the IDA localization record from PMID:21231916 and with the IBA phylogenetic inference. While high-throughput proteomics can have contaminants, the fact that DNAJB8 is testis-enriched and independently represented in GOA as nuclear by direct assay supports the biological relevance of this detection. The annotation is well corroborated. Supporting Evidence: PMID:21630459 With this approach, 403 different proteins have been identified from the isolated sperm nuclei. The most abundant family of proteins identified are the histones, for which several novel members had not been reported previously as present in the spermatogenic cell line or in the human mature spermatozoa. |
| GO:0006457 protein folding | IDA PMID:23612975 The DNAJB6 and DNAJB8 protein chaperones prevent intracellul... | ACCEPT | Summary: IDA annotation for protein folding (GO:0006457) from Gillis et al. (PMID:23612975). The GOA qualifier is 'acts_upstream_of_or_within', which indicates involvement in the broader process. Gillis et al. demonstrated that DNAJB8 prevents intracellular aggregation of polyQ peptides by directly binding the polyQ tract, placing it in the protein folding quality control pathway. DNAJB8 functions as a holdase rather than a foldase -- Hageman et al. (PMID:21231916) showed that chaperones suppressing polyQ aggregation could not stimulate luciferase refolding. The term protein folding is therefore somewhat imprecise for DNAJB8, but the broader proteostasis role is real and the qualifier 'acts_upstream_of_or_within' appropriately captures the indirect relationship. Reason: DNAJB8 is a J-domain co-chaperone that participates in protein folding quality control, specifically by preventing aggregation of misfolded proteins. The qualifier 'acts_upstream_of_or_within' is appropriate as DNAJB8 acts within the protein folding process by suppressing aggregation rather than directly catalyzing productive folding. The IDA evidence from Gillis et al. (PMID:23612975) directly demonstrates this anti-aggregation activity in cellular assays with polyQ peptides. Supporting Evidence: PMID:23612975 we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. In contrast, HSPA/Hsp70 and DNAJB1, also members of the DNAJ chaperone family, did not prevent peptide-initiated aggregation. PMID:21231916 Overexpressed chaperones that suppressed polyQ aggregation were found not to be able to stimulate luciferase refolding. Inversely, chaperones that supported luciferase refolding were poor suppressors of polyQ aggregation. |
| GO:0044183 protein folding chaperone | IDA PMID:23612975 The DNAJB6 and DNAJB8 protein chaperones prevent intracellul... | ACCEPT | Summary: IDA annotation for protein folding chaperone (GO:0044183) from Gillis et al. (PMID:23612975). This study demonstrated that DNAJB8 prevents intracellular aggregation of polyQ peptides by directly binding the polyQ tract, establishing DNAJB8 as a chaperone that binds proteins to assist in the protein folding process (by preventing misfolding/aggregation). Hageman et al. (PMID:20159555) previously established that DNAJB8 is a superior suppressor of polyQ protein aggregation with activity residing in the C-terminal serine-rich region. This is a core molecular function of DNAJB8 and GO:0044183 is the best available GO term for its holdase chaperone activity. Reason: GO:0044183 (protein folding chaperone) is the most appropriate molecular function term for DNAJB8. The IDA evidence from Gillis et al. (PMID:23612975) directly demonstrates that DNAJB8 binds polyQ substrates to prevent their aggregation, which falls within the scope of this term. This is also the recommended replacement term for the obsoleting GO:0051082 (unfolded protein binding). The annotation is independently supported by IBA phylogenetic inference for the same term. Supporting Evidence: PMID:23612975 we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract. PMID:20159555 members of a subclass of the DNAJB family (particularly DNAJB6b and DNAJB8) are superior suppressors of aggregation and toxicity of disease-associated polyglutamine proteins. The antiaggregation activity is largely independent of the N-terminal Hsp70-interacting J-domain. |
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Download this section (compressed HTML)Q: What is the structural basis for DNAJB8 substrate binding by the C-terminal serine-rich (SSF-SST) region, and how does it discriminate polyQ tracts and other amyloidogenic sequences from non-aggregating clients?
Q: How is the intramolecular J-domain/CTD autoinhibition of DNAJB8 regulated in vivo, and which signals (substrate load, post-translational modifications, HDAC interactions) release the J-domain to recruit Hsp70?
Q: Given that DNAJB8 oligomerization (mediated by the AFSSFN steric-zipper motif) is dispensable for substrate binding and anti-aggregation, what is the physiological purpose of the puncta-forming oligomeric state?
Q: How does the testis-restricted/cancer-testis-antigen expression of DNAJB8 relate to its tumor-initiating activity in renal and colorectal cancer stem-like cells, and is its anti-aggregation activity required for tumorigenicity?
Experiment: Cross-linking mass spectrometry and NMR mapping of full-length DNAJB8 in apo, substrate-bound (polyQ peptide), and Hsp70-engaged states to define the structural basis of the holdase mechanism and the JD-CTD autoinhibitory contact.
Experiment: Compare DNAJB8 wild-type, J-domain HPD-mutant, monomerizing AFSSFN-mutant, and SSF-SST/TTK-LKS substrate-binding-deficient variants in cellular polyQ, FlucDM amorphous-aggregate, and proteasomal-degradation assays to quantify the contribution of each module to fibrillar versus amorphous client handling.
Experiment: Knock-out DNAJB8 in renal and colorectal cancer stem-like cell models and rescue with chaperone-active versus chaperone-dead variants to test whether the holdase activity is required for tumor-initiating capacity, side population maintenance, and oxaliplatin resistance via the TP53/MDR1 pathway.
Experiment: Define the in vivo DNAJB8 interactome by proximity labeling (BioID/TurboID) in spermatogenic cells and cancer stem-like cell models, then validate candidate amyloidogenic clients with biochemical aggregation assays to test whether DNAJB8 has tissue-restricted substrate sets distinct from polyQ.
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