HSCB (also known as HSC20 or DNAJC20) is a mitochondrial J-domain co-chaperone essential for iron-sulfur (Fe-S) cluster biogenesis. It functions as the co-chaperone for the Hsp70 chaperone HSPA9 (mitochondrial mortalin), mediating the transfer of nascent Fe-S clusters from the scaffold protein ISCU to recipient apoproteins. The J-domain of HSCB stimulates HSPA9 ATPase activity (up to ~400-fold in bacterial orthologs), driving conformational changes that promote cluster release from ISCU and transfer to client proteins. HSCB recognizes LYR motifs on recipient proteins and accessory factors (e.g., SDHAF1, LYRM7), conferring substrate specificity in Fe-S cluster delivery to respiratory chain complexes I, II, and III. A cytoplasmic form (C-HSC20) has been identified that may integrate mitochondrial Fe-S assembly with cytosolic Fe-S delivery via CIAO1.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0044571 [2Fe-2S] cluster assembly | IBA GO_REF:0000033 | MODIFY | Summary: IBA annotation from phylogenetic inference. HSCB is essential for Fe-S cluster biogenesis but functions specifically in the transfer/delivery phase rather than de novo assembly. The deep research confirms HSCB "acts after NFS1-ISCU assembly of the nascent cluster" and "facilitates transfer to clients" (Maio & Rouault 2022). Reason: HSCB does not directly assemble Fe-S clusters; it mediates their transfer from the ISCU scaffold to recipient proteins. The more accurate term is the parent GO:0016226 'iron-sulfur cluster assembly' which encompasses both assembly and transfer phases, or better yet, a term reflecting its co-chaperone role in cluster transfer. Since the IBA annotation correctly associates HSCB with Fe-S cluster biogenesis, keeping as non-core is appropriate. Proposed replacements: iron-sulfur cluster assembly Supporting Evidence: PMID:20668094 hHSC20 is an integral component of the human ISC biosynthetic machinery file:human/HSCB/HSCB-deep-research-falcon.md model: Edison Scientific Literature |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for mitochondrial localization. Strongly supported by multiple experimental studies showing HSCB localizes to mitochondria where it functions in Fe-S cluster biogenesis. Reason: Multiple lines of evidence confirm mitochondrial localization. PMID:20668094 states "hHSC20...localizes mainly to the mitochondria in HeLa cells." This is the primary site of HSCB function in Fe-S cluster biogenesis. Supporting Evidence: PMID:20668094 hHSC20 is expressed in various human tissues and localizes mainly to the mitochondria in HeLa cells PMID:12938016 encodes a conserved 235-amino-acid protein, including a putative mitochondrial import leader |
| GO:0001671 ATPase activator activity | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation from InterPro mapping. This annotation captures a core molecular function of HSCB - its J-domain stimulates the ATPase activity of HSPA9 (mitochondrial Hsp70), which is essential for Fe-S cluster transfer. Reason: This is a core function of HSCB. The deep research indicates "the J-protein cochaperone (HscB/Jac1/HSCB) dramatically increases stimulation (reported up to ~400-fold in bacterial systems)" and "HSCB binds ISCU and engages HSPA9 via its J-domain HPD motif to stimulate ATP hydrolysis" (Maio & Rouault 2022). This ATPase activation is mechanistically central to Fe-S cluster transfer. Supporting Evidence: PMID:20668094 the putative human homolog of the specialized DnaJ type co-chaperones |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from UniProt subcellular location. Some HSCB is detected extra-mitochondrially as C-HSC20 (cytosolic form). Reason: Experimental evidence supports cytoplasmic localization. PMID:20668094 states "small amounts were also detected extra-mitochondrially" and a cytoplasmic form (C-HSC20) has been characterized in PMID:29309586. The cytosolic form may integrate with CIAO1-mediated cytosolic Fe-S delivery. Supporting Evidence: PMID:20668094 However, small amounts were also detected extra-mitochondrially |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from UniProt subcellular location mapping. Reason: Duplicate of IBA annotation above. Mitochondrial localization is well-supported by multiple experimental studies. Supporting Evidence: PMID:20668094 hHSC20 is expressed in various human tissues and localizes mainly to the mitochondria in HeLa cells |
| GO:0044571 [2Fe-2S] cluster assembly | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: IEA annotation from InterPro. Duplicate of IBA annotation. Same concerns apply about HSCB's role in transfer rather than direct assembly. Reason: While HSCB is involved in Fe-S cluster biogenesis, it specifically functions in the transfer step rather than de novo assembly. The broader parent term GO:0016226 would be more accurate. Keeping as non-core acknowledges the relationship to Fe-S metabolism without implying direct assembly activity. Supporting Evidence: PMID:20668094 hHSC20 is an integral component of the human ISC biosynthetic machinery |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: IEA annotation from UniProt keyword mapping. HSCB contains a unique cysteine-rich N-terminal domain that may bind metal ions. Reason: HSCB has a cysteine-rich N-terminal domain (the "HscB_4_cys" domain, IPR040682) that distinguishes it from fungal and bacterial homologs. PMID:20668094 notes this domain "was found to be important for the integrity and function of the human co-chaperone." However, direct metal ion binding is not the primary molecular function - Fe-S cluster delivery via its co-chaperone activity is. Supporting Evidence: PMID:20668094 A cysteine-rich N-terminal domain, which clearly distinguishes hHSC20 from the specialized DnaJ type III proteins of fungi and most bacteria, was found to be important for the integrity and function of the human co-chaperone |
| GO:0051087 protein-folding chaperone binding | IEA GO_REF:0000002 | MODIFY | Summary: IEA annotation from InterPro. HSCB binds to the Hsp70 chaperone HSPA9 as its cognate co-chaperone partner. Reason: While HSCB does bind to a chaperone (HSPA9), the more specific term GO:0030544 'Hsp70 protein binding' would be more accurate since HSPA9 is specifically an Hsp70 family member. The interaction is well-documented: PMID:24606901 shows "HSC20 physically interacts...and works together with its cognate chaperone, HSPA9." Proposed replacements: Hsp70 protein binding Supporting Evidence: PMID:24606901 HSC20 physically interacts with SDHB, and works together with its cognate chaperone, HSPA9, to enhance transfer of Fe-S clusters from the main scaffold ISCU directly to SDHB |
| GO:0051259 protein complex oligomerization | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: IEA annotation from InterPro domain match (HscB_oligo_C, IPR009073). HSCB has been shown to dimerize. Reason: HSCB dimerization has been noted in the literature. PMID:24606901 states "The previously observed propensity of HSC20 to dimerize may allow two holo-ISCU molecules at neighboring binding sites to reorganize their adjacent [2Fe-2S] centers." However, this is not a primary function but rather a structural property that facilitates its main Fe-S cluster delivery role. Supporting Evidence: PMID:24606901 The previously observed propensity of HSC20 to dimerize may allow two holo-ISCU molecules at neighboring binding sites to reorganize their adjacent [2Fe-2S] centers |
| GO:0005515 protein binding | IPI PMID:24606901 Cochaperone binding to LYR motifs confers specificity of iro... | REMOVE | Summary: IPI annotation from IntAct based on protein interactions detected in PMID:24606901. This paper identified LYR motif-containing proteins as HSC20 interacting partners. Reason: GO:0005515 'protein binding' is uninformative. The actual binding activities of HSCB are more specific: it binds ISCU (scaffold), HSPA9 (Hsp70 chaperone), and LYR motif-containing recipient proteins. These specific interactions should be annotated with more precise terms like GO:0140767 'enzyme-substrate adaptor activity' or GO:0030544 'Hsp70 protein binding'. Supporting Evidence: PMID:24606901 direct binding of specific targets to the cochaperone HSC20 is mediated by affinity of its C-terminus for proteins that bear the LYR motif |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: IPI annotation from high-throughput interactome mapping study. Reason: GO:0005515 'protein binding' is too vague and uninformative. This high-throughput study provides interaction data but the term does not capture HSCB's specific functional binding activities in Fe-S cluster delivery. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network. |
| GO:0005515 protein binding | IPI PMID:26749241 Disease-Causing SDHAF1 Mutations Impair Transfer of Fe-S Clu... | REMOVE | Summary: IPI annotation from study on SDHAF1 mutations affecting Fe-S cluster transfer. Shows HSCB interaction with Fe-S delivery machinery components. Reason: GO:0005515 is uninformative. The interaction context (SDHAF1 and Fe-S cluster delivery to SDHB) supports more specific terms like enzyme-substrate adaptor activity. The underlying biology is valuable but needs proper term assignment. Supporting Evidence: PMID:26749241 2015 Dec 31. Disease-Causing SDHAF1 Mutations Impair Transfer of Fe-S Clusters to SDHB. |
| GO:0005515 protein binding | IPI PMID:28380382 A Single Adaptable Cochaperone-Scaffold Complex Delivers Nas... | REMOVE | Summary: IPI annotation from study showing HSCB delivers Fe-S clusters to respiratory chain complexes I-III via a single cochaperone-scaffold complex. Reason: GO:0005515 is too general. This study provides excellent support for GO:0140767 'enzyme-substrate adaptor activity' as HSCB acts as an adaptor bringing ISCU (with Fe-S cluster) to recipient respiratory chain subunits. Supporting Evidence: PMID:28380382 A Single Adaptable Cochaperone-Scaffold Complex Delivers Nascent Iron-Sulfur Clusters to Mammalian Respiratory Chain Complexes I-III. |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | REMOVE | Summary: IPI annotation from genetic variant study on protein interactions. Reason: GO:0005515 is uninformative and should not be used when more specific terms are available. This high-throughput study does not add functional specificity. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations. |
| GO:0042802 identical protein binding | IPI PMID:24606901 Cochaperone binding to LYR motifs confers specificity of iro... | KEEP AS NON CORE | Summary: IPI annotation indicating HSCB homodimerization detected in PMID:24606901. Reason: HSCB dimerization is documented and may facilitate its function by allowing coordination of two ISCU-bound clusters for [4Fe-4S] cluster formation. PMID:24606901 notes the "propensity of HSC20 to dimerize" may enable cluster reorganization. While homodimerization is experimentally supported, it is a structural property supporting the main function rather than a core function. Supporting Evidence: PMID:24606901 The previously observed propensity of HSC20 to dimerize may allow two holo-ISCU molecules at neighboring binding sites to reorganize their adjacent [2Fe-2S] centers, enabling them to coalesce into the [4Fe-4S] and [3Fe-4S] clusters of mature SDHB |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: IDA annotation from HPA immunofluorescence curation. Reason: Mitochondrial localization is a core characteristic of HSCB. This is strongly supported by multiple publications and is essential for its function in mitochondrial Fe-S cluster biogenesis. Supporting Evidence: PMID:20668094 hHSC20 is expressed in various human tissues and localizes mainly to the mitochondria in HeLa cells |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: IDA annotation from HPA immunofluorescence curation for cytosolic localization. Reason: A cytosolic form of HSCB (C-HSC20) has been characterized. PMID:29309586 describes "Cytosolic HSC20 integrates de novo iron-sulfur cluster biogenesis with the CIAO1-mediated transfer to recipients." This represents a functionally relevant localization. Supporting Evidence: PMID:20668094 However, small amounts were also detected extra-mitochondrially |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: HTP annotation from high-confidence mitochondrial proteome study. Reason: Further confirmation of mitochondrial localization from quantitative proteomics. HSCB was identified as part of the high-confidence human mitochondrial proteome. Supporting Evidence: PMID:34800366 Epub 2021 Nov 19. Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context. |
| GO:0005515 protein binding | IPI PMID:29309586 Cytosolic HSC20 integrates de novo iron-sulfur cluster bioge... | REMOVE | Summary: IPI annotation from study on cytosolic HSC20 integrating with CIAO1-mediated Fe-S cluster transfer. Reason: GO:0005515 is uninformative. The actual finding (interaction with CIAO1 and integration of mitochondrial and cytosolic Fe-S biogenesis) would be better represented by more specific terms. Supporting Evidence: PMID:29309586 Cytosolic HSC20 integrates de novo iron-sulfur cluster biogenesis with the CIAO1-mediated transfer to recipients. |
| GO:0005515 protein binding | IPI PMID:23940031 Human mitochondrial chaperone (mtHSP70) and cysteine desulfu... | REMOVE | Summary: IPI annotation from study showing HSC20 binds preferentially to structured ISCU while HSPA9/NFS1 prefer disordered ISCU. Reason: GO:0005515 is too vague. This study provides mechanistic insight into how HSC20 preferentially recognizes the structured (holo) form of ISCU scaffold, which supports its role in Fe-S cluster transfer. Should be replaced with enzyme-substrate adaptor activity annotation. Supporting Evidence: PMID:23940031 2013 Aug 12. Human mitochondrial chaperone (mtHSP70) and cysteine desulfurase (NFS1) bind preferentially to the disordered conformation, whereas co-chaperone (HSC20) binds to the structured conformation of the iron-sulfur cluster scaffold protein (ISCU). |
| GO:0060215 primitive hemopoiesis | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS annotation transferred from mouse ortholog. HSCB depletion affects hematopoiesis in mouse models. Reason: This is a downstream phenotypic effect of HSCB's core function in Fe-S cluster biogenesis. Primitive hematopoiesis requires functional mitochondrial Fe-S proteins (especially for heme synthesis and respiratory function). The annotation is pleiotropic rather than core. |
| GO:0060319 primitive erythrocyte differentiation | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS annotation transferred from mouse ortholog. Related to effects on erythropoiesis. Reason: This is a downstream effect of HSCB's role in Fe-S cluster biogenesis. Erythrocyte differentiation requires heme synthesis, which depends on Fe-S cluster-containing enzymes. The annotation reflects pleiotropic effects rather than core molecular function. |
| GO:0005515 protein binding | IPI PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | REMOVE | Summary: IPI annotation from foundational study characterizing human HSC20 interactions with ISCU and HSPA9. Reason: GO:0005515 is uninformative. This paper demonstrates specific interactions with ISCU and HSPA9 that should be represented by more specific terms (e.g., Hsp70 protein binding, enzyme-substrate adaptor activity). Supporting Evidence: PMID:20668094 interacts with its proposed human partners, hISCU and hHSPA9 |
| GO:0005737 cytoplasm | IDA PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | ACCEPT | Summary: IDA annotation for cytoplasmic localization based on experimental observation in PMID:20668094. Reason: Direct experimental evidence for extra-mitochondrial HSCB. The paper explicitly states that "small amounts were also detected extra-mitochondrially." This supports the existence of the cytoplasmic form C-HSC20. Supporting Evidence: PMID:20668094 However, small amounts were also detected extra-mitochondrially |
| GO:0005739 mitochondrion | IDA PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | ACCEPT | Summary: IDA annotation for mitochondrial localization from PMID:20668094 experimental characterization of human HSC20. Reason: Primary experimental evidence for mitochondrial localization of HSCB. This is a core annotation as the mitochondrion is the primary site of HSCB function. Supporting Evidence: PMID:20668094 hHSC20 is expressed in various human tissues and localizes mainly to the mitochondria in HeLa cells |
| GO:0016226 iron-sulfur cluster assembly | IMP PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | ACCEPT | Summary: IMP annotation based on knockdown studies showing HSCB is required for Fe-S protein activities in both mitochondria and cytosol. Reason: Core biological process annotation. PMID:20668094 demonstrates that "RNA interference-mediated depletion of hHSC20 specifically reduced the activities of both mitochondrial and cytosolic ISC-containing enzymes." HSCB is essential for Fe-S cluster biogenesis. Supporting Evidence: PMID:20668094 RNA interference-mediated depletion of hHSC20 specifically reduced the activities of both mitochondrial and cytosolic ISC-containing enzymes |
| GO:0005739 mitochondrion | TAS PMID:12938016 Identification of a novel candidate gene in the iron-sulfur ... | ACCEPT | Summary: TAS annotation for mitochondrial localization based on the initial identification of human HSCB gene. Reason: The original paper identifying human HSCB described a mitochondrial targeting sequence: "encodes a conserved 235-amino-acid protein, including a putative mitochondrial import leader." Supporting Evidence: PMID:12938016 encodes a conserved 235-amino-acid protein, including a putative mitochondrial import leader |
| GO:0003674 molecular_function | ND GO_REF:0000015 | REMOVE | Summary: ND (No biological Data) annotation indicating no MF annotation was available at time of curation. Reason: This placeholder annotation is obsolete. HSCB has well-characterized molecular functions including ATPase activator activity (GO:0001671), enzyme-substrate adaptor activity (GO:0140767), and Hsp70 protein binding (GO:0030544). |
| GO:0140767 enzyme-substrate adaptor activity | IDA PMID:24606901 Cochaperone binding to LYR motifs confers specificity of iro... | NEW | Summary: NEW annotation to capture the core molecular function of HSCB as an adaptor that brings together the Fe-S cluster-loaded ISCU scaffold and recipient apoproteins via recognition of LYR motifs. Reason: HSCB functions as an enzyme-substrate adaptor, recruiting Fe-S cluster recipients to the ISCU-HSPA9 transfer machinery. PMID:24606901 demonstrates that "direct binding of specific targets to the cochaperone HSC20 is mediated by affinity of its C-terminus for proteins that bear the LYR motif" and that HSC20 "utilizes its own LYR motif to position an ISCU-HSC20-HSPA9 complex near to chaperone complexes directly associated with the LYR binding site of SDHB." This adaptor function is central to HSCB's role in Fe-S cluster delivery. Supporting Evidence: PMID:24606901 direct binding of specific targets to the cochaperone HSC20 is mediated by affinity of its C-terminus for proteins that bear the LYR motif, a tripeptide that constitutes a major molecular signature of distinctive Fe-S recipients PMID:24606901 we uncovered molecular details of how SDHB acquires its three Fe-S centers, and how assembly of Complex II is contingent upon successful biogenesis of SDHB |
| GO:0030544 Hsp70 protein binding | IPI PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | NEW | Summary: NEW annotation to capture the specific interaction of HSCB's J-domain with HSPA9 (mitochondrial Hsp70). Reason: HSCB specifically interacts with HSPA9 (mitochondrial Hsp70) as its cognate chaperone partner. This interaction, mediated by HSCB's J-domain, is essential for Fe-S cluster transfer. PMID:20668094 shows HSCB "interacts with its proposed human partners, hISCU and hHSPA9." This is more specific than generic protein binding and captures a core functional interaction. Supporting Evidence: PMID:20668094 interacts with its proposed human partners, hISCU and hHSPA9 PMID:24606901 HSC20 physically interacts with SDHB, and works together with its cognate chaperone, HSPA9, to enhance transfer of Fe-S clusters from the main scaffold ISCU directly to SDHB |
| GO:1990230 iron-sulfur cluster transfer complex | IDA PMID:24606901 Cochaperone binding to LYR motifs confers specificity of iro... | NEW | Summary: NEW annotation for the complex containing HSCB that mediates Fe-S cluster transfer. GO defines this complex as containing "HSPA9, HSCB, GLRX5, ABCB7 and GFER" in humans. Reason: HSCB is a core component of the Fe-S cluster transfer complex. PMID:24606901 demonstrates the ISCU-HSC20-HSPA9 complex is responsible for Fe-S cluster delivery to recipient proteins. The GO definition explicitly lists HSCB as a component of this complex. Supporting Evidence: PMID:24606901 interaction of SDHB with the chaperone-cochaperone complex precedes association with SDHA |
| GO:0005759 mitochondrial matrix | IDA PMID:20668094 Characterization of the human HSC20, an unusual DnaJ type II... | NEW | Summary: NEW annotation to specify the sub-mitochondrial localization of HSCB in the matrix where Fe-S cluster biogenesis occurs. Reason: HSCB functions in the mitochondrial matrix where the ISC machinery resides. The deep research confirms "The operative system is the mitochondrial matrix, where HSCB, HSPA9, ISCU, NFS1/ISD11, and FXN reside and act in ISC biogenesis" (Maio & Rouault 2022). This is a more specific localization than generic mitochondrion. Supporting Evidence: PMID:20668094 hHSC20 is an integral component of the human ISC biosynthetic machinery |
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Download this section (compressed HTML)Q: What is the exact structure and function of the cysteine-rich N-terminal domain unique to human HSCB?
Q: How does cytosolic C-HSC20 coordinate with CIAO1 to integrate mitochondrial and cytosolic Fe-S biogenesis?
Q: Are there disease-causing mutations in HSCB and what phenotypes do they cause?
Experiment: Structural studies (cryo-EM/X-ray) of the human ISCU-HSC20-HSPA9 complex
Experiment: Identification of the complete set of LYR motif-containing proteins that depend on HSCB for Fe-S cluster delivery
Experiment: Characterization of the specific mechanism by which C-HSC20 interfaces with CIAO1
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