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 Ξ΄-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).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016887 ATP hydrolysis activity | IBA GO_REF:0000033 | ACCEPT | 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. Reason: ATP hydrolysis activity is a core molecular function of CLPX, directly demonstrated for the human protein and conserved across orthologs. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Reaction=ATP + H2O = ADP + phosphate + H(+); ... EC=3.6.4.10 |
| GO:0005524 ATP binding | IBA GO_REF:0000033 | ACCEPT | Summary: CLPX binds ATP via its P-loop AAA+ module; ATP binding is required for hexamer assembly and for substrate engagement. Reason: ATP binding is a core molecular function consistent with the AAA+ architecture and the demonstrated ATP-dependent hexamerization of CLPX. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Homohexamer that forms a ring structure; this hexamerization requires ATP binding. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based transfer of ATP binding, consistent with the experimentally supported IBA/ISS annotations for the same activity. Reason: Redundant with the IBA ATP binding annotation but correct; ATP binding is core to CLPX function. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Homohexamer that forms a ring structure; this hexamerization requires ATP binding. |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: CLPX is a mitochondrial protein; this IEA captures the broad mitochondrial localization, which is more precisely the mitochondrial matrix. Reason: The mitochondrion term is correct but less specific than mitochondrial matrix, which is the experimentally supported and core compartment for CLPX. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion ... Mitochondrion matrix, mitochondrion nucleoid |
| GO:0006457 protein folding | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: PMID:25957689 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. |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated transfer of ATP hydrolysis activity, redundant with the experimentally supported IDA/EXP/IBA annotations. Reason: ATP hydrolysis activity is a core, experimentally demonstrated molecular function of CLPX. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Reaction=ATP + H2O = ADP + phosphate + H(+); ... EC=3.6.4.10 |
| GO:0042645 mitochondrial nucleoid | IEA GO_REF:0000044 | KEEP AS NON CORE | 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. Reason: Nucleoid association is genuine but represents a peripheral/regulatory localization; the core compartment is the mitochondrial matrix where CLPX performs unfolding and proteolysis. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Mitochondrion matrix, mitochondrion nucleoid |
| GO:0140662 ATP-dependent protein folding chaperone | IEA GO_REF:0000002 | ACCEPT | 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. 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. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt ATP-dependent chaperone that functions as an unfoldase. PMID:25957689 Mcx1 accelerates formation of active Hem1 by stimulating cofactor binding to the apoenzyme. |
| GO:0005515 protein binding | IPI PMID:15522782 Crystallography and mutagenesis point to an essential role f... | MARK AS OVER ANNOTATED | Summary: This IPI reflects the CLPXβCLPP 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. Reason: Bare protein binding does not describe a function; the underlying CLPXβCLPP interaction is more informatively represented by peptidase activator activity and the ClpXP complex part_of terms. Supporting Evidence: PMID:15522782 positioned to interact with unfolded substrates translocated there by the associated ClpX chaperone. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Immunofluorescence-based localization of CLPX to mitochondria; correct but less specific than mitochondrial matrix. Reason: Mitochondrion is correct but subsumed by the more precise mitochondrial matrix localization, which is core. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | NAS PMID:15522782 Crystallography and mutagenesis point to an essential role f... | ACCEPT | Summary: CLPX is a soluble mitochondrial-matrix protein where it carries out ATP-dependent unfolding and feeds substrates to the matrix-localized CLPP peptidase. Reason: Mitochondrial matrix is the core, experimentally supported compartment for CLPX function. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Mitochondrion matrix, mitochondrion nucleoid |
| GO:0006508 proteolysis | IDA PMID:15522782 Crystallography and mutagenesis point to an essential role f... | ACCEPT | Summary: As the ATPase/unfoldase subunit of ClpXP, CLPX is required for ATP-dependent proteolysis; it unfolds and delivers substrates to CLPP for cleavage. 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. Supporting Evidence: PMID:11923310 hClpXP displays both ATP-dependent proteolytic activity and ATP- or ATPgammaS-dependent peptidase activity. |
| GO:0009368 endopeptidase Clp complex | IPI PMID:15522782 Crystallography and mutagenesis point to an essential role f... | KEEP AS NON CORE | 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. Reason: Correct complex membership, but the human-specific mitochondrial endopeptidase Clp complex term (GO:0009841) more precisely captures CLPX localization and identity. Supporting Evidence: PMID:11923310 Complexes of a double heptameric ring of hClpP with hexameric hClpX rings bound on each side are stable in the presence of ATP. |
| GO:0016887 ATP hydrolysis activity | EXP PMID:28874591 Mutation in human CLPX elevates levels of Ξ΄-aminolevulinate ... | ACCEPT | Summary: Experimental evidence that human CLPX hydrolyzes ATP; the EPP2 variant p.Gly298Asp inactivates ATPase activity, directly demonstrating this catalytic function. Reason: Directly demonstrated core catalytic activity of CLPX. Supporting Evidence: PMID:28874591 The mutation in CLPX inactivates its ATPase activity, resulting in coassembly of mutant and WT protomers to form an enzyme with reduced activity. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput mitochondrial proteome assignment of CLPX to mitochondria; correct but less specific than mitochondrial matrix. Reason: Correct compartment, but redundant with and less precise than the mitochondrial matrix annotation. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9838035 | ACCEPT | Summary: Reactome event placing CLPXP (and CLPX) in the mitochondrial matrix where it binds matrix protein substrates. Reason: Mitochondrial matrix is the core compartment for CLPX; consistent with experimental evidence. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Mitochondrion matrix, mitochondrion nucleoid |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9838289 | ACCEPT | Summary: Reactome event placing CLPXP in the mitochondrial matrix where it degrades matrix protein substrates. Reason: Mitochondrial matrix is the core compartment for CLPX; consistent with experimental evidence. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Mitochondrion matrix, mitochondrion nucleoid |
| GO:0016887 ATP hydrolysis activity | IDA PMID:22710082 Substrate recognition and processing by a Walker B mutant of... | ACCEPT | Summary: Direct biochemical characterization of human CLPX ATPase activity; the Walker B mutant (E359A) abolishes ATP hydrolysis, confirming this activity. Reason: Directly demonstrated core catalytic activity of CLPX. Supporting Evidence: PMID:22710082 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. |
| GO:0046034 ATP metabolic process | IDA PMID:22710082 Substrate recognition and processing by a Walker B mutant of... | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: PMID:25957689 ClpX unfolds substrate proteins by ATP-driven translocation of the polypeptide chain through the central pore of its hexameric assembly. |
| GO:0005759 mitochondrial matrix | IDA PMID:10525407 Mitochondrial localization and oligomeric structure of HClpP... | ACCEPT | 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). Reason: Mitochondrial matrix is the core, experimentally supported compartment for CLPX and the ClpXP system. Supporting Evidence: PMID:10525407 the mammalian homologue of ClpP is located in the mitochondrial matrix with a tendency to be found in association with the inner mitochondrial membrane. |
| GO:0009368 endopeptidase Clp complex | IDA PMID:11923310 Functional proteolytic complexes of the human mitochondrial ... | KEEP AS NON CORE | 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. Reason: Correct complex membership; the mitochondrial endopeptidase Clp complex term (GO:0009841) is the more precise representation for human CLPX. Supporting Evidence: PMID:11923310 Our results establish that human ClpX and ClpP constitute a bone fide ATP-dependent protease. |
| GO:0009368 endopeptidase Clp complex | IDA PMID:22710082 Substrate recognition and processing by a Walker B mutant of... | KEEP AS NON CORE | 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. Reason: Correct complex membership; the mitochondrial endopeptidase Clp complex term (GO:0009841) more precisely captures human CLPX. Supporting Evidence: PMID:22710082 it retains the ability to mediate casein degradation by hCLPP, in a fashion similar to the small molecule ClpP-activator, ADEP. |
| GO:0004176 ATP-dependent peptidase activity | IDA PMID:16115876 Human mitochondrial ClpP is a stable heptamer that assembles... | ACCEPT | 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. Reason: The contributes_to qualifier correctly attributes the ATP-dependent peptidase activity to the ClpXP complex, with CLPX providing the ATP-dependent activation/assembly. Supporting Evidence: PMID:16115876 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. |
| GO:0030163 protein catabolic process | IDA PMID:16115876 Human mitochondrial ClpP is a stable heptamer that assembles... | ACCEPT | Summary: CLPX, as the ATPase/unfoldase of ClpXP, is required for ATP-dependent protein degradation in the mitochondrial matrix (protein quality control). Reason: Protein catabolic process via the ClpXP protease is a core biological process for CLPX. Supporting Evidence: PMID:16115876 The hClpXP complex has protease activity and greatly increased peptidase activity. |
| GO:0042645 mitochondrial nucleoid | IDA PMID:18063578 The layered structure of human mitochondrial DNA nucleoids. | KEEP AS NON CORE | 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. Reason: Nucleoid association is a genuine but secondary localization tied to a regulatory role; the core compartment is the mitochondrial matrix. Supporting Evidence: PMID:18063578 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 |
| GO:0005524 ATP binding | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity transfer (from mouse Clpx) of ATP binding, consistent with the AAA+ P-loop module and the experimentally supported ATP-dependent hexamerization. Reason: ATP binding is a core molecular function of CLPX. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Homohexamer that forms a ring structure; this hexamerization requires ATP binding. |
| GO:0005743 mitochondrial inner membrane | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: file:human/CLPX/CLPX-uniprot.txt Mitochondrion matrix, mitochondrion nucleoid |
| GO:0005515 protein binding | IPI PMID:11923310 Functional proteolytic complexes of the human mitochondrial ... | MARK AS OVER ANNOTATED | Summary: This IPI reflects the CLPXβCLPP interaction (WITH/FROM CLPP, Q16740). Bare "protein binding" is uninformative; the specific functional consequence is captured by peptidase activator activity and ClpXP complex membership. Reason: Bare protein binding does not convey function; the CLPXβCLPP interaction underlying it is more informatively represented by peptidase activator activity and the Clp complex part_of terms. Supporting Evidence: PMID:11923310 Our results establish that human ClpX and ClpP constitute a bone fide ATP-dependent protease. |
| GO:0005739 mitochondrion | IDA PMID:11003706 Human and mouse mitochondrial orthologs of bacterial ClpX. | KEEP AS NON CORE | 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. Reason: Correct mitochondrial localization, subsumed by the more precise mitochondrial matrix annotation. Supporting Evidence: PMID:11003706 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. |
| GO:0016504 peptidase activator activity | IDA PMID:11923310 Functional proteolytic complexes of the human mitochondrial ... | ACCEPT | 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 CLPXβCLPP interaction. Reason: Peptidase activator activity is an informative, experimentally supported molecular function describing how CLPX enables CLPP catalysis. Supporting Evidence: PMID:11923310 hClpXP displays both ATP-dependent proteolytic activity and ATP- or ATPgammaS-dependent peptidase activity. |
| GO:0005515 protein binding | IPI PMID:16115876 Human mitochondrial ClpP is a stable heptamer that assembles... | MARK AS OVER ANNOTATED | Summary: This IPI reflects the CLPXβCLPP interaction (WITH/FROM CLPP, Q16740). Bare "protein binding" is uninformative; the functional consequence (CLPP activation/assembly) is captured elsewhere. Reason: Bare protein binding does not describe a function; the CLPXβCLPP interaction is better represented by peptidase activator activity and the mitochondrial Clp complex part_of term. Supporting Evidence: PMID:16115876 In the presence of ATP, hClpX interacts with hClpP forming a complex. |
| GO:0009841 mitochondrial endopeptidase Clp complex | IDA PMID:16115876 Human mitochondrial ClpP is a stable heptamer that assembles... | ACCEPT | 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. Reason: This is the most precise and correct complex annotation for human CLPX, representing its core structural/functional context. Supporting Evidence: PMID:16115876 Electron microscopy confirmed that the complex consisted of a double ring of hClpP with an hClpX ring axially aligned on each end. |
| GO:0016504 peptidase activator activity | IDA PMID:16115876 Human mitochondrial ClpP is a stable heptamer that assembles... | ACCEPT | 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. Reason: Peptidase activator activity is an experimentally supported, informative molecular function of CLPX toward CLPP. Supporting Evidence: PMID:16115876 hClpX must exert an allosteric effect on hClpP to promote a conformation that stabilizes the tetradecamer. |
| GO:0006783 heme biosynthetic process | IDA PMID:25957689 Mitochondrial ClpX Activates a Key Enzyme for Heme Biosynthe... | NEW | 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. 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. Supporting Evidence: PMID:25957689 mtClpX directly stimulates ALA synthase in vitro by catalyzing incorporation of its cofactor, pyridoxal phosphate. PMID:28874591 the mitochondrial AAA+ unfoldase ClpX promotes heme biosynthesis by activation of Ξ΄-aminolevulinate synthase (ALAS), which catalyzes the first step of heme synthesis. |
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Download this section (compressed HTML)Q: Beyond ALAS, what is the full repertoire of physiological CLPXP substrates in the human mitochondrial matrix, and which are degraded versus remodeled/activated?
Suggested experts: Baker TA, Dougan DA
Q: 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?
Suggested experts: Kardon JR, Paw BH
Experiment: 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.
Hypothesis: CLPX has additional matrix substrates whose degradation or remodeling depends on its ATP-driven translocation activity.
Type: substrate-trapping proteomics
Experiment: 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.
Hypothesis: The balance between CLPX-mediated ALAS activation (PLP incorporation) and ALAS turnover determines heme output and EPP2 phenotype severity.
Type: in vitro reconstitution and erythroid cell metabolic assay
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