UQCRFS1

---
id: P47985
gene_symbol: UQCRFS1
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  UQCRFS1 encodes the Rieske iron-sulfur protein (RISP), one of three catalytic
  subunits of the mitochondrial cytochrome bc1 complex (Complex III, CIII2).
  UQCRFS1 contains a [2Fe-2S] cluster that is essential for electron transfer
  from ubiquinol to cytochrome c1 during the Q-cycle. The protein is
  nuclear-encoded, imported into mitochondria, and inserted as the penultimate
  subunit during CIII assembly via BCS1L-mediated translocation. The catalytic
  globular domain resides in the intermembrane space, attached to the complex
  by a single transmembrane helix. After insertion, the N-terminal
  mitochondrial targeting sequence is cleaved to generate subunit 9, a small
  fragment that remains associated with the complex. Bi-allelic pathogenic
  variants cause mitochondrial Complex III deficiency (MC3DN10) with
  cardiomyopathy, alopecia totalis, and lactic acidosis (PMID:31883641).
  Deep research review (UQCRFS1-deep-research-falcon.md) confirms UQCRFS1 as
  a late-incorporating catalytic subunit essential for Q-cycle chemistry and
  ROS control, with assembly depending on LYRM7 and BCS1L. Recent in situ
  cryo-EM structures have directly visualized the Rieske head domain movement
  during catalytic electron transfer (Zheng et al. 2024, Nature).
existing_annotations:
# ============================================================
# IBA ANNOTATIONS (phylogenetically inferred)
# ============================================================
  - term:
      id: GO:0016491
      label: oxidoreductase activity
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        IBA annotation for oxidoreductase activity. UQCRFS1 is a catalytic subunit
        of Complex III that participates in oxidoreduction during the Q-cycle,
        transferring electrons from ubiquinol to cytochrome c1 via its [2Fe-2S]
        cluster (PMID:28380382). This term is correct but very broad. The more
        specific molecular function of UQCRFS1 as an individual subunit is
        electron transfer activity (GO:0009055), while the whole-complex activity
        is quinol-cytochrome-c reductase activity (GO:0008121). Since
        oxidoreductase activity is a parent of both of these, and the IBA
        inference is phylogenetically sound, this annotation is acceptable but
        could be made more specific. Deep research (UQCRFS1-deep-research-falcon.md)
        confirms the Rieske protein accepts an electron at the Qo site via its
        2Fe-2S cluster and undergoes head-domain movement during the Q-cycle.
      action: MODIFY
      reason: >-
        UQCRFS1 is indeed an oxidoreductase as part of Complex III, but this term
        is too broad for informative annotation. The subunit-specific molecular
        function is electron transfer activity (GO:0009055) -- the Rieske protein
        transfers electrons via its [2Fe-2S] cluster from ubiquinol at the Qo site
        to cytochrome c1. This is well established from structural and biochemical
        studies (PMID:28380382). GO:0016491 is a valid parent term but does not
        convey the specific mechanism.
      proposed_replacement_terms:
        - id: GO:0009055
          label: electron transfer activity
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            The iron-sulfur (Fe-S) cluster of the Rieske protein, UQCRFS1, is essential
            for
            Complex III (CIII) activity, though the mechanism for Fe-S cluster transfer
            has
            not previously been elucidated.
        - reference_id: PMID:28380382
          supporting_text: >-
            Energy transduction by Complex III (CIII) follows the Q cycle mechanism,
            whereby oxidation of a membrane-localized ubiquinol is coupled to proton
            pumping across the inner mitochondrial membrane into the intermembrane
            space.
        - reference_id: file:human/UQCRFS1/UQCRFS1-deep-research-falcon.md
          supporting_text: >-
            Deep research review confirms UQCRFS1 as the Rieske iron-sulfur protein
            participating in ubiquinol oxidation at the Qo site during the Q-cycle,
            with recent in situ cryo-EM resolving head-domain movements.
  - term:
      id: GO:0045275
      label: respiratory chain complex III
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        IBA annotation for respiratory chain complex III. UQCRFS1 is a core
        catalytic subunit of Complex III. The GO definition of respiratory chain
        complex III (GO:0045275) explicitly names the Rieske iron sulfur protein
        as one of the three catalytic subunits. This is firmly established across
        multiple lines of evidence including direct biochemical studies
        (PMID:23168492), disease genetics (PMID:31883641), and cryo-EM structures
        (PDB:5XTE).
      action: ACCEPT
      reason: >-
        This is a core annotation. UQCRFS1 is unambiguously a structural component
        of respiratory chain complex III. The GO term definition explicitly names
        the Rieske ISP as one of the three catalytic subunits. Multiple
        experimental studies confirm this localization.
      supported_by:
        - reference_id: PMID:23168492
          supporting_text: >-
            human LYRM7, which we propose to be renamed MZM1L (MZM1-like), works as
            a
            human Rieske
            Fe-S protein (UQCRFS1) chaperone, binding to this subunit within the
            mitochondrial matrix and stabilizing it prior to its translocation and
            insertion
            into the late CIII dimeric intermediate within the mitochondrial inner
            membrane.
        - reference_id: PMID:31883641
          supporting_text: >-
            Here we describe rare bi-allelic
            variants in the gene of a catalytic subunit of CIII, UQCRFS1, which encodes
            the
            Rieske iron-sulfur protein, in two unrelated individuals.
  - term:
      id: GO:0006122
      label: mitochondrial electron transport, ubiquinol to cytochrome c
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        IBA annotation for the core biological process of UQCRFS1. This is the
        precise biological process catalyzed by Complex III, and UQCRFS1 is one
        of the three catalytic subunits directly involved in this electron
        transfer step. The Rieske protein accepts an electron from ubiquinol at
        the Qo site and transfers it to cytochrome c1 via its [2Fe-2S] cluster
        (PMID:28380382).
      action: ACCEPT
      reason: >-
        This is the core biological process for UQCRFS1. The Rieske protein is
        one of three catalytic subunits of Complex III that directly participates
        in electron transfer from ubiquinol to cytochrome c. This is the most
        specific and accurate BP annotation for this gene. Phylogenetic inference
        is well supported.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            Ubiquinol-cytochrome c oxidoreductase (E.C. 1.10.2.2, also known as
            cytochrome bc1 complex or Complex III), a central component of the
            mitochondrial respiratory chain, consists of 11 different subunits

# ============================================================
# IEA ANNOTATIONS (electronically inferred)
# ============================================================
  - term:
      id: GO:1902600
      label: proton transmembrane transport
    evidence_type: IEA
    original_reference_id: GO_REF:0000108
    review:
      summary: >-
        IEA annotation inferred from quinol-cytochrome-c reductase activity
        (GO:0008121) via logical inference. Complex III couples electron transfer
        to proton translocation across the inner mitochondrial membrane during
        the Q-cycle (PMID:28380382). UQCRFS1 contributes to this activity as a
        catalytic subunit, though the proton translocation itself is primarily
        mediated through the quinone chemistry in cytochrome b rather than the
        Rieske protein directly.
      action: KEEP_AS_NON_CORE
      reason: >-
        Proton transmembrane transport is a downstream consequence of the Q-cycle
        catalyzed by Complex III as a whole. UQCRFS1 contributes to the overall
        Q-cycle mechanism but proton translocation is more directly attributable
        to the quinone chemistry at the Qo and Qi sites of cytochrome b. This is
        an IEA inference from the whole-complex activity and is not wrong but is
        not the core function of the Rieske subunit specifically.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            Energy transduction by Complex III (CIII) follows the Q cycle mechanism,
            whereby oxidation of a membrane-localized ubiquinol is coupled to proton
            pumping across the inner mitochondrial membrane into the intermembrane
            space.
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: >-
        IEA annotation from UniProt subcellular location mapping. UQCRFS1 is an
        integral membrane protein of the mitochondrial inner membrane with a
        single transmembrane helix (residues 104-140, PDB:5XTE). UniProt
        explicitly annotates the subcellular location as mitochondrion inner
        membrane. This is well established.
      action: ACCEPT
      reason: >-
        UQCRFS1 is an integral single-pass membrane protein of the mitochondrial
        inner membrane, confirmed by cryo-EM structures (PDB:5XTE) and UniProt
        annotation. The IEA mapping is correct and consistent with experimental
        evidence.
  - term:
      id: GO:0008121
      label: quinol-cytochrome-c reductase activity
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: >-
        IEA annotation for the whole-complex enzymatic activity of Complex III
        (EC 7.1.1.8). UQCRFS1 is one of three catalytic subunits of Complex III
        that together catalyze the quinol-cytochrome-c reductase reaction.
        However, as a single subunit, UQCRFS1 does not independently catalyze
        this full reaction -- it contributes to the complex activity. In GO
        annotation practice, individual subunits of a complex should use the
        contributes_to qualifier for the whole-complex activity. The more
        specific subunit-level MF is electron transfer activity (GO:0009055).
      action: MODIFY
      reason: >-
        This annotation is correct in substance -- UQCRFS1 is a catalytic subunit
        of the complex that performs this reaction. However, the GOA file shows
        this with the enables qualifier, and UQCRFS1 alone cannot catalyze the
        full quinol-cytochrome-c reductase reaction. The subunit-specific MF is
        electron transfer activity, while GO:0008121 should be retained only as
        a contributes_to activity of Complex III.
      proposed_replacement_terms:
        - id: GO:0009055
          label: electron transfer activity
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            The catalytic activity of CIII depends on three highly conserved subunits
            that contain redox active centers, cytochrome b (MT-CYB), cytochrome c
            1
            (CYC1), and the Rieske iron-sulfur
  - term:
      id: GO:0016020
      label: membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    review:
      summary: >-
        IEA annotation from InterPro domain mapping. UQCRFS1 has a single
        transmembrane helix (residues 104-140) and is an integral membrane
        protein. This term is correct but extremely generic. More specific terms
        (mitochondrial inner membrane, GO:0005743) are already annotated from
        other sources.
      action: ACCEPT
      reason: >-
        This is a correct but very generic CC annotation. UQCRFS1 is indeed a
        membrane protein with a transmembrane helix. More specific annotations
        (mitochondrial inner membrane) are present from other evidence lines.
        As a broad IEA it is acceptable to retain, though it adds little
        information beyond what is captured by the more specific terms.
  - term:
      id: GO:0022904
      label: respiratory electron transport chain
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        IEA annotation from UniProt keyword mapping (KW-0679, Respiratory chain).
        UQCRFS1 is a core component of the mitochondrial respiratory electron
        transport chain as a catalytic subunit of Complex III. This is correct
        and well supported, though the more specific child term GO:0006122
        (mitochondrial electron transport, ubiquinol to cytochrome c) is the
        precise process.
      action: ACCEPT
      reason: >-
        This is a correct and broader parent annotation. UQCRFS1 unambiguously
        participates in the respiratory electron transport chain. The more
        specific term GO:0006122 is already annotated via IBA. As an IEA from
        keyword mapping, this broader annotation is acceptable.
  - term:
      id: GO:0031966
      label: mitochondrial membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000117
    review:
      summary: >-
        IEA annotation from ARBA machine learning model. UQCRFS1 is located in
        the mitochondrial inner membrane specifically. This term (mitochondrial
        membrane) is a parent of mitochondrial inner membrane (GO:0005743),
        which is already annotated. Correct but less informative.
      action: ACCEPT
      reason: >-
        Correct but generic. UQCRFS1 is specifically in the mitochondrial inner
        membrane, and the more specific term GO:0005743 is already annotated
        from multiple sources. This broader IEA is acceptable to retain.
  - term:
      id: GO:0045275
      label: respiratory chain complex III
    evidence_type: IEA
    original_reference_id: GO_REF:0000117
    review:
      summary: >-
        IEA annotation from ARBA for respiratory chain complex III. Duplicate
        of the IBA annotation with the same GO ID. UQCRFS1 is unambiguously a
        subunit of Complex III. Both annotations (IBA and IEA) are valid.
      action: ACCEPT
      reason: >-
        Correct. This duplicates the IBA annotation for the same term, which is
        fine -- independent evidence lines supporting the same conclusion. UQCRFS1
        is a core catalytic subunit of respiratory chain complex III.
  - term:
      id: GO:0046872
      label: metal ion binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        IEA annotation from UniProt keyword mapping (KW-0479, Iron). UQCRFS1
        binds iron as part of its [2Fe-2S] cluster. This term is correct but
        very broad. The more specific term GO:0051537 (2 iron, 2 sulfur cluster
        binding) is already annotated and is far more informative.
      action: ACCEPT
      reason: >-
        Correct but very generic. UQCRFS1 does bind metal ions (iron in its
        [2Fe-2S] cluster). The more specific child term GO:0051537 is already
        annotated from other sources. As a broad IEA from keyword mapping, this
        is acceptable to retain, though it provides minimal additional
        information.
  - term:
      id: GO:0051536
      label: iron-sulfur cluster binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        IEA annotation from UniProt keyword mapping (KW-0411, Iron-sulfur).
        UQCRFS1 binds a [2Fe-2S] cluster, which is a type of iron-sulfur
        cluster. Correct but less specific than GO:0051537 (2 iron, 2 sulfur
        cluster binding) which is already annotated.
      action: ACCEPT
      reason: >-
        Correct. UQCRFS1 binds an iron-sulfur cluster (specifically [2Fe-2S]).
        The more specific child term GO:0051537 is already present. This broader
        IEA is acceptable.
  - term:
      id: GO:0051537
      label: 2 iron, 2 sulfur cluster binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: >-
        IEA annotation from combined automated methods. UQCRFS1 binds a single
        [2Fe-2S] cluster per subunit, coordinated by Cys217, Cys219, Cys236,
        His239, and His241 in the Rieske domain (UniProt FT BINDING entries).
        This is a well-characterized cofactor essential for the electron
        transfer function of UQCRFS1 (PMID:28380382).
      action: ACCEPT
      reason: >-
        Core molecular function annotation. UQCRFS1 binds one [2Fe-2S] cluster
        per subunit, which is essential for its electron transfer activity.
        This is confirmed by the UniProt record, structural data, and multiple
        experimental studies. The IEA annotation is correct and well supported.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            The iron-sulfur (Fe-S) cluster of the Rieske protein, UQCRFS1, is essential
            for
            Complex III (CIII) activity

# ============================================================
# IPI ANNOTATIONS (protein binding)
# ============================================================
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:27499296
    review:
      summary: >-
        IPI annotation for protein binding based on mitochondrial protein
        interaction mapping study (Floyd et al. 2016). The GOA WITH/FROM column
        shows UniProtKB:Q5U5X0 (LYRM7). LYRM7 is a well-characterized UQCRFS1
        chaperone that stabilizes the apo-Rieske protein in the mitochondrial
        matrix prior to [2Fe-2S] cluster insertion and BCS1L-mediated
        translocation into Complex III (PMID:23168492, PMID:28380382). This is
        a real interaction but protein binding is uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        The UQCRFS1-LYRM7 interaction is genuine and functionally important --
        LYRM7 chaperones UQCRFS1 during Complex III assembly. However, protein
        binding (GO:0005515) conveys no useful functional information about this
        interaction. Per GO curation guidelines, protein binding should be
        avoided when more specific terms are available. The interaction is better
        captured by the assembly process annotation (GO:0034551) and CC
        annotations.
      supported_by:
        - reference_id: PMID:27499296
          supporting_text: >-
            we assessed condition-specific protein-protein
            interactions for 50 select MXPs using affinity enrichment mass spectrometry.
            Our
            data connect MXPs to diverse mitochondrial processes, including multiple
            aspects
            of respiratory chain function.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:28380382
    review:
      summary: >-
        IPI annotation for protein binding from Maio et al. 2017 (Cell Metab).
        WITH/FROM is Q5U5X0 (LYRM7). This study demonstrated that LYRM7 directly
        binds UQCRFS1 in a pre-assembly intermediate and recruits the Fe-S
        transfer complex (HSC20/HSPA9/ISCU) for [2Fe-2S] cluster delivery.
        The interaction is mechanistically important but protein binding is
        uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Same interaction as above (UQCRFS1-LYRM7), confirmed with more
        mechanistic detail in this study. Protein binding does not capture the
        functional significance. The interaction is part of the Fe-S cluster
        biogenesis and Complex III assembly pathway. Better captured by other
        annotations.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            a transient subcomplex involved in CIII assembly,
            composed of LYRM7 bound to UQCRFS1, interacts with components of an Fe-S
            transfer complex, consisting of HSC20, its cognate chaperone HSPA9, and
            the
            holo-scaffold ISCU.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:33961781
    review:
      summary: >-
        IPI annotation for protein binding from Huttlin et al. 2021 (Cell),
        the BioPlex 3.0 proteome-scale interaction network. WITH/FROM is Q5U5X0
        (LYRM7). This is a high-throughput interactome study that independently
        detected the UQCRFS1-LYRM7 interaction via affinity purification mass
        spectrometry. The interaction is genuine but this is the same interaction
        already captured by more focused studies.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Same UQCRFS1-LYRM7 interaction detected in a high-throughput interactome
        screen. Protein binding remains uninformative. The functional
        significance of this interaction is better captured by assembly process
        annotations.
      supported_by:
        - reference_id: PMID:33961781
          supporting_text: >-
            Through affinity-purification
            mass spectrometry, we have created two proteome-scale, cell-line-specific
            interaction networks.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:40205054
    review:
      summary: >-
        IPI annotation for protein binding from Schaffer et al. 2025 (Nature),
        multimodal cell maps. WITH/FROM is Q5U5X0 (LYRM7). Another independent
        detection of the UQCRFS1-LYRM7 interaction in a large-scale study.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Same UQCRFS1-LYRM7 interaction. Protein binding is uninformative
        per GO curation guidelines. The functional context (chaperone interaction
        during Complex III assembly) is not captured by this generic term.
      supported_by:
        - reference_id: PMID:40205054
          supporting_text: >-
            we construct a global map of human subcellular architecture
            through joint measurement of biophysical interactions and immunofluorescence
            images for over 5,100 proteins in U2OS osteosarcoma cells.

# ============================================================
# EXPERIMENTAL ANNOTATIONS (HTP, IDA, IMP, IC, TAS, HDA)
# ============================================================
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: HTP
    original_reference_id: PMID:34800366
    review:
      summary: >-
        HTP annotation for mitochondrial localization from Morgenstern et al.
        2021 (Cell Metab), a quantitative high-confidence human mitochondrial
        proteome study. UQCRFS1 is unambiguously a mitochondrial protein -- it
        is a core subunit of mitochondrial Complex III. This is well established.
      action: ACCEPT
      reason: >-
        UQCRFS1 is a bona fide mitochondrial protein, confirmed by multiple
        independent experimental approaches. This HTP annotation from a
        high-quality mitochondrial proteome study is correct, though more
        specific CC annotations (mitochondrial inner membrane, respiratory chain
        complex III) are also present.
      supported_by:
        - reference_id: PMID:34800366
          supporting_text: >-
            We classified
            >8,000 proteins in mitochondrial preparations of human cells and defined
            a
            mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP).
  - term:
      id: GO:0045275
      label: respiratory chain complex III
    evidence_type: IDA
    original_reference_id: PMID:23168492
    review:
      summary: >-
        IDA annotation for Complex III localization from Sanchez et al. 2013
        (BBA). This study characterized LYRM7 as a UQCRFS1 chaperone and in
        the process directly demonstrated UQCRFS1 incorporation into the Complex
        III dimer by BN-PAGE and immunodetection. UQCRFS1 was shown to co-migrate
        with assembled CIII2 and supercomplexes.
      action: ACCEPT
      reason: >-
        Direct experimental evidence (IDA) for UQCRFS1 as part of Complex III.
        The study used BN-PAGE to demonstrate UQCRFS1 incorporation into assembled
        Complex III. This is a core CC annotation with strong experimental support.
      supported_by:
        - reference_id: PMID:23168492
          supporting_text: >-
            LYRM7/MZM1L is a novel human CIII assembly factor involved in the UQCRFS1
            insertion step, which enables formation of the mature and functional CIII
            enzyme.
  - term:
      id: GO:0045275
      label: respiratory chain complex III
    evidence_type: IC
    original_reference_id: PMID:31883641
    review:
      summary: >-
        IC (inferred by curator) annotation for Complex III membership from
        Gusic et al. 2020 (Am J Hum Genet). The GOA WITH/FROM column shows
        GO:0034551 (mitochondrial respiratory chain complex III assembly),
        indicating the curator inferred Complex III membership from assembly
        defects observed in patient fibroblasts with UQCRFS1 variants. Patient
        cells showed reduced UQCRFS1 abundance and impaired CIII assembly.
      action: ACCEPT
      reason: >-
        Valid curator inference. If UQCRFS1 variants impair Complex III assembly
        and reduce CIII activity, it follows that UQCRFS1 is part of Complex III.
        This is well supported by the disease genetics study and consistent with
        all other evidence.
      supported_by:
        - reference_id: PMID:31883641
          supporting_text: >-
            Studies in
            proband-derived fibroblasts showed a deleterious effect of the variants
            on
            UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular
            respiration.
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9866272
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-9866272 (2Fe-2S is inserted
        in UQCRFS1). This Reactome reaction models the [2Fe-2S] cluster insertion
        step during UQCRFS1 maturation, which occurs at the mitochondrial inner
        membrane. UQCRFS1 is indeed located in the inner membrane after assembly.
      action: ACCEPT
      reason: >-
        Correct. UQCRFS1 is a single-pass integral protein of the mitochondrial
        inner membrane. The Reactome annotation from the Fe-S cluster insertion
        pathway is consistent with the known biology.
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9906017
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-9906017 (Unknown peptidase
        cleaves UQCRFS1 subunit). This reaction models the proteolytic processing
        of UQCRFS1 after its insertion into Complex III at the inner membrane.
        The localization is correct.
      action: ACCEPT
      reason: >-
        Correct. The proteolytic processing of UQCRFS1 occurs after its insertion
        into the inner membrane Complex III dimer.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9866253
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-9866253 (apo-UQCRFS1 binds
        LYRM7). This reaction models the chaperone binding step where apo-UQCRFS1
        binds LYRM7 in the mitochondrial matrix prior to Fe-S cluster insertion
        and BCS1L-mediated translocation. The matrix is the transient location
        of the apo-protein during assembly, not the final functional location.
      action: KEEP_AS_NON_CORE
      reason: >-
        Correct but non-core for the assembly intermediate. After import into
        mitochondria, apo-UQCRFS1 resides transiently in the mitochondrial matrix
        where it binds LYRM7 and receives its [2Fe-2S] cluster before being
        translocated by BCS1L into the pre-CIII complex in the inner membrane.
        The final functional location is the mitochondrial inner membrane, so
        matrix localization should not be treated as a core active location.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            Binding of HSC20 to the LYR motif of LYRM7 in a
            pre-assembled UQCRFS1-LYRM7 intermediate in the mitochondrial matrix facilitates
            Fe-S cluster transfer to UQCRFS1.
        - reference_id: PMID:23168492
          supporting_text: >-
            binding to this subunit within the
            mitochondrial matrix and stabilizing it prior to its translocation and
            insertion
            into the late CIII dimeric intermediate within the mitochondrial inner
            membrane.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9866272
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-9866272 (2Fe-2S is inserted
        in UQCRFS1). This reaction occurs in the mitochondrial matrix where
        the Fe-S transfer complex delivers the [2Fe-2S] cluster to
        apo-UQCRFS1 bound to LYRM7. As above, matrix localization is a
        transient assembly state.
      action: KEEP_AS_NON_CORE
      reason: >-
        Correct but non-core for the assembly intermediate. The Fe-S cluster
        insertion into UQCRFS1 occurs in the mitochondrial matrix via the
        HSC20/HSPA9/ISCU transfer complex. This is a valid transient localization
        during UQCRFS1 biogenesis, but the final functional location is the
        mitochondrial inner membrane.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            Binding of HSC20 to the LYR motif of LYRM7 in a
            pre-assembled UQCRFS1-LYRM7 intermediate in the mitochondrial matrix facilitates
            Fe-S cluster transfer to UQCRFS1.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: IDA
    original_reference_id: PMID:31883641
    review:
      summary: >-
        IDA annotation for mitochondrial localization from Gusic et al. 2020.
        This study showed that wild-type UQCRFS1 localizes to mitochondria, and
        the V14D pathogenic variant causes mislocalization to the cytosol and
        nucleus. Direct immunofluorescence microscopy confirmed the mitochondrial
        localization of wild-type UQCRFS1.
      action: ACCEPT
      reason: >-
        Direct experimental demonstration that UQCRFS1 localizes to mitochondria.
        The disease study provided direct evidence via immunofluorescence showing
        mitochondrial localization of the wild-type protein and mislocalization
        of the V14D mutant.
      supported_by:
        - reference_id: PMID:31883641
          supporting_text: >-
            Studies in
            proband-derived fibroblasts showed a deleterious effect of the variants
            on
            UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular
            respiration.
  - term:
      id: GO:0022904
      label: respiratory electron transport chain
    evidence_type: IMP
    original_reference_id: PMID:31883641
    review:
      summary: >-
        IMP annotation for respiratory electron transport chain from Gusic et al.
        2020. Bi-allelic UQCRFS1 variants caused impaired cellular respiration in
        patient fibroblasts, which was rescued by lentiviral complementation with
        wild-type UQCRFS1. This demonstrates that UQCRFS1 is required for
        respiratory chain function.
      action: ACCEPT
      reason: >-
        Valid IMP annotation. The mutant phenotype (impaired cellular respiration)
        directly demonstrates involvement in the respiratory electron transport
        chain. Complementation with wild-type UQCRFS1 rescued the defect,
        confirming causality. This is also a parent term of GO:0006122 which is
        the more specific annotation.
      supported_by:
        - reference_id: PMID:31883641
          supporting_text: >-
            Complementation studies via lentiviral transduction and
            overexpression of wild-type UQCRFS1 restored mitochondrial function and
            rescued
            the cellular phenotype, confirming UQCRFS1 variants as causative for CIII
            deficiency.
  - term:
      id: GO:0034551
      label: mitochondrial respiratory chain complex III assembly
    evidence_type: IMP
    original_reference_id: PMID:31883641
    review:
      summary: >-
        IMP annotation for Complex III assembly from Gusic et al. 2020. Patient
        fibroblasts with bi-allelic UQCRFS1 variants showed impaired CIII
        assembly by BN-PAGE. UQCRFS1 is the penultimate subunit incorporated
        during CIII assembly, so its loss directly impairs the assembly process.
        This is a real involvement but is not the core evolved function of the
        Rieske protein -- the core function is electron transfer.
      action: KEEP_AS_NON_CORE
      reason: >-
        UQCRFS1 is incorporated as the penultimate step in Complex III assembly,
        and its loss impairs CIII assembly. However, UQCRFS1 is not an assembly
        factor per se -- it is a structural/catalytic subunit whose incorporation
        is required for full assembly. The assembly process is better attributed
        to assembly factors like BCS1L, LYRM7, and TTC19. The annotation is
        correct (UQCRFS1 variants do disrupt assembly) but it describes a
        consequence of subunit loss rather than the core function of the protein.
      supported_by:
        - reference_id: PMID:31883641
          supporting_text: >-
            Studies in
            proband-derived fibroblasts showed a deleterious effect of the variants
            on
            UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular
            respiration.
        - reference_id: PMID:28380382
          supporting_text: >-
            Incorporation of the Rieske protein UQCRFS1 is the penultimate step in
            CIII assembly, followed only by the insertion of a small supernumerary
            subunit (UQCR10 in mammalian cells).
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:23168492
    review:
      summary: >-
        IPI annotation for protein binding from Sanchez et al. 2013 (BBA).
        WITH/FROM is Q5U5X0 (LYRM7). This study identified LYRM7 as a human
        UQCRFS1 chaperone. The interaction was demonstrated by co-purification
        and other biochemical approaches. The interaction is genuine and
        functionally important (LYRM7 stabilizes apo-UQCRFS1 in the matrix).
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Same UQCRFS1-LYRM7 interaction as the other IPI annotations. Protein
        binding is uninformative per GO guidelines. The functional significance
        of this chaperone interaction is better captured by the assembly-related
        annotations.
      supported_by:
        - reference_id: PMID:23168492
          supporting_text: >-
            We conclude that human
            LYRM7, which we propose to be renamed MZM1L (MZM1-like), works as a human
            Rieske
            Fe-S protein (UQCRFS1) chaperone
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: HDA
    original_reference_id: PMID:20833797
    review:
      summary: >-
        HDA (high-throughput direct assay) annotation for mitochondrial
        localization from Zhao et al. 2011 (Mol Cell Proteomics). This
        phosphoproteomics study isolated functional mitochondria from human
        skeletal muscle and identified UQCRFS1 among the mitochondrial
        phosphoproteins by mass spectrometry.
      action: ACCEPT
      reason: >-
        Correct. UQCRFS1 was identified by mass spectrometry in purified
        mitochondrial fractions from human skeletal muscle. This is consistent
        with all other evidence for mitochondrial localization.
      supported_by:
        - reference_id: PMID:20833797
          supporting_text: >-
            We performed a phosphoproteomics study of functional
            mitochondria isolated from human muscle biopsies with the aim to obtain
            a
            comprehensive overview of mitochondrial phosphoproteins.
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-164651
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-164651 (Electron transfer
        from ubiquinol to cytochrome c of complex III). This reaction represents
        the core catalytic function of Complex III occurring at the inner
        membrane, where UQCRFS1 participates as the Rieske iron-sulfur subunit.
      action: ACCEPT
      reason: >-
        Correct. UQCRFS1 functions at the mitochondrial inner membrane as part
        of Complex III during electron transfer from ubiquinol to cytochrome c.
        This Reactome pathway correctly models the functional localization.
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9906042
    review:
      summary: >-
        TAS annotation from Reactome pathway R-HSA-9906042 (TTC19 clears UQCRFS1
        fragments from Complex III). This reaction models the quality control
        step where TTC19 removes UQCRFS1-derived fragments from the inner
        membrane Complex III. The localization is correct.
      action: ACCEPT
      reason: >-
        Correct. The TTC19-mediated clearance of UQCRFS1 fragments occurs at
        the mitochondrial inner membrane where Complex III resides. This is
        consistent with the known biology of UQCRFS1 processing.

# ============================================================
# NEW ANNOTATIONS (suggested additions)
# ============================================================
  - term:
      id: GO:0009055
      label: electron transfer activity
    evidence_type: ISS
    original_reference_id: PMID:28380382
    review:
      summary: >-
        NEW annotation. UQCRFS1 is the Rieske iron-sulfur protein that transfers
        electrons from ubiquinol (at the Qo site of cytochrome b) to cytochrome
        c1 via its [2Fe-2S] cluster. This is the subunit-specific molecular
        function that is missing from the current annotation set. The existing
        annotations include the whole-complex activity (GO:0008121,
        quinol-cytochrome-c reductase activity) and the broad parent
        (GO:0016491, oxidoreductase activity), but the specific electron
        transfer activity of the Rieske subunit is not captured. Deep research
        (UQCRFS1-deep-research-falcon.md) confirms that in situ cryo-EM
        structures captured the Rieske head domain positions across catalytic
        states during the Q-cycle (Zheng et al. 2024, Nature).
      action: NEW
      reason: >-
        The existing annotations lack a subunit-specific MF term for UQCRFS1.
        Electron transfer activity (GO:0009055) precisely describes what the
        Rieske protein does -- it transfers electrons between ubiquinol and
        cytochrome c1 using its [2Fe-2S] cluster. This is well established
        from structural and biochemical studies and is the most informative
        MF annotation for this subunit.
      supported_by:
        - reference_id: PMID:28380382
          supporting_text: >-
            The iron-sulfur (Fe-S) cluster of the Rieske protein, UQCRFS1, is essential
            for
            Complex III (CIII) activity
core_functions:
  - molecular_function:
      id: GO:0009055
      label: electron transfer activity
    contributes_to_molecular_function:
      id: GO:0008121
      label: quinol-cytochrome-c reductase activity
    directly_involved_in:
      - id: GO:0006122
        label: mitochondrial electron transport, ubiquinol to cytochrome c
    locations:
      - id: GO:0005743
        label: mitochondrial inner membrane
    description: >-
      UQCRFS1 is the Rieske iron-sulfur protein, one of three catalytic subunits
      of mitochondrial Complex III (cytochrome bc1 complex). Its primary
      subunit-specific molecular function is electron transfer activity
      (GO:0009055), mediated by its [2Fe-2S] cluster which accepts an electron
      from ubiquinol bound at the Qo site of cytochrome b and transfers it to
      cytochrome c1. The Rieske head domain undergoes a conformational swing
      between the b-site and c-site positions during each catalytic cycle. This
      electron transfer contributes to the overall quinol-cytochrome-c reductase
      activity (GO:0008121, EC 7.1.1.8) of the Complex III homodimer. UQCRFS1
      also binds a [2Fe-2S] cluster (GO:0051537). The protein is located in
      respiratory chain complex III (GO:0045275) at the mitochondrial inner
      membrane (GO:0005743), with the catalytic globular domain in the
      intermembrane space and a single transmembrane helix anchoring it to the
      complex. The core biological process is mitochondrial electron transport,
      ubiquinol to cytochrome c (GO:0006122).
    supported_by:
      - reference_id: PMID:28380382
        supporting_text: >-
          The iron-sulfur (Fe-S) cluster of the Rieske protein, UQCRFS1, is essential
          for
          Complex III (CIII) activity
      - reference_id: PMID:28380382
        supporting_text: >-
          The catalytic activity of CIII depends on three highly conserved subunits
          that contain redox active centers, cytochrome b (MT-CYB), cytochrome c 1
          (CYC1), and the Rieske iron-sulfur
      - reference_id: PMID:31883641
        supporting_text: >-
          Affected children
          presented with low CIII activity in fibroblasts, lactic acidosis, fetal
          bradycardia, hypertrophic cardiomyopathy, and alopecia totalis.
    in_complex:
      id: GO:0045275
      label: respiratory chain complex III
references:
  - id: GO_REF:0000002
    title: Gene Ontology annotation through association of InterPro records with GO
      terms
    findings:
      - statement: >-
          InterPro2GO mapping of the Rieske domain (IPR005805/IPR014349) and
          ubiquinol-cytochrome c reductase iron-sulfur subunit signatures assigns
          UQCRFS1 the cellular component term membrane (GO:0016020) consistent
          with its single transmembrane helix.
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings:
      - statement: >-
          PANTHER phylogenetic propagation transfers the conserved Rieske-subunit
          functions across UQCRFS1 orthologues, including oxidoreductase activity
          (GO:0016491), respiratory chain complex III (GO:0045275), and
          mitochondrial electron transport, ubiquinol to cytochrome c
          (GO:0006122).
  - id: GO_REF:0000043
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
    findings:
      - statement: >-
          UniProtKB keywords KW-0679 (Respiratory chain), KW-0411 (Iron-sulfur),
          and KW-0479 (Iron) on UQCRFS1 are mapped to GO:0022904 (respiratory
          electron transport chain), GO:0051536 (iron-sulfur cluster binding),
          and GO:0046872 (metal ion binding) respectively.
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
      vocabulary mapping, accompanied by conservative changes to GO terms applied
      by UniProt
    findings:
      - statement: >-
          UniProtKB Subcellular Location annotation for UQCRFS1 (Mitochondrion
          inner membrane; single-pass membrane protein) is mapped to
          mitochondrial inner membrane (GO:0005743).
  - id: GO_REF:0000108
    title: Automatic assignment of GO terms using logical inference, based on on inter-ontology
      links
    findings:
      - statement: >-
          Inter-ontology logical inference from the EC 7.1.1.8
          (quinol-cytochrome-c reductase) reaction propagates proton
          transmembrane transport (GO:1902600) to UQCRFS1 as part of the Q-cycle
          activity of Complex III.
  - id: GO_REF:0000117
    title: Electronic Gene Ontology annotations created by ARBA machine learning models
    findings:
      - statement: >-
          ARBA rule-based annotation assigns UQCRFS1 the broader cellular
          component terms mitochondrial membrane (GO:0031966) and respiratory
          chain complex III (GO:0045275), consistent with experimental and
          phylogenetic evidence for inner-membrane Complex III subunit
          membership.
  - id: GO_REF:0000120
    title: Combined Automated Annotation using Multiple IEA Methods
    findings:
      - statement: >-
          Combined automated IEA pipelines annotate UQCRFS1 with quinol-
          cytochrome-c reductase activity (GO:0008121) and 2 iron, 2 sulfur
          cluster binding (GO:0051537) from converging UniProt feature, family,
          and reaction-based evidence.
  - id: PMID:20833797
    title: Phosphoproteome analysis of functional mitochondria isolated from resting
      human muscle reveals extensive phosphorylation of inner membrane protein complexes
      and enzymes.
    findings:
      - statement: UQCRFS1 identified by mass spectrometry in mitochondria isolated
          from human skeletal muscle.
        supporting_text: >-
          We performed a phosphoproteomics study of functional
          mitochondria isolated from human muscle biopsies with the aim to obtain
          a
          comprehensive overview of mitochondrial phosphoproteins.
  - id: PMID:23168492
    title: LYRM7/MZM1L is a UQCRFS1 chaperone involved in the last steps of mitochondrial
      Complex III assembly in human cells.
    findings:
      - statement: LYRM7 (MZM1L) identified as a chaperone for UQCRFS1, binding the
          Rieske protein in the mitochondrial matrix and stabilizing it prior to translocation
          into Complex III.
        supporting_text: >-
          We conclude that human
          LYRM7, which we propose to be renamed MZM1L (MZM1-like), works as a human
          Rieske
          Fe-S protein (UQCRFS1) chaperone, binding to this subunit within the
          mitochondrial matrix and stabilizing it prior to its translocation and insertion
          into the late CIII dimeric intermediate within the mitochondrial inner membrane.
      - statement: UQCRFS1 demonstrated to co-migrate with assembled CIII2 and supercomplexes
          by BN-PAGE.
        supporting_text: >-
          LYRM7/MZM1L is a novel human CIII assembly factor involved in the UQCRFS1
          insertion step, which enables formation of the mature and functional CIII
          enzyme.
  - id: PMID:27499296
    title: Mitochondrial Protein Interaction Mapping Identifies Regulators of Respiratory
      Chain Function.
    findings:
      - statement: UQCRFS1-LYRM7 interaction detected by affinity enrichment mass
          spectrometry in mitochondrial protein interaction mapping.
        supporting_text: >-
          we assessed condition-specific protein-protein
          interactions for 50 select MXPs using affinity enrichment mass spectrometry.
  - id: PMID:28380382
    title: A Single Adaptable Cochaperone-Scaffold Complex Delivers Nascent Iron-Sulfur
      Clusters to Mammalian Respiratory Chain Complexes I-III.
    findings:
      - statement: HSC20 co-chaperone binds LYRM7 LYR motif in pre-assembled UQCRFS1-LYRM7
          intermediate to facilitate [2Fe-2S] cluster transfer to UQCRFS1.
        supporting_text: >-
          Binding of HSC20 to the LYR motif of LYRM7 in a
          pre-assembled UQCRFS1-LYRM7 intermediate in the mitochondrial matrix facilitates
          Fe-S cluster transfer to UQCRFS1.
      - statement: UQCRFS1 is the penultimate subunit incorporated during Complex
          III assembly.
        supporting_text: >-
          Incorporation of the Rieske protein UQCRFS1 is the penultimate step in
          CIII assembly, followed only by the insertion of a small supernumerary
          subunit (UQCR10 in mammalian cells).
      - statement: The Fe-S cluster of UQCRFS1 is essential for Complex III activity.
        supporting_text: >-
          The iron-sulfur (Fe-S) cluster of the Rieske protein, UQCRFS1, is essential
          for
          Complex III (CIII) activity
  - id: PMID:31883641
    title: Bi-Allelic UQCRFS1 Variants Are Associated with Mitochondrial Complex III
      Deficiency, Cardiomyopathy, and Alopecia Totalis.
    findings:
      - statement: Bi-allelic UQCRFS1 variants cause isolated Complex III deficiency
          with cardiomyopathy, alopecia totalis, and lactic acidosis.
        supporting_text: >-
          Affected children
          presented with low CIII activity in fibroblasts, lactic acidosis, fetal
          bradycardia, hypertrophic cardiomyopathy, and alopecia totalis.
      - statement: Patient fibroblasts showed reduced UQCRFS1 abundance, impaired
          mitochondrial import, defective CIII assembly, and decreased cellular respiration.
        supporting_text: >-
          Studies in
          proband-derived fibroblasts showed a deleterious effect of the variants
          on
          UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular
          respiration.
      - statement: Wild-type UQCRFS1 complementation via lentiviral transduction rescued
          the cellular phenotype.
        supporting_text: >-
          Complementation studies via lentiviral transduction and
          overexpression of wild-type UQCRFS1 restored mitochondrial function and
          rescued
          the cellular phenotype, confirming UQCRFS1 variants as causative for CIII
          deficiency.
      - statement: UQCRFS1 localizes to mitochondria by immunofluorescence; V14D variant
          causes cytosolic mislocalization.
        supporting_text: >-
          Studies in
          proband-derived fibroblasts showed a deleterious effect of the variants
          on
          UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular
          respiration.
  - id: PMID:33961781
    title: Dual proteome-scale networks reveal cell-specific remodeling of the human
      interactome.
    findings:
      - statement: UQCRFS1-LYRM7 interaction independently detected in BioPlex 3.0
          proteome-scale interaction network.
        supporting_text: >-
          Through affinity-purification
          mass spectrometry, we have created two proteome-scale, cell-line-specific
          interaction networks.
  - id: PMID:34800366
    title: Quantitative high-confidence human mitochondrial proteome and its dynamics
      in cellular context.
    findings:
      - statement: UQCRFS1 identified as high-confidence mitochondrial protein.
        supporting_text: >-
          We classified
          >8,000 proteins in mitochondrial preparations of human cells and defined
          a
          mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP).
  - id: PMID:40205054
    title: Multimodal cell maps as a foundation for structural and functional genomics.
    findings:
      - statement: UQCRFS1-LYRM7 interaction detected in multimodal cell mapping study.
        supporting_text: >-
          we construct a global map of human subcellular architecture
          through joint measurement of biophysical interactions and immunofluorescence
          images for over 5,100 proteins in U2OS osteosarcoma cells.
  - id: Reactome:R-HSA-164651
    title: Electron transfer from ubiquinol to cytochrome c of complex III
    findings:
      - statement: UQCRFS1 participates in the core catalytic reaction of Complex
          III at the inner membrane.
        supporting_text: >-
          The protonmotive Q cycle is the mechanism by which complex III transfers
          electrons from ubiquinol to cytochrome c, linking this process to translocation
          of protons across the membrane.
  - id: Reactome:R-HSA-9866253
    title: apo-UQCRFS1 binds LYRM7
    findings:
      - statement: Apo-UQCRFS1 binds chaperone LYRM7 in the mitochondrial matrix during
          Complex III biogenesis.
        supporting_text: >-
          the Complex III subunit UQCRFS1 (Rieske protein) binds to and is stabilized
          by the chaperone LYRM7 (MZM1L)
  - id: Reactome:R-HSA-9866272
    title: 2Fe-2S is inserted in UQCRFS1
    findings:
      - statement: The [2Fe-2S] cluster is inserted into UQCRFS1 at the mitochondrial
          inner membrane during assembly.
        supporting_text: >-
          Frataxin (FXN) subunit of the 2Fe-2S transfer complex catalyzes the insertion
          of one 2Fe-2S cluster in the Complex III subunit UQCRFS1 (Rieske protein)
          as
          part of a UQCRFS1:LYRM7 complex.
  - id: Reactome:R-HSA-9906017
    title: Unknown peptidase cleaves UQCRFS1 subunit
    findings:
      - statement: UQCRFS1 undergoes proteolytic processing after insertion into Complex
          III.
        supporting_text: >-
          Proteolytic processing is necessary for the correct insertion of UQCRFS1
          in the complex III dimer. An unknown peptidase cleaves the N-terminal 78
          amino acids of UQCRFS1
  - id: Reactome:R-HSA-9906042
    title: TTC19 clears UQCRFS1 fragments from Complex III
    findings:
      - statement: TTC19 mediates clearance of UQCRFS1-derived fragments from Complex
          III at the inner membrane.
        supporting_text: >-
          N-terminal cleavage fragments of UQCRFS1 are cleared by TTC19, stabilizing
          the final complex.
  - id: file:human/UQCRFS1/UQCRFS1-deep-research-falcon.md
    title: Deep research review for UQCRFS1 (Falcon/Edison)
    findings:
      - statement: >-
          UQCRFS1 is the catalytic Rieske iron-sulfur subunit of mitochondrial complex
          III
          (CIII2), participating in ubiquinol (QH2) oxidation at the Qo site during
          the
          Q-cycle and shuttling an electron via its 2Fe-2S cluster to cytochrome c1.
      - statement: >-
          High-resolution in situ cryo-EM resolved multiple native supercomplex
          organizations and captured distinct Qo-site Q10/QH2 binding states and
          corresponding Rieske-domain movements, providing direct structural support
          for the Q-cycle mechanism (Zheng et al. 2024, Nature).
      - statement: >-
          UQCRFS1 is the last subunit incorporated into CIII; LYRM7 (MZM1L) chaperones
          Fe-S cluster insertion, BCS1L translocates the folded ISP into pre-CIII
          for
          late-stage incorporation, and TTC19 mediates turnover of N-terminal peptides
          for CIII integrity.
suggested_questions:
  - question: >-
      What controls the kinetics and timing of the Rieske head-domain swing between
      the b-position (electron acceptance from ubiquinol at the Qo site) and the
      c-position (electron donation to cytochrome c1) during the Q-cycle, and how
      is this coupled to proton translocation in human Complex III?
  - question: >-
      How is the LYRM7-bound apo-UQCRFS1 intermediate handed off to the
      HSC20/HSPA9/ISCU Fe-S transfer complex, and what determines the order of
      Fe-S cluster insertion versus BCS1L-mediated translocation into the inner
      membrane Complex III pre-assembly?
  - question: >-
      What is the role of the cleaved N-terminal peptide (subunit 9/UQCR11) and
      its TTC19-mediated turnover in stabilising the mature Complex III dimer,
      and does its accumulation contribute to disease in TTC19-deficient
      patients?
  - question: >-
      How do disease-associated UQCRFS1 variants (e.g. V14D, R63H) selectively
      impair mitochondrial import, [2Fe-2S] cluster acquisition, or BCS1L
      translocation, and which step is rate-limiting in the tissue-specific
      cardiomyopathy and alopecia phenotype?
suggested_experiments:
  - description: >-
      Reconstitute the human apo-UQCRFS1/LYRM7/HSC20/HSPA9/ISCU Fe-S transfer
      pathway in vitro with purified components and chemically reconstituted
      [2Fe-2S] donors, then measure cluster transfer kinetics by EPR/UV-vis
      under wild-type versus disease-mutant conditions to identify the
      rate-limiting transfer step.
  - description: >-
      Use BCS1L-overexpression and CRISPR-engineered UQCRFS1 variant cell lines
      to follow Rieske-protein translocation kinetics across the inner membrane
      by pulse-chase split-fluorescent-protein complementation, distinguishing
      import, matrix folding, Fe-S insertion, and BCS1L-driven translocation
      defects.
  - description: >-
      Capture in situ time-resolved cryo-electron tomography of mitochondrial
      cristae from human cardiomyocytes carrying UQCRFS1 disease variants to
      determine whether Rieske head-domain swing is impaired or whether
      Complex III/IV supercomplex architecture is altered.
  - description: >-
      Generate a UQCRFS1 conditional-knockout/rescue mouse model with
      tissue-restricted expression of WT versus catalytically dead Rieske
      variants to dissect the contribution of electron transfer activity versus
      structural assembly support to the cardiomyopathy and alopecia phenotype.