id: O76031
gene_symbol: CLPX
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: CLPX is the regulatory AAA+ ATPase/unfoldase subunit of the mitochondrial-matrix
  ClpXP protease. It is imported into mitochondria via an N-terminal transit peptide,
  assembles into a homohexameric ring (ATP-dependent), and pairs with two heptameric
  rings of the CLPP peptidase to form the symmetry-mismatched ClpXP protease. Using
  energy from ATP binding and hydrolysis, CLPX recognizes specific substrate proteins,
  unfolds them, and translocates the unfolded polypeptide through its central pore
  into the CLPP proteolytic chamber for degradation, contributing to mitochondrial
  protein quality control. Independently of CLPP-coupled degradation, CLPX also acts
  as a chaperone/unfoldase that remodels and activates δ-aminolevulinate synthase (ALAS)
  by accelerating incorporation of the pyridoxal 5'-phosphate (PLP) cofactor, thereby
  stimulating the first, rate-limiting step of heme biosynthesis and supporting erythropoiesis;
  it also contributes to heme-induced turnover of ALAS, so the balance of activation
  and degradation tunes ALAS activity. CLPX contains a ClpX-type zinc-binding domain
  and a P-loop AAA+ ATPase module, and mutations affecting its ATPase activity cause
  autosomal dominant erythropoietic protoporphyria 2 (EPP2).
existing_annotations:
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: ATP hydrolysis is the central catalytic activity that powers CLPX unfolding/translocation;
      this is experimentally established for human CLPX (EC 3.6.4.10) and supported by
      phylogenetic transfer across the ClpX family.
    action: ACCEPT
    reason: ATP hydrolysis activity is a core molecular function of CLPX, directly demonstrated
      for the human protein and conserved across orthologs.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: 'Reaction=ATP + H2O = ADP + phosphate + H(+); ... EC=3.6.4.10'
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: CLPX binds ATP via its P-loop AAA+ module; ATP binding is required for hexamer
      assembly and for substrate engagement.
    action: ACCEPT
    reason: ATP binding is a core molecular function consistent with the AAA+ architecture
      and the demonstrated ATP-dependent hexamerization of CLPX.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Homohexamer that forms a ring structure; this hexamerization requires
        ATP binding.
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro-based transfer of ATP binding, consistent with the experimentally
      supported IBA/ISS annotations for the same activity.
    action: ACCEPT
    reason: Redundant with the IBA ATP binding annotation but correct; ATP binding is core
      to CLPX function.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Homohexamer that forms a ring structure; this hexamerization requires
        ATP binding.
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: CLPX is a mitochondrial protein; this IEA captures the broad mitochondrial
      localization, which is more precisely the mitochondrial matrix.
    action: KEEP_AS_NON_CORE
    reason: The mitochondrion term is correct but less specific than mitochondrial matrix,
      which is the experimentally supported and core compartment for CLPX.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion ... Mitochondrion matrix, mitochondrion
        nucleoid'
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: CLPX is an ATP-dependent unfoldase/chaperone, not a protein-folding catalyst;
      the bare "protein folding" InterPro transfer mischaracterizes its mechanism. The
      more specific molecular function GO:0140662 (ATP-dependent protein folding chaperone)
      is separately annotated.
    action: MARK_AS_OVER_ANNOTATED
    reason: ClpX actively unfolds and translocates substrates rather than promoting folding;
      this generic InterPro BP transfer is an over-annotation. Its chaperone role is better
      captured by the ATP-dependent protein folding chaperone MF term and by protein quality
      control / heme biosynthesis process terms.
    supported_by:
    - reference_id: PMID:25957689
      supporting_text: The prokaryotic AAA+ unfoldase ClpX is particularly specialized for
        regulatory unfolding ... ClpX unfolds substrate proteins by ATP-driven translocation
        of the polypeptide chain through the central pore of its hexameric assembly.
      reference_section_type: RESULTS
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Automated transfer of ATP hydrolysis activity, redundant with the experimentally
      supported IDA/EXP/IBA annotations.
    action: ACCEPT
    reason: ATP hydrolysis activity is a core, experimentally demonstrated molecular function
      of CLPX.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: 'Reaction=ATP + H2O = ADP + phosphate + H(+); ... EC=3.6.4.10'
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0042645
    label: mitochondrial nucleoid
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: CLPX has been detected among mitochondrial nucleoid-associated proteins and
      regulates mtDNA nucleoid distribution via TFAM, but this is a secondary, regulatory
      association rather than its core matrix localization.
    action: KEEP_AS_NON_CORE
    reason: Nucleoid association is genuine but represents a peripheral/regulatory localization;
      the core compartment is the mitochondrial matrix where CLPX performs unfolding and
      proteolysis.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Mitochondrion matrix, mitochondrion nucleoid
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0140662
    label: ATP-dependent protein folding chaperone
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: CLPX is an ATP-dependent chaperone/unfoldase; it remodels substrates (e.g.,
      activates ALAS by promoting PLP cofactor incorporation) and assists protein quality
      control using ATP-driven conformational work.
    action: ACCEPT
    reason: This MF term best captures the ATP-dependent chaperone activity of CLPX, including
      its CLPP-independent remodeling/activation role; it is preferable to the generic
      "protein folding" BP transfer.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: ATP-dependent chaperone that functions as an unfoldase.
      reference_section_type: DATABASE_ENTRY
    - reference_id: PMID:25957689
      supporting_text: Mcx1 accelerates formation of active Hem1 by stimulating cofactor
        binding to the apoenzyme.
      reference_section_type: RESULTS
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:15522782
  qualifier: enables
  review:
    summary: This IPI reflects the CLPX–CLPP interaction (WITH/FROM CLPP). Bare "protein
      binding" is uninformative; the specific functional consequence (CLPP peptidase activation
      and Clp complex assembly) is captured by other annotations.
    action: MARK_AS_OVER_ANNOTATED
    reason: Bare protein binding does not describe a function; the underlying CLPX–CLPP
      interaction is more informatively represented by peptidase activator activity and
      the ClpXP complex part_of terms.
    supported_by:
    - reference_id: PMID:15522782
      supporting_text: positioned to interact with unfolded substrates translocated there
        by the associated ClpX chaperone.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Immunofluorescence-based localization of CLPX to mitochondria; correct but
      less specific than mitochondrial matrix.
    action: KEEP_AS_NON_CORE
    reason: Mitochondrion is correct but subsumed by the more precise mitochondrial matrix
      localization, which is core.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion'
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: NAS
  original_reference_id: PMID:15522782
  qualifier: located_in
  review:
    summary: CLPX is a soluble mitochondrial-matrix protein where it carries out ATP-dependent
      unfolding and feeds substrates to the matrix-localized CLPP peptidase.
    action: ACCEPT
    reason: Mitochondrial matrix is the core, experimentally supported compartment for CLPX
      function.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Mitochondrion matrix, mitochondrion nucleoid
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0006508
    label: proteolysis
  evidence_type: IDA
  original_reference_id: PMID:15522782
  qualifier: involved_in
  review:
    summary: As the ATPase/unfoldase subunit of ClpXP, CLPX is required for ATP-dependent
      proteolysis; it unfolds and delivers substrates to CLPP for cleavage.
    action: ACCEPT
    reason: Participation in proteolysis is a core process for CLPX as part of the ClpXP
      protease, though "protein catabolic process" / "protein quality control" terms capture
      the biology more specifically.
    supported_by:
    - reference_id: PMID:11923310
      supporting_text: hClpXP displays both ATP-dependent proteolytic activity and ATP-
        or ATPgammaS-dependent peptidase activity.
      reference_section_type: ABSTRACT
- term:
    id: GO:0009368
    label: endopeptidase Clp complex
  evidence_type: IPI
  original_reference_id: PMID:15522782
  qualifier: part_of
  review:
    summary: CLPX is a bona fide component of the ClpXP protease complex (CLPX hexamer +
      CLPP tetradecamer). The generic "endopeptidase Clp complex" parent is correct but
      less specific than the mitochondrial Clp complex term.
    action: KEEP_AS_NON_CORE
    reason: Correct complex membership, but the human-specific mitochondrial endopeptidase
      Clp complex term (GO:0009841) more precisely captures CLPX localization and identity.
    supported_by:
    - reference_id: PMID:11923310
      supporting_text: Complexes of a double heptameric ring of hClpP with hexameric hClpX
        rings bound on each side are stable in the presence of ATP.
      reference_section_type: ABSTRACT
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: EXP
  original_reference_id: PMID:28874591
  qualifier: enables
  review:
    summary: Experimental evidence that human CLPX hydrolyzes ATP; the EPP2 variant p.Gly298Asp
      inactivates ATPase activity, directly demonstrating this catalytic function.
    action: ACCEPT
    reason: Directly demonstrated core catalytic activity of CLPX.
    supported_by:
    - reference_id: PMID:28874591
      supporting_text: The mutation in CLPX inactivates its ATPase activity, resulting in
        coassembly of mutant and WT protomers to form an enzyme with reduced activity.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: High-throughput mitochondrial proteome assignment of CLPX to mitochondria;
      correct but less specific than mitochondrial matrix.
    action: KEEP_AS_NON_CORE
    reason: Correct compartment, but redundant with and less precise than the mitochondrial
      matrix annotation.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion'
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9838035
  qualifier: located_in
  review:
    summary: Reactome event placing CLPXP (and CLPX) in the mitochondrial matrix where it
      binds matrix protein substrates.
    action: ACCEPT
    reason: Mitochondrial matrix is the core compartment for CLPX; consistent with experimental
      evidence.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Mitochondrion matrix, mitochondrion nucleoid
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9838289
  qualifier: located_in
  review:
    summary: Reactome event placing CLPXP in the mitochondrial matrix where it degrades
      matrix protein substrates.
    action: ACCEPT
    reason: Mitochondrial matrix is the core compartment for CLPX; consistent with experimental
      evidence.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Mitochondrion matrix, mitochondrion nucleoid
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0016887
    label: ATP hydrolysis activity
  evidence_type: IDA
  original_reference_id: PMID:22710082
  qualifier: enables
  review:
    summary: Direct biochemical characterization of human CLPX ATPase activity; the Walker
      B mutant (E359A) abolishes ATP hydrolysis, confirming this activity.
    action: ACCEPT
    reason: Directly demonstrated core catalytic activity of CLPX.
    supported_by:
    - reference_id: PMID:22710082
      supporting_text: we report the characterization of a Walker B mutation in human CLPX
        ... Although this mutant lacks ATPase activity, it retains the ability to mediate
        casein degradation by hCLPP.
      reference_section_type: ABSTRACT
- term:
    id: GO:0046034
    label: ATP metabolic process
  evidence_type: IDA
  original_reference_id: PMID:22710082
  qualifier: involved_in
  review:
    summary: This BP annotation derives from the ATPase assay, but CLPX biology is ATP-driven
      protein unfolding and proteolysis, not ATP metabolism per se. The ATPase activity
      is already captured as a molecular function.
    action: MARK_AS_OVER_ANNOTATED
    reason: Annotating CLPX to ATP metabolic process is a generic over-annotation; ATP hydrolysis
      is the energy source for its unfoldase function and is appropriately captured by the
      ATP hydrolysis activity MF term rather than an ATP-metabolism process.
    supported_by:
    - reference_id: PMID:25957689
      supporting_text: ClpX unfolds substrate proteins by ATP-driven translocation of the
        polypeptide chain through the central pore of its hexameric assembly.
      reference_section_type: RESULTS
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: IDA
  original_reference_id: PMID:10525407
  qualifier: located_in
  review:
    summary: Localization of the human Clp protease system to the mitochondrial matrix (this
      paper primarily characterizes the partner peptidase hClpP, which colocalizes with
      CLPX in the matrix).
    action: ACCEPT
    reason: Mitochondrial matrix is the core, experimentally supported compartment for CLPX
      and the ClpXP system.
    supported_by:
    - reference_id: PMID:10525407
      supporting_text: the mammalian homologue of ClpP is located in the mitochondrial matrix
        with a tendency to be found in association with the inner mitochondrial membrane.
      reference_section_type: ABSTRACT
- term:
    id: GO:0009368
    label: endopeptidase Clp complex
  evidence_type: IDA
  original_reference_id: PMID:11923310
  qualifier: part_of
  review:
    summary: CLPX assembles with CLPP into the ATP-dependent ClpXP protease; the generic
      Clp complex term is correct but less specific than the mitochondrial Clp complex
      term.
    action: KEEP_AS_NON_CORE
    reason: Correct complex membership; the mitochondrial endopeptidase Clp complex term
      (GO:0009841) is the more precise representation for human CLPX.
    supported_by:
    - reference_id: PMID:11923310
      supporting_text: Our results establish that human ClpX and ClpP constitute a bone
        fide ATP-dependent protease.
      reference_section_type: ABSTRACT
- term:
    id: GO:0009368
    label: endopeptidase Clp complex
  evidence_type: IDA
  original_reference_id: PMID:22710082
  qualifier: part_of
  review:
    summary: CLPX is part of the ClpXP protease complex with CLPP; the generic Clp complex
      parent is correct but less specific than the mitochondrial Clp complex term.
    action: KEEP_AS_NON_CORE
    reason: Correct complex membership; the mitochondrial endopeptidase Clp complex term
      (GO:0009841) more precisely captures human CLPX.
    supported_by:
    - reference_id: PMID:22710082
      supporting_text: it retains the ability to mediate casein degradation by hCLPP, in
        a fashion similar to the small molecule ClpP-activator, ADEP.
      reference_section_type: ABSTRACT
- term:
    id: GO:0004176
    label: ATP-dependent peptidase activity
  evidence_type: IDA
  original_reference_id: PMID:16115876
  qualifier: contributes_to
  review:
    summary: Peptidase activity resides in CLPP; CLPX contributes by ATP-dependent binding
      that drives assembly of the active CLPP tetradecamer and greatly increases its peptidase
      activity. The contributes_to qualifier is appropriate.
    action: ACCEPT
    reason: The contributes_to qualifier correctly attributes the ATP-dependent peptidase
      activity to the ClpXP complex, with CLPX providing the ATP-dependent activation/assembly.
    supported_by:
    - reference_id: PMID:16115876
      supporting_text: The hClpXP complex has protease activity and greatly increased peptidase
        activity, indicating that interaction with hClpX affects the conformation of the
        hClpP catalytic active site.
      reference_section_type: ABSTRACT
- term:
    id: GO:0030163
    label: protein catabolic process
  evidence_type: IDA
  original_reference_id: PMID:16115876
  qualifier: involved_in
  review:
    summary: CLPX, as the ATPase/unfoldase of ClpXP, is required for ATP-dependent protein
      degradation in the mitochondrial matrix (protein quality control).
    action: ACCEPT
    reason: Protein catabolic process via the ClpXP protease is a core biological process
      for CLPX.
    supported_by:
    - reference_id: PMID:16115876
      supporting_text: The hClpXP complex has protease activity and greatly increased peptidase
        activity.
      reference_section_type: ABSTRACT
- term:
    id: GO:0042645
    label: mitochondrial nucleoid
  evidence_type: IDA
  original_reference_id: PMID:18063578
  qualifier: located_in
  review:
    summary: CLPX was identified among mitochondrial nucleoid-associated proteins in a nucleoid
      proteomics/cross-linking study; this reflects a regulatory association (control of
      mtDNA nucleoid distribution via TFAM) rather than the core matrix localization.
    action: KEEP_AS_NON_CORE
    reason: Nucleoid association is a genuine but secondary localization tied to a regulatory
      role; the core compartment is the mitochondrial matrix.
    supported_by:
    - reference_id: PMID:18063578
      supporting_text: A set of core nucleoid proteins is found in both native and cross-linked
        nucleoids, including 13 proteins with known roles in mtDNA transactions. Several
        other metabolic proteins and chaperones identified in native nucleoids
      reference_section_type: ABSTRACT
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: Sequence-similarity transfer (from mouse Clpx) of ATP binding, consistent with
      the AAA+ P-loop module and the experimentally supported ATP-dependent hexamerization.
    action: ACCEPT
    reason: ATP binding is a core molecular function of CLPX.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Homohexamer that forms a ring structure; this hexamerization requires
        ATP binding.
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ISS transfer from mouse Clpx of inner-membrane localization. CLPX is a soluble
      matrix protein that may associate peripherally with the matrix face of the inner
      membrane, but the inner membrane is not its primary compartment.
    action: MARK_AS_OVER_ANNOTATED
    reason: CLPX is a matrix-soluble AAA+ protein; inner-membrane localization is at best
      a weak/peripheral association inferred by similarity and is not supported as a core
      or direct location. The matrix annotation captures the biology.
    supported_by:
    - reference_id: file:human/CLPX/CLPX-uniprot.txt
      supporting_text: Mitochondrion matrix, mitochondrion nucleoid
      reference_section_type: DATABASE_ENTRY
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:11923310
  qualifier: enables
  review:
    summary: This IPI reflects the CLPX–CLPP interaction (WITH/FROM CLPP, Q16740). Bare
      "protein binding" is uninformative; the specific functional consequence is captured
      by peptidase activator activity and ClpXP complex membership.
    action: MARK_AS_OVER_ANNOTATED
    reason: Bare protein binding does not convey function; the CLPX–CLPP interaction underlying
      it is more informatively represented by peptidase activator activity and the Clp
      complex part_of terms.
    supported_by:
    - reference_id: PMID:11923310
      supporting_text: Our results establish that human ClpX and ClpP constitute a bone
        fide ATP-dependent protease.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:11003706
  qualifier: located_in
  review:
    summary: Cloning and characterization of human CLPX showed an N-terminal mitochondrial
      transit peptide and import of the tagged protein into mitochondria; correct but less
      specific than mitochondrial matrix.
    action: KEEP_AS_NON_CORE
    reason: Correct mitochondrial localization, subsumed by the more precise mitochondrial
      matrix annotation.
    supported_by:
    - reference_id: PMID:11003706
      supporting_text: The polypeptide contains an N-terminal putative mitochondrial transit
        peptide, and expression of a full-length ClpX cDNA tagged at its C-terminus (Myc-His)
        shows that the polypeptide is transported into mitochondria.
      reference_section_type: ABSTRACT
- term:
    id: GO:0016504
    label: peptidase activator activity
  evidence_type: IDA
  original_reference_id: PMID:11923310
  qualifier: enables
  review:
    summary: CLPX activates the CLPP peptidase in an ATP-dependent manner, conferring proteolytic
      activity and greatly increased peptidase activity on the ClpXP complex; this captures
      the specific functional meaning of the CLPX–CLPP interaction.
    action: ACCEPT
    reason: Peptidase activator activity is an informative, experimentally supported molecular
      function describing how CLPX enables CLPP catalysis.
    supported_by:
    - reference_id: PMID:11923310
      supporting_text: hClpXP displays both ATP-dependent proteolytic activity and ATP-
        or ATPgammaS-dependent peptidase activity.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16115876
  qualifier: enables
  review:
    summary: This IPI reflects the CLPX–CLPP interaction (WITH/FROM CLPP, Q16740). Bare
      "protein binding" is uninformative; the functional consequence (CLPP activation/assembly)
      is captured elsewhere.
    action: MARK_AS_OVER_ANNOTATED
    reason: Bare protein binding does not describe a function; the CLPX–CLPP interaction
      is better represented by peptidase activator activity and the mitochondrial Clp complex
      part_of term.
    supported_by:
    - reference_id: PMID:16115876
      supporting_text: In the presence of ATP, hClpX interacts with hClpP forming a complex.
      reference_section_type: ABSTRACT
- term:
    id: GO:0009841
    label: mitochondrial endopeptidase Clp complex
  evidence_type: IDA
  original_reference_id: PMID:16115876
  qualifier: part_of
  review:
    summary: CLPX is a component of the mitochondrial ClpXP protease; in the presence of
      ATP it drives assembly of CLPP heptamers into the active tetradecamer, forming the
      double-ring complex flanked by CLPX hexamers.
    action: ACCEPT
    reason: This is the most precise and correct complex annotation for human CLPX, representing
      its core structural/functional context.
    supported_by:
    - reference_id: PMID:16115876
      supporting_text: Electron microscopy confirmed that the complex consisted of a double
        ring of hClpP with an hClpX ring axially aligned on each end.
      reference_section_type: ABSTRACT
- term:
    id: GO:0016504
    label: peptidase activator activity
  evidence_type: IDA
  original_reference_id: PMID:16115876
  qualifier: enables
  review:
    summary: CLPX exerts an allosteric, ATP-dependent effect on CLPP that stabilizes the
      active tetradecamer and greatly increases peptidase activity, directly demonstrating
      peptidase activator activity.
    action: ACCEPT
    reason: Peptidase activator activity is an experimentally supported, informative molecular
      function of CLPX toward CLPP.
    supported_by:
    - reference_id: PMID:16115876
      supporting_text: hClpX must exert an allosteric effect on hClpP to promote a conformation
        that stabilizes the tetradecamer.
      reference_section_type: ABSTRACT
- term:
    id: GO:0006783
    label: heme biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:25957689
  qualifier: involved_in
  review:
    summary: CLPX promotes heme biosynthesis by ATP-dependent activation of ALAS, the first
      and rate-limiting enzyme of the pathway, via accelerated incorporation of the PLP
      cofactor. This is a well-established but CLPP-independent chaperone role of CLPX that
      is not captured in the current GOA.
    action: NEW
    reason: The CLPX role in stimulating heme biosynthesis through ALAS activation is strongly
      supported experimentally (yeast to mammals) and underlies the human EPP2 disease phenotype;
      it should be annotated as a biological process for CLPX.
    supported_by:
    - reference_id: PMID:25957689
      supporting_text: mtClpX directly stimulates ALA synthase in vitro by catalyzing incorporation
        of its cofactor, pyridoxal phosphate.
      reference_section_type: ABSTRACT
    - reference_id: PMID:28874591
      supporting_text: the mitochondrial AAA+ unfoldase ClpX promotes heme biosynthesis by
        activation of δ-aminolevulinate synthase (ALAS), which catalyzes the first step
        of heme synthesis.
      reference_section_type: ABSTRACT
core_functions:
- description: CLPX is the ATP-dependent unfoldase subunit of the mitochondrial ClpXP protease.
    It recognizes specific matrix protein substrates and, using ATP binding and hydrolysis,
    unfolds them and translocates the polypeptide through its hexameric pore into the CLPP
    proteolytic chamber for degradation, supporting mitochondrial protein quality control.
  molecular_function:
    id: GO:0016887
    label: ATP hydrolysis activity
  directly_involved_in:
  - id: GO:0030163
    label: protein catabolic process
  - id: GO:0006508
    label: proteolysis
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  in_complex:
    id: GO:0009841
    label: mitochondrial endopeptidase Clp complex
  supported_by:
  - reference_id: file:human/CLPX/CLPX-uniprot.txt
    supporting_text: ATP-dependent chaperone that functions as an unfoldase. As part of
      the ClpXP protease complex, it recognizes specific protein substrates, unfolds them
      using energy derived from ATP hydrolysis, and then translocates them to the proteolytic
      subunit (CLPP) of the ClpXP complex for degradation.
    reference_section_type: DATABASE_ENTRY
  - reference_id: PMID:22710082
    supporting_text: human CLPXP exhibits a similar mode of substrate recognition and is
      deregulated by ADEPs.
    reference_section_type: ABSTRACT
- description: CLPX activates the CLPP peptidase. In an ATP-dependent manner it binds CLPP
    heptamers, drives their assembly into the active tetradecamer, and allosterically increases
    CLPP peptidase activity, providing the regulatory ATPase that gives the ClpXP complex
    its substrate specificity.
  molecular_function:
    id: GO:0016504
    label: peptidase activator activity
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  in_complex:
    id: GO:0009841
    label: mitochondrial endopeptidase Clp complex
  supported_by:
  - reference_id: PMID:16115876
    supporting_text: hClpX must exert an allosteric effect on hClpP to promote a conformation
      that stabilizes the tetradecamer.
    reference_section_type: ABSTRACT
  - reference_id: PMID:11923310
    supporting_text: substrate selection, which differs between human and E. coli ClpX,
      is dependent solely on the Clp ATPase.
    reference_section_type: ABSTRACT
- description: Independently of CLPP-coupled degradation, CLPX acts as an ATP-dependent chaperone
    that remodels and activates δ-aminolevulinate synthase (ALAS) by accelerating incorporation
    of the pyridoxal 5'-phosphate cofactor, thereby stimulating the first, rate-limiting
    step of heme biosynthesis and supporting erythropoiesis. Mutations affecting CLPX ATPase
    activity dysregulate ALAS and cause erythropoietic protoporphyria.
  molecular_function:
    id: GO:0140662
    label: ATP-dependent protein folding chaperone
  directly_involved_in:
  - id: GO:0006783
    label: heme biosynthetic process
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:25957689
    supporting_text: mtClpX directly stimulates ALA synthase in vitro by catalyzing incorporation
      of its cofactor, pyridoxal phosphate.
    reference_section_type: ABSTRACT
  - reference_id: PMID:28874591
    supporting_text: We previously discovered that the mitochondrial AAA+ unfoldase ClpX
      promotes heme biosynthesis by activation of δ-aminolevulinate synthase (ALAS), which
      catalyzes the first step of heme synthesis.
    reference_section_type: ABSTRACT
proposed_new_terms: []
suggested_questions:
- question: Beyond ALAS, what is the full repertoire of physiological CLPXP substrates in
    the human mitochondrial matrix, and which are degraded versus remodeled/activated?
  experts:
  - Baker TA
  - Dougan DA
- question: Is the CLPP-independent chaperone/unfoldase activity of CLPX (e.g., ALAS activation,
    nucleoid/TFAM regulation) significant enough in human cells to warrant its own MF/BP
    annotations distinct from the ClpXP proteolytic role?
  experts:
  - Kardon JR
  - Paw BH
suggested_experiments:
- hypothesis: CLPX has additional matrix substrates whose degradation or remodeling depends
    on its ATP-driven translocation activity.
  description: Use trapping mutants (Walker B E359A or pore-1 substitutions) to capture and
    identify CLPX-bound substrates by co-immunoprecipitation/mass spectrometry in human
    mitochondria, comparing WT, ATPase-dead, and translocation-defective variants.
  experiment_type: substrate-trapping proteomics
- hypothesis: The balance between CLPX-mediated ALAS activation (PLP incorporation) and ALAS
    turnover determines heme output and EPP2 phenotype severity.
  description: Reconstitute apo-ALAS2 activation and degradation with purified human CLPX/CLPXP
    (WT and G298D), quantifying PLP incorporation, ALAS unfolding/degradation, and ALA/PPIX
    levels in erythroid cell models.
  experiment_type: in vitro reconstitution and erythroid cell metabolic assay
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- 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: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:10525407
  title: Mitochondrial localization and oligomeric structure of HClpP, the human homologue
    of E. coli ClpP.
  findings:
  - statement: The mammalian homologue of ClpP localizes to the mitochondrial matrix, with
      a tendency to associate with the inner mitochondrial membrane; mitochondrial ClpP
      forms two heptameric rings.
    reference_section_type: ABSTRACT
- id: PMID:11003706
  title: Human and mouse mitochondrial orthologs of bacterial ClpX.
  findings:
  - statement: Human CLPX encodes a 633-aa precursor with an N-terminal mitochondrial transit
      peptide; the tagged protein is imported into mitochondria and contains an AAA+ domain
      and a C4 zinc-finger motif. Expression is highest in skeletal muscle and heart.
    reference_section_type: ABSTRACT
- id: PMID:11923310
  title: Functional proteolytic complexes of the human mitochondrial ATP-dependent
    protease, hClpXP.
  findings:
  - statement: hClpX forms a hexameric ring and hClpP a heptameric ring; together with ATP
      they form a symmetry-mismatched ClpXP complex with ATP-dependent proteolytic and peptidase
      activity. Substrate selection depends solely on the Clp ATPase (CLPX).
    reference_section_type: ABSTRACT
- id: PMID:15522782
  title: Crystallography and mutagenesis point to an essential role for the N-terminus
    of human mitochondrial ClpP.
  findings:
  - statement: The N-terminal peptides of CLPP line the axial channel, positioned to interact
      with unfolded substrates translocated by the associated CLPX chaperone; the CLPX–CLPP
      interaction is essential for ClpX-mediated protein and peptide degradation.
    reference_section_type: ABSTRACT
- id: PMID:16115876
  title: Human mitochondrial ClpP is a stable heptamer that assembles into a tetradecamer
    in the presence of ClpX.
  findings:
  - statement: In the presence of ATP, hClpX binds hClpP and allosterically drives assembly
      of the active tetradecamer, conferring protease activity and greatly increased peptidase
      activity; CLPX thus regulates appearance of CLPP peptidase activity.
    reference_section_type: ABSTRACT
- id: PMID:18063578
  title: The layered structure of human mitochondrial DNA nucleoids.
  findings:
  - statement: CLPX is among the chaperones/metabolic proteins identified in mitochondrial
      DNA nucleoid preparations, consistent with a nucleoid-associated localization.
    reference_section_type: ABSTRACT
- id: PMID:22710082
  title: Substrate recognition and processing by a Walker B mutant of the human mitochondrial
    AAA+ protein CLPX.
  findings:
  - statement: A Walker B mutant (E359A) of human CLPX lacks ATPase activity but still mediates
      casein degradation by hCLPP; most model substrates are recognized by the N-terminal
      domain, and human CLPXP is deregulated by ADEPs.
    reference_section_type: ABSTRACT
- id: PMID:25957689
  title: Mitochondrial ClpX Activates a Key Enzyme for Heme Biosynthesis and Erythropoiesis.
  findings:
  - statement: Mitochondrial ClpX directly activates ALA synthase (ALAS) by accelerating
      ATP-dependent incorporation of the pyridoxal phosphate cofactor into the apoenzyme,
      using its central polypeptide-translocating (unfoldase) activity; this activity is
      conserved from yeast to mammals and is required for efficient erythropoiesis.
    reference_section_type: RESULTS
- id: PMID:28874591
  title: Mutation in human CLPX elevates levels of δ-aminolevulinate synthase and
    protoporphyrin IX to promote erythropoietic protoporphyria.
  findings:
  - statement: A dominant p.Gly298Asp mutation in the CLPX ATPase active site inactivates
      ATPase activity; mutant/WT coassembly yields reduced-activity enzyme that increases
      ALAS stability and ALA/PPIX levels, causing erythropoietic protoporphyria (EPP2).
      CLPX both activates and promotes turnover of ALAS.
    reference_section_type: ABSTRACT
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings:
  - statement: CLPX is identified as part of the high-confidence human mitochondrial proteome.
    reference_section_type: RESULTS
- id: Reactome:R-HSA-9838035
  title: CLPXP binds mitochondrial matrix proteins
  findings: []
- id: Reactome:R-HSA-9838289
  title: CLPXP degrades mitochondrial matrix proteins
  findings: []