SCO1 is an inner mitochondrial membrane copper metallochaperone required for maturation of the CuA site of cytochrome c oxidase subunit II (MT-CO2/COX2). Together with SCO2 and other COX2-module factors, SCO1 participates in the mitochondrial copper relay that enables Complex IV assembly. SCO2 appears to act upstream as a thiol-disulfide oxidoreductase affecting the redox state of SCO1's copper-coordinating cysteines, whereas SCO1 is best treated as the copper-handling/metallochaperone step for COX2 CuA-site maturation. SCO1 is an assembly factor rather than a structural subunit of mature Complex IV. It also has a secondary role in cellular copper homeostasis, including regulation of CTR1 abundance/localization. Pathogenic variants cause mitochondrial Complex IV deficiency, nuclear type 4.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005507 copper ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: SCO1 binds copper through conserved SCO-family metal-binding residues as part of the mitochondrial copper relay for COX2 CuA-site maturation. Reason: Core biochemical property supporting copper chaperone activity. The existing annotation is IEA, but patient-cell and pathway-ordering evidence support the functional importance of the CxxxC copper-coordinating motif in SCO1-dependent COX2 maturation. Supporting Evidence: PMID:19336478 The subsequent maturation of CO II is contingent upon the formation of a complex that includes both SCO proteins, each with a functional CxxxC copper-coordinating motif. file:human/SCO1/SCO1-deep-research-falcon.md SCO1 has a thioredoxin-like fold and a copper-binding CxxxC motif characteristic of SCO-family proteins; available context states that cysteine-based copper binding requires the cysteines to remain reduced. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: SCO1 is a single-pass mitochondrial inner membrane protein with its functional domain exposed for COX2 maturation. Reason: Correct core localization. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md Human SCO1 is described as a mitochondrial inner membrane transmembrane protein with a short N-terminal matrix-facing tail and most of the polypeptide exposed to the IMS, consistent with its role in receiving copper from IMS-localized COX17 and delivering copper to the IMS-facing domain of COX2. |
| GO:0006878 intracellular copper ion homeostasis | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: SCO1 participates in copper handling and has reported effects on cellular copper homeostasis, but its primary evolved role is copper delivery during Complex IV assembly. Reason: Keep as a valid secondary function, not the core annotation. Falcon-summarized 2023-2024 work in hepatocyte-specific Sco1 loss models links SCO1 deficiency to hepatic copper deficiency, elevated circulating copper/ceruloplasmin, and copper-dependent immunosuppression signaling, supporting a real but secondary role in systemic copper homeostasis. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md 2023-2024 work links hepatocyte-specific Sco1 loss to hepatic copper deficiency, elevated circulating copper/ceruloplasmin, AFP-dependent copper-requiring immunosuppression, and bone-marrow lymphoid progenitor defects, expanding SCO1 biology from complex IV metallation to systemic pathophysiology in mitochondrial disease models. |
| GO:0008535 respiratory chain complex IV assembly | IEA GO_REF:0000002 | ACCEPT | Summary: SCO1 is required for respiratory chain Complex IV assembly through COX2 copper-site maturation. Reason: Correct biological-process annotation. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md SCO1 is the COX2-specific copper metallochaperone for cytochrome c oxidase/complex IV biogenesis. Current understanding is that it inserts copper into the CuA site of COX2 rather than catalyzing a classic enzyme reaction; its role is metallation/assembly, with redox competence of cysteines required for function. |
| GO:0016531 copper chaperone activity | IEA GO_REF:0000002 | ACCEPT | Summary: SCO1 functions as a copper chaperone/metallochaperone in the COX2 CuA-site maturation pathway. Reason: Core molecular function. OpenScientist review of the focused core-function hypothesis judged this term supported, while highlighting that the current GOA term is IEA-only and should be upgraded or complemented with experimental support from patient-cell studies and pathway-ordering work. Supporting Evidence: PMID:15229189 Our results demonstrate that the human SCO proteins have non-overlapping, cooperative functions in mitochondrial copper delivery. PMID:19336478 Based on these data we present a model in which each SCO protein fulfills distinct, stage-specific functions during CO II synthesis and CuA site maturation. file:human/SCO1/SCO1-hypotheses/core-function-1-go-0016531/openscientist.md OpenScientist judged copper chaperone activity (GO:0016531) to be supported as a core molecular function of SCO1 and recommended upgrading the IEA-only molecular-function support. file:human/SCO1/SCO1-deep-research-falcon.md Across recent reviews, SCO1 is described as a COX2-specific copper metallochaperone required for insertion of copper into the CuA site of COX2 during complex IV assembly. SCO1 is not described as catalyzing a small-molecule reaction; instead, its functional output is metal delivery/metallation and enabling maturation of complex IV. |
| GO:0033617 mitochondrial respiratory chain complex IV assembly | IEA GO_REF:0000117 | ACCEPT | Summary: SCO1 participates specifically in mitochondrial respiratory chain Complex IV assembly. Reason: Correct and appropriately specific biological-process annotation. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md Recent reviews converge on a mechanistic consensus that SCO1 is a core CuA-site metallochaperone operating at the IMS face of the inner membrane, receiving copper from COX17 and cooperating with SCO2/COA6-driven redox processes to enable copper insertion into COX2 and thus complex IV maturation. |
| GO:0005515 protein binding | IPI PMID:18458339 Mitochondrial copper(I) transfer from Cox17 to Sco1 is coupl... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:29568061 An AP-MS- and BioID-compatible MAC-tag enables comprehensive... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005739 mitochondrion | IEA GO_REF:0000107 | ACCEPT | Summary: SCO1 is a mitochondrial Complex IV assembly factor. Reason: Correct broad localization; mitochondrial inner membrane is more specific. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: SCO1 is a mitochondrial Complex IV assembly factor. Reason: Correct broad localization; mitochondrial inner membrane is more specific. Supporting Evidence: file:human/SCO1/SCO1-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865449 | ACCEPT | Summary: SCO1 is a single-pass mitochondrial inner membrane protein with its functional domain exposed for COX2 maturation. Reason: Correct core localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865579 | ACCEPT | Summary: SCO1 is a single-pass mitochondrial inner membrane protein with its functional domain exposed for COX2 maturation. Reason: Correct core localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865630 | ACCEPT | Summary: SCO1 is a single-pass mitochondrial inner membrane protein with its functional domain exposed for COX2 maturation. Reason: Correct core localization. |
| GO:0005739 mitochondrion | IDA PMID:9878253 Identification and characterization of human cDNAs specific ... | ACCEPT | Summary: SCO1 is a mitochondrial Complex IV assembly factor. Reason: Correct broad localization; mitochondrial inner membrane is more specific. Supporting Evidence: file:human/SCO1/SCO1-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion inner membrane |
| GO:0005515 protein binding | IPI PMID:29381136 COX16 promotes COX2 metallation and assembly during respirat... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0033617 mitochondrial respiratory chain complex IV assembly | IMP PMID:15229189 Human SCO1 and SCO2 have independent, cooperative functions ... | ACCEPT | Summary: SCO1 participates specifically in mitochondrial respiratory chain Complex IV assembly. Reason: Correct and appropriately specific biological-process annotation. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md SCO1 is described as a COX2-specific copper metallochaperone required for insertion of copper into the CuA site of COX2 during complex IV assembly. |
| GO:0005743 mitochondrial inner membrane | IDA PMID:15229189 Human SCO1 and SCO2 have independent, cooperative functions ... | ACCEPT | Summary: SCO1 is a single-pass mitochondrial inner membrane protein with its functional domain exposed for COX2 maturation. Reason: Correct core localization. Supporting Evidence: file:human/SCO1/SCO1-deep-research-falcon.md Human SCO1 is described as a mitochondrial inner membrane transmembrane protein with a short N-terminal matrix-facing tail and most of the polypeptide exposed to the IMS |
| GO:0005515 protein binding | IPI PMID:24403053 Human COX20 cooperates with SCO1 and SCO2 to mature COX2 and... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:28330871 Human mitochondrial cytochrome c oxidase assembly factor COX... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:29154948 The mitochondrial TMEM177 associates with COX20 during COX2 ... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0005515 protein binding | IPI PMID:26160915 COA6 is a mitochondrial complex IV assembly factor critical ... | MARK AS OVER ANNOTATED | Summary: The protein-binding annotations report physical interactions among COX2-module assembly factors or broad interactome hits. These do not define SCO1's function. Reason: Generic protein binding is less informative than copper chaperone activity and Complex IV assembly; mark as over-annotated rather than core. |
| GO:0030016 myofibril | IDA PMID:20864674 Unexpected vascular enrichment of SCO1 over SCO2 in mammalia... | KEEP AS NON CORE | Summary: The myofibril annotation reflects reported localization/enrichment in a tissue context but does not describe SCO1's primary mitochondrial assembly-factor role. Reason: Keep as non-core pending stronger mechanistic interpretation. |
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Download this section (compressed HTML)Q: Should GO add or request an experimental human SCO1 annotation to GO:0016531 based on PMID:15229189 and PMID:19336478, rather than leaving the molecular-function support as IEA-only?
Q: Does the evidence for SCO2 support retaining copper chaperone activity, or should SCO2 be reviewed separately as an upstream thiol-disulfide oxidoreductase acting on SCO1?
Q: Is there direct human biochemical evidence for copper transfer from purified SCO1 to a COX2 CuA-site substrate, or should SCO1 copper chaperone activity remain supported primarily by IMP/pathway-ordering evidence?
Experiment: Reconstitute copper transfer from purified human SCO1 to a COX2 CuA-site substrate or COX2 maturation module, with CxxxC/His-site mutants as negative controls.
Hypothesis: SCO1 directly transfers copper to the COX2 CuA-site maturation intermediate.
Type: in vitro copper-transfer assay
Experiment: Perform rescue experiments in SCO1-deficient human cells with wild-type SCO1 and copper-coordinating motif mutants, measuring COX2 metallation, Complex IV assembly, and COX activity separately.
Hypothesis: SCO1's copper-coordinating motif is required for COX2 metallation rather than only for SCO1 stability or protein-protein interaction.
Type: CRISPR rescue and mitochondrial biogenesis assay
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