CKMT1A

UniProt ID: P12532
Organism: Homo sapiens
Review Status: COMPLETE
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Gene Description

CKMT1A encodes the ubiquitous mitochondrial creatine kinase (U-MtCK / Mia-CK), a member of the ATP:guanido phosphotransferase (phosphagen kinase) family that reversibly catalyzes phosphoryl transfer between ATP and creatine (creatine + ATP = N-phosphocreatine + ADP + H+; EC 2.7.3.2). The protein is synthesized with an N-terminal mitochondrial transit peptide and, after import, assembles into homo-octamers (four homodimers) that reside in the mitochondrial intermembrane space as peripheral membrane proteins on the intermembrane (outer) face of the inner membrane. Octamers bind the inner-membrane phospholipid cardiolipin via an N-terminal region, can bridge the inner and outer membranes at contact sites, and are functionally coupled to the adenine nucleotide translocator and to porin/VDAC. Using ATP exported from oxidative phosphorylation, the enzyme loads high-energy phosphate onto creatine to make phosphocreatine, constituting the mitochondrial (phosphate-loading) limb of the phosphocreatine shuttle that buffers cytosolic ATP at sites of high, fluctuating energy demand such as heart, skeletal muscle and brain. Mitochondrial creatine kinase octamers also contribute to mitochondrial structure and to regulation of the mitochondrial permeability transition. In humans the protein is encoded by two near-identical duplicated genes, CKMT1A and CKMT1B, both represented by UniProt entry P12532; it is distinct from the muscle-restricted sarcomeric mitochondrial creatine kinase (CKMT2) and from the cytosolic isoenzymes (CKM, CKB).

Existing Annotations Review

GO Term Evidence Action Reason
GO:0004111 creatine kinase activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetically inferred creatine kinase activity. This is the precise, correct molecular function of CKMT1A and is the core enzymatic activity of the protein, consistent with the EC 2.7.3.2 assignment and the crystal/cryo-EM structures.
Reason: Creatine kinase activity is the defining, experimentally and structurally supported function of the mitochondrial creatine kinase family; this is a core function.
Supporting Evidence:
PMID:2914937
Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
GO:0005739 mitochondrion
IBA
GO_REF:0000033 		KEEP AS NON CORE
Summary: Phylogenetically inferred mitochondrial localization. Correct but non-specific; the protein is more precisely localized to the mitochondrial intermembrane space / intermembrane face of the inner membrane.
Reason: Accurate but superseded by the more specific intermembrane-space / inner-membrane location captured in core_functions; retained as a true but coarse statement.
GO:0046314 phosphocreatine biosynthetic process
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetically inferred involvement in phosphocreatine biosynthesis. This is the direct biological process carried out by the creatine kinase reaction (forming N-phosphocreatine) and is a core process for the mitochondrial enzyme, which supplies the phosphate-loading limb of the phosphocreatine shuttle.
Reason: The creatine kinase reaction produces phosphocreatine; this process annotation is directly entailed by the molecular function and is core.
Supporting Evidence:
PMID:2914937
support the phosphocreatine shuttle hypothesis
GO:0003824 catalytic activity
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: Root-level catalytic activity term from InterPro2GO. Correct but maximally generic; the specific creatine kinase activity (GO:0004111) is already annotated.
Reason: Uninformative parent of the specific GO:0004111 already present; adds no curation value.
GO:0004111 creatine kinase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Creatine kinase activity inferred electronically from the EC 2.7.3.2 / RHEA:17157 mapping. Same correct molecular function as the IBA and TAS annotations of this term.
Reason: Correct specific molecular function, redundant with but consistent with the IBA/TAS creatine kinase activity annotations; core.
GO:0005743 mitochondrial inner membrane
IEA
GO_REF:0000120 		ACCEPT
Summary: Electronically inferred inner-membrane localization from the UniProt subcellular-location keyword. Consistent with experimental evidence: the octamer is a peripheral membrane protein on the intermembrane (outer) face of the inner membrane and binds cardiolipin.
Reason: Matches the curated UniProt subcellular location (Mitochondrion inner membrane; peripheral; intermembrane side); correct compartment.
GO:0016301 kinase activity
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: Generic kinase activity from InterPro2GO. Correct but a non-specific parent of creatine kinase activity (GO:0004111).
Reason: Subsumed by the specific GO:0004111 already annotated; over-general.
GO:0016772 transferase activity, transferring phosphorus-containing groups
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: Generic phosphotransferase grouping term from InterPro2GO. Correct but a high-level parent of the specific creatine kinase activity.
Reason: Non-specific ancestor of GO:0004111; adds no value beyond the specific term.
GO:0016775 phosphotransferase activity, nitrogenous group as acceptor
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: Phosphotransferase with a nitrogenous (guanidino) acceptor — the family-level activity captured by InterPro2GO. Accurate (creatine is the nitrogenous acceptor) but still a parent of the specific GO:0004111.
Reason: Correct but subsumed by the specific creatine kinase activity already annotated.
GO:0046314 phosphocreatine biosynthetic process
IEA
GO_REF:0000002 		ACCEPT
Summary: Phosphocreatine biosynthetic process inferred electronically from InterPro. Same correct core process as the IBA annotation of this term.
Reason: Correct biological process directly entailed by creatine kinase activity; core, consistent with the IBA annotation.
GO:0005515 protein binding
IPI
PMID:32296183
A reference map of the human binary protein interactome. 		KEEP AS NON CORE
Summary: Bare protein-binding annotation from a high-throughput binary interactome map (HuRI). Not informative about molecular function; the relevant binding partners are better captured as specific interactions rather than as a generic MF term.
Reason: Per curation guidelines, bare 'protein binding' is uninformative as a molecular function and is not a core function; the underlying high-throughput interaction is retained but non-core.
GO:0005515 protein binding
IPI
PMID:32814053
Interactome Mapping Provides a Network of Neurodegenerative ... 		KEEP AS NON CORE
Summary: Bare protein-binding annotation from a neurodegeneration interactome-mapping study. Uninformative as a molecular function.
Reason: Generic 'protein binding' from a high-throughput screen; not a core molecular function.
GO:0005515 protein binding
IPI
PMID:33961781
Dual proteome-scale networks reveal cell-specific remodeling... 		KEEP AS NON CORE
Summary: Bare protein-binding annotation from a proteome-scale AP-MS network (BioPlex). Uninformative as a molecular function.
Reason: Generic 'protein binding' from a high-throughput screen; not a core molecular function.
GO:0005515 protein binding
IPI
PMID:40205054
Multimodal cell maps as a foundation for structural and func... 		KEEP AS NON CORE
Summary: Bare protein-binding annotation from a multimodal cell-mapping study. Uninformative as a molecular function.
Reason: Generic 'protein binding' from a high-throughput screen; not a core molecular function.
GO:0005739 mitochondrion
HTP
PMID:34800366
Quantitative high-confidence human mitochondrial proteome an... 		KEEP AS NON CORE
Summary: High-throughput proteomic detection of the protein in mitochondria. Correct compartment but non-specific relative to the intermembrane-space / inner-membrane location.
Reason: Accurate but coarse; the specific location is captured in core_functions.
GO:0005743 mitochondrial inner membrane
TAS
Reactome:R-HSA-200326 		ACCEPT
Summary: Reactome-asserted inner-membrane localization for the creatine kinase octamer reaction. Consistent with the peripheral, intermembrane-side inner-membrane localization of U-MtCK.
Reason: Matches curated localization (intermembrane face of the inner membrane); correct compartment.
GO:0004111 creatine kinase activity
TAS
PMID:2914937
Isolation and characterization of the gene and cDNA encoding... 		ACCEPT
Summary: Author-asserted creatine kinase activity from the paper that isolated the human mitochondrial CK gene/cDNA. This is the core, correct molecular function.
Reason: Traceable author statement of the defining enzymatic activity (EC 2.7.3.2); core.
Supporting Evidence:
PMID:2914937
Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
GO:0005739 mitochondrion
TAS
PMID:2914937
Isolation and characterization of the gene and cDNA encoding... 		KEEP AS NON CORE
Summary: Author-asserted mitochondrial localization. Correct but non-specific; the precise location is the intermembrane space / intermembrane face of the inner membrane.
Reason: Accurate but coarse compartment; the specific localization is captured in core_functions.
GO:0005758 mitochondrial intermembrane space
IC
UniProtKB:P12532 		NEW
Summary: The catalytically active CKMT1A octamer is a peripheral inner-membrane protein on the intermembrane side, i.e. it faces the mitochondrial intermembrane space. This is the precise compartment for its function and is proposed as a more specific localization than mitochondrion/inner membrane. GO:0005758 was not present in GOA; added here for consistency with the parallel CKMT2 review and with this review's core_functions.
Reason: More specific localization for the active octamer, inferred from the curated UniProt subcellular location (inner membrane, peripheral, intermembrane side).
Supporting Evidence:
UniProtKB:P12532
Mitochondrion inner membrane; Peripheral membrane

Core Functions

Reversibly phosphorylates creatine using OXPHOS-derived ATP to produce N-phosphocreatine, the phosphate-loading reaction of the mitochondrial creatine kinase octamer in the intermembrane space. This activity constitutes the mitochondrial limb of the phosphocreatine shuttle.

Molecular Function:
creatine kinase activity
Directly Involved In:
phosphocreatine biosynthetic process
Cellular Locations:
mitochondrial intermembrane space mitochondrial inner membrane
Supporting Evidence:
  • PMID:2914937
    Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
  • PMID:2914937
    support the phosphocreatine shuttle hypothesis

References

UniProtKB:P12532
Creatine kinase U-type, mitochondrial (human)
  • CKMT1A is a peripheral mitochondrial inner-membrane protein on the intermembrane side, consistent with the active octamer facing the mitochondrial intermembrane space.
    "Mitochondrion inner membrane; Peripheral membrane"
GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:2914937
Isolation and characterization of the gene and cDNA encoding human mitochondrial creatine kinase.
  • Isolated the human mitochondrial creatine kinase gene and cDNA (the ubiquitous CKMT1 locus); the gene contains 9 exons encoding a 416-residue protein including a 38-residue mitochondrial transit peptide, and the data support the phosphocreatine shuttle hypothesis.
    "support the phosphocreatine shuttle hypothesis"
PMID:10737943
Crystal structure of human ubiquitous mitochondrial creatine kinase.
PMID:32296183
A reference map of the human binary protein interactome.
PMID:32814053
Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
PMID:33961781
Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
PMID:34800366
Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.
PMID:40205054
Multimodal cell maps as a foundation for structural and functional genomics.
PMID:24522192
CKMT1 regulates the mitochondrial permeability transition pore in a process that provides evidence for alternative forms of the complex.
Reactome:R-HSA-200326
creatine + ATP => phosphocreatine + ADP [CK octamer]

Suggested Questions for Experts

Q: Is the regulation of the mitochondrial permeability transition by CKMT1 a direct structural/gating role of the octamer at contact sites, or an indirect consequence of altered local ATP/ADP and cardiolipin organization?

Q: Do CKMT1A and CKMT1B (the duplicated genes encoding identical protein) show any tissue- or context-specific differences in expression or regulation despite producing the same enzyme?

Suggested Experiments

Experiment: Reconstitute purified CKMT1 octamers vs. dimer-stabilized mutants with cardiolipin-containing proteoliposomes containing ANT/VDAC and measure ATP/ADP channeling and phosphocreatine production relative to bulk substrate.

Hypothesis: Octamer formation and cardiolipin binding are required for functional coupling of CKMT1 to the adenine nucleotide translocator and for efficient phosphocreatine export.

Type: biochemical reconstitution

📚 Additional Documentation

Notes

(CKMT1A-notes.md)

CKMT1A (P12532) — Creatine kinase U-type, mitochondrial (ubiquitous, U-MtCK / Mia-CK)

Identity and gene duplication

  • UniProt P12532 (KCRU_HUMAN), "Creatine kinase U-type, mitochondrial"; EC 2.7.3.2; AltNames "Acidic-type
    mitochondrial creatine kinase (Mia-CK)" and "Ubiquitous mitochondrial creatine kinase (U-MtCK)".
  • The UniProt entry P12532 covers BOTH genes CKMT1A (HGNC:31736) and CKMT1B (HGNC:1995): the GN block lists
    Name=CKMT1A; Synonyms=CKMT ... and ... Name=CKMT1B; Synonyms=CKMT. These are near-identical duplicated
    genes on chromosome 15 encoding the same 417-aa protein (precursor with a 1..39 mitochondrial transit peptide;
    mature chain 40..417) [UniProt P12532].
  • One of two mitochondrial CK genes in human; the other is CKMT2 (P17540, sarcomeric, S-MtCK). The cytosolic CK
    isoforms are CKM (muscle) and CKB (brain).

Catalytic function

  • Creatine kinase: reversibly catalyzes phosphoryl transfer between ATP and creatine:
    creatine + ATP = N-phosphocreatine + ADP + H(+) (Rhea:17157, EC 2.7.3.2) [UniProt P12532 CATALYTIC ACTIVITY].
  • "Creatine kinase isoenzymes play a central role in energy transduction in tissues with large, fluctuating energy
    demands, such as skeletal muscle, heart, brain and spermatozoa." [UniProt P12532 FUNCTION].
  • The mitochondrial isoforms reside in the intermembrane space and use OXPHOS-derived ATP to phosphorylate creatine:
    "uMtCK and sMtCK ... are octameric enzymes residing in the intermembrane space of mitochondria that catalyze the
    reversible transfer of the phosphoryl group from ATP generated by mitochondria to creatine (Cr) to form ADP and
    phosphocreatine (pCr)" [bioRxiv 2024.06.18.598884, mitochondrial CK structure review framing].
  • Member of the ATP:guanido phosphotransferase (phosphagen kinase) family; two phosphagen-kinase domains (N-terminal
    45..131, C-terminal 158..400); multiple ATP-binding residues annotated [UniProt P12532 DOMAIN/BINDING].
  • The 1989 paper that isolated the human gene/cDNA encoding mitochondrial CK ("placental MtCK", i.e. the ubiquitous
    CKMT1 locus): "The human placental MtCK gene contains 9 exons encoding 416 amino acids, including a 38-amino acid
    transit peptide" and the data "support the phosphocreatine shuttle hypothesis" [PMID:2914937 abstract].

Octamer assembly and structure

  • "Exists as an octamer composed of four MTCK homodimers." [UniProt P12532 SUBUNIT; ECO:0000269|PubMed:10737943].
  • Crystal structure of human U-MtCK solved at 2.7 Å (PDB 1QK1) [PMID:10737943; UniProt PDB cross-refs].
  • Mitochondrial CK occurs in two oligomeric forms (dimers and octamers), unlike the exclusively dimeric cytosolic CKs;
    only the octamer bridges the inner and outer mitochondrial membranes [ScienceDirect "Mitochondrial Creatine Kinase"
    overview; Biochemistry 1997 bi961838v octamer membrane-binding/cross-linking].
  • Recent cryo-EM structures of the human octamer (PDB 9B04/9B05/9B0T/9B0U/9B14/9B16, ~2.2–2.9 Å) define substrate
    binding, catalysis and covalent-inhibitor binding for the breast-cancer-target mitochondrial CK [bioRxiv
    2024.06.18.598884; UniProt PDB/EMDB cross-refs].

Localization, cardiolipin binding, membrane bridging

  • Subcellular location: "Mitochondrion inner membrane; Peripheral membrane protein; Intermembrane side."
    i.e. it faces the intermembrane space [UniProt P12532 SUBCELLULAR LOCATION].
  • "Mitochondrial creatine kinase binds cardiolipin." [UniProt P12532 MISCELLANEOUS]; an N-terminal cardiolipin-binding
    region (residues 40..64) is annotated [UniProt P12532 REGION].
  • It localizes between the inner and outer mitochondrial membranes, bound to the cardiolipin-rich inner-membrane leaflet
    [ScienceDirect overview]. Octamer–membrane interaction with cardiolipin liposomes is a two-step adsorption/insertion
    process [Eur Biophys J 2010, s00249-010-0600-4].
  • The octamer can simultaneously bind two membranes and promote cardiolipin-dependent lipid transfer between liposomes
    mimicking contact-site composition (≈30 mol% cardiolipin); transfer is lost when cardiolipin is replaced by
    phosphatidylglycerol [PMC1697860 "Novel Lipid Transfer Property of Two Mitochondrial Proteins that Bridge the Inner
    and Outer Membranes"].

Functional coupling to ANT / VDAC and the phosphocreatine shuttle

  • MtCK octamers form contact sites that functionally couple oxidative phosphorylation to phosphocreatine export: at the
    inner membrane the enzyme is coupled (via cardiolipin) to the adenine nucleotide translocator (ANT/SLC25A4), receiving
    matrix-derived ATP and returning ADP for re-phosphorylation; at the outer membrane it associates with VDAC/porin to
    release phosphocreatine to the cytosol [ScienceDirect overview; review framing of the phosphocreatine circuit].
  • This is the "phosphate-loading" limb of the phosphocreatine (creatine/PCr) shuttle, replenishing the cytosolic PCr pool
    that buffers ATP at sites of high, fluctuating energy demand [PMID:2914937 "support the phosphocreatine shuttle
    hypothesis"; UniProt FUNCTION].

Mitochondrial permeability transition

  • CKMT1 regulates the mitochondrial permeability transition pore (PTP): "Mitochondrial creatine kinase-1 (CKMT1) is a
    universal and functionally necessary gatekeeper of the permeability transition pore (PT-pore), as its depletion induces
    mitochondrial depolarization and apoptotic cell death"; this regulation appears independent of the classical CypD/VDAC1
    components, indicating an alternative PTP form [PMID:24522192, J Cell Sci 2014].

Tissue expression

  • Ubiquitous expression (hence "U-MtCK"); the 1989 RNA blot found abundant MtCK mRNA in adult ventricle and skeletal
    muscle, lower in placenta and small intestine PMID:2914937. Human Protein Atlas: tissue-enhanced in brain, esophagus,
    intestine; Bgee: expressed in cerebellum and ~92 other tissues [UniProt P12532 HPA/Bgee cross-refs]. Distinct from the
    sarcomeric S-MtCK (CKMT2), which is restricted to heart/skeletal muscle.

Interactions (IntAct, from binary/AP-MS interactome screens)

  • UniProt lists interactions including CKM (P06732), CKMT2 (P17540), ASB9 (Q96DX5), RXRG (P48443), NUTF2 (P61970),
    HPCA (P84074), OCM (P0CE72), PPP1R16A (Q96I34), CTNNA3, JPH3, PSMC3, WFS1 [UniProt P12532 INTERACTION]. ASB9 is a
    known substrate-recognition subunit of an E3 ligase that targets creatine kinases — biologically plausible.
  • GOA protein binding (GO:0005515) IPI annotations derive from high-throughput interactome maps: PMID:32296183 (HuRI
    binary interactome), PMID:33961781 (BioPlex dual proteome network), PMID:32814053 (neurodegeneration interactome),
    PMID:40205054 (multimodal cell maps). These are uninformative as molecular-function statements.

Annotation review orientation

  • Core, well-supported: creatine kinase activity (GO:0004111, MF), phosphocreatine biosynthetic process (GO:0046314, BP),
    mitochondrial intermembrane space / inner-membrane intermembrane side localization (GO:0005758 / GO:0005743 CC).
  • IEA grandparent MF terms (catalytic activity GO:0003824; kinase activity GO:0016301; transferase transferring
    phosphorus GO:0016772; phosphotransferase nitrogenous acceptor GO:0016775) are correct but generic — over-annotations
    relative to the specific GO:0004111.
  • protein binding (GO:0005515) IPI entries: keep as non-core; uninformative bare binding from HT screens.

📄 View Raw YAML

 
id: P12532
gene_symbol: CKMT1A
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  CKMT1A encodes the ubiquitous mitochondrial creatine kinase (U-MtCK / Mia-CK), a member of
  the ATP:guanido phosphotransferase (phosphagen kinase) family that reversibly catalyzes
  phosphoryl transfer between ATP and creatine (creatine + ATP = N-phosphocreatine + ADP + H+;
  EC 2.7.3.2). The protein is synthesized with an N-terminal mitochondrial transit peptide and,
  after import, assembles into homo-octamers (four homodimers) that reside in the mitochondrial
  intermembrane space as peripheral membrane proteins on the intermembrane (outer) face of the
  inner membrane. Octamers bind the inner-membrane phospholipid cardiolipin via an N-terminal
  region, can bridge the inner and outer membranes at contact sites, and are functionally coupled
  to the adenine nucleotide translocator and to porin/VDAC. Using ATP exported from oxidative
  phosphorylation, the enzyme loads high-energy phosphate onto creatine to make phosphocreatine,
  constituting the mitochondrial (phosphate-loading) limb of the phosphocreatine shuttle that
  buffers cytosolic ATP at sites of high, fluctuating energy demand such as heart, skeletal muscle
  and brain. Mitochondrial creatine kinase octamers also contribute to mitochondrial structure and
  to regulation of the mitochondrial permeability transition. In humans the protein is encoded by
  two near-identical duplicated genes, CKMT1A and CKMT1B, both represented by UniProt entry P12532;
  it is distinct from the muscle-restricted sarcomeric mitochondrial creatine kinase (CKMT2) and
  from the cytosolic isoenzymes (CKM, CKB).
alternative_products:
- name: '1'
  id: P12532-1
- name: '2'
  id: P12532-2
  sequence_note: VSP_038045
existing_annotations:
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetically inferred creatine kinase activity. This is the precise, correct molecular
      function of CKMT1A and is the core enzymatic activity of the protein, consistent with the
      EC 2.7.3.2 assignment and the crystal/cryo-EM structures.
    action: ACCEPT
    reason: >-
      Creatine kinase activity is the defining, experimentally and structurally supported function
      of the mitochondrial creatine kinase family; this is a core function.
    supported_by:
    - reference_id: PMID:2914937
      supporting_text: Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetically inferred mitochondrial localization. Correct but non-specific; the protein is
      more precisely localized to the mitochondrial intermembrane space / intermembrane face of the
      inner membrane.
    action: KEEP_AS_NON_CORE
    reason: >-
      Accurate but superseded by the more specific intermembrane-space / inner-membrane location
      captured in core_functions; retained as a true but coarse statement.
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Phylogenetically inferred involvement in phosphocreatine biosynthesis. This is the direct
      biological process carried out by the creatine kinase reaction (forming N-phosphocreatine) and
      is a core process for the mitochondrial enzyme, which supplies the phosphate-loading limb of
      the phosphocreatine shuttle.
    action: ACCEPT
    reason: >-
      The creatine kinase reaction produces phosphocreatine; this process annotation is directly
      entailed by the molecular function and is core.
    supported_by:
    - reference_id: PMID:2914937
      supporting_text: support the phosphocreatine shuttle hypothesis
      reference_section_type: ABSTRACT
- term:
    id: GO:0003824
    label: catalytic activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Root-level catalytic activity term from InterPro2GO. Correct but maximally generic; the specific
      creatine kinase activity (GO:0004111) is already annotated.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Uninformative parent of the specific GO:0004111 already present; adds no curation value.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Creatine kinase activity inferred electronically from the EC 2.7.3.2 / RHEA:17157 mapping. Same
      correct molecular function as the IBA and TAS annotations of this term.
    action: ACCEPT
    reason: >-
      Correct specific molecular function, redundant with but consistent with the IBA/TAS creatine
      kinase activity annotations; core.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      Electronically inferred inner-membrane localization from the UniProt subcellular-location keyword.
      Consistent with experimental evidence: the octamer is a peripheral membrane protein on the
      intermembrane (outer) face of the inner membrane and binds cardiolipin.
    action: ACCEPT
    reason: >-
      Matches the curated UniProt subcellular location (Mitochondrion inner membrane; peripheral;
      intermembrane side); correct compartment.
- term:
    id: GO:0016301
    label: kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Generic kinase activity from InterPro2GO. Correct but a non-specific parent of creatine kinase
      activity (GO:0004111).
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Subsumed by the specific GO:0004111 already annotated; over-general.
- term:
    id: GO:0016772
    label: transferase activity, transferring phosphorus-containing groups
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Generic phosphotransferase grouping term from InterPro2GO. Correct but a high-level parent of
      the specific creatine kinase activity.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Non-specific ancestor of GO:0004111; adds no value beyond the specific term.
- term:
    id: GO:0016775
    label: phosphotransferase activity, nitrogenous group as acceptor
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Phosphotransferase with a nitrogenous (guanidino) acceptor — the family-level activity captured
      by InterPro2GO. Accurate (creatine is the nitrogenous acceptor) but still a parent of the
      specific GO:0004111.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Correct but subsumed by the specific creatine kinase activity already annotated.
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      Phosphocreatine biosynthetic process inferred electronically from InterPro. Same correct core
      process as the IBA annotation of this term.
    action: ACCEPT
    reason: >-
      Correct biological process directly entailed by creatine kinase activity; core, consistent with
      the IBA annotation.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: >-
      Bare protein-binding annotation from a high-throughput binary interactome map (HuRI). Not
      informative about molecular function; the relevant binding partners are better captured as
      specific interactions rather than as a generic MF term.
    action: KEEP_AS_NON_CORE
    reason: >-
      Per curation guidelines, bare 'protein binding' is uninformative as a molecular function and is
      not a core function; the underlying high-throughput interaction is retained but non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32814053
  qualifier: enables
  review:
    summary: >-
      Bare protein-binding annotation from a neurodegeneration interactome-mapping study. Uninformative
      as a molecular function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Generic 'protein binding' from a high-throughput screen; not a core molecular function.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: >-
      Bare protein-binding annotation from a proteome-scale AP-MS network (BioPlex). Uninformative as a
      molecular function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Generic 'protein binding' from a high-throughput screen; not a core molecular function.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:40205054
  qualifier: enables
  review:
    summary: >-
      Bare protein-binding annotation from a multimodal cell-mapping study. Uninformative as a molecular
      function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Generic 'protein binding' from a high-throughput screen; not a core molecular function.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      High-throughput proteomic detection of the protein in mitochondria. Correct compartment but
      non-specific relative to the intermembrane-space / inner-membrane location.
    action: KEEP_AS_NON_CORE
    reason: >-
      Accurate but coarse; the specific location is captured in core_functions.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-200326
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted inner-membrane localization for the creatine kinase octamer reaction. Consistent
      with the peripheral, intermembrane-side inner-membrane localization of U-MtCK.
    action: ACCEPT
    reason: >-
      Matches curated localization (intermembrane face of the inner membrane); correct compartment.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: TAS
  original_reference_id: PMID:2914937
  qualifier: enables
  review:
    summary: >-
      Author-asserted creatine kinase activity from the paper that isolated the human mitochondrial CK
      gene/cDNA. This is the core, correct molecular function.
    action: ACCEPT
    reason: >-
      Traceable author statement of the defining enzymatic activity (EC 2.7.3.2); core.
    supported_by:
    - reference_id: PMID:2914937
      supporting_text: Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
      reference_section_type: ABSTRACT
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: TAS
  original_reference_id: PMID:2914937
  qualifier: located_in
  review:
    summary: >-
      Author-asserted mitochondrial localization. Correct but non-specific; the precise location is the
      intermembrane space / intermembrane face of the inner membrane.
    action: KEEP_AS_NON_CORE
    reason: >-
      Accurate but coarse compartment; the specific localization is captured in core_functions.
- term:
    id: GO:0005758
    label: mitochondrial intermembrane space
  evidence_type: IC
  original_reference_id: UniProtKB:P12532
  qualifier: located_in
  review:
    summary: >-
      The catalytically active CKMT1A octamer is a peripheral inner-membrane protein on the
      intermembrane side, i.e. it faces the mitochondrial intermembrane space. This is the
      precise compartment for its function and is proposed as a more specific localization than
      mitochondrion/inner membrane. GO:0005758 was not present in GOA; added here for consistency
      with the parallel CKMT2 review and with this review's core_functions.
    action: NEW
    reason: >-
      More specific localization for the active octamer, inferred from the curated UniProt
      subcellular location (inner membrane, peripheral, intermembrane side).
    supported_by:
    - reference_id: UniProtKB:P12532
      supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
core_functions:
- description: >-
    Reversibly phosphorylates creatine using OXPHOS-derived ATP to produce N-phosphocreatine, the
    phosphate-loading reaction of the mitochondrial creatine kinase octamer in the intermembrane space.
    This activity constitutes the mitochondrial limb of the phosphocreatine shuttle.
  molecular_function:
    id: GO:0004111
    label: creatine kinase activity
  directly_involved_in:
  - id: GO:0046314
    label: phosphocreatine biosynthetic process
  locations:
  - id: GO:0005758
    label: mitochondrial intermembrane space
  - id: GO:0005743
    label: mitochondrial inner membrane
  supported_by:
  - reference_id: PMID:2914937
    supporting_text: Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy metabolism.
    reference_section_type: ABSTRACT
  - reference_id: PMID:2914937
    supporting_text: support the phosphocreatine shuttle hypothesis
    reference_section_type: ABSTRACT
references:
- id: UniProtKB:P12532
  title: Creatine kinase U-type, mitochondrial (human)
  findings:
  - statement: >-
      CKMT1A is a peripheral mitochondrial inner-membrane protein on the intermembrane side,
      consistent with the active octamer facing the mitochondrial intermembrane space.
    supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:2914937
  title: Isolation and characterization of the gene and cDNA encoding human mitochondrial creatine kinase.
  findings:
  - statement: >-
      Isolated the human mitochondrial creatine kinase gene and cDNA (the ubiquitous CKMT1 locus); the
      gene contains 9 exons encoding a 416-residue protein including a 38-residue mitochondrial transit
      peptide, and the data support the phosphocreatine shuttle hypothesis.
    supporting_text: support the phosphocreatine shuttle hypothesis
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified abstract; the cited paper isolated the human mitochondrial creatine kinase
      gene/cDNA (placental/ubiquitous MtCK = CKMT1) and is the source of the TAS creatine kinase
      activity and mitochondrion annotations. Abstract-only in cache (full_text_available: false).
- id: PMID:10737943
  title: Crystal structure of human ubiquitous mitochondrial creatine kinase.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Source of the UniProt SUBUNIT octamer assertion (octamer of four homodimers) and PDB 1QK1;
      directly characterizes the human U-MtCK protein reviewed here. Not cached locally.
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput binary interactome map (HuRI); source of bare protein-binding IPI annotations.
      Correctly cited but uninformative for molecular function.
- id: PMID:32814053
  title: Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput interactome mapping; source of bare protein-binding IPI annotations. Uninformative
      for molecular function.
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      BioPlex AP-MS network; source of bare protein-binding IPI annotations. Uninformative for molecular
      function.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      High-throughput mitochondrial proteome; corroborates mitochondrial localization of the protein.
- id: PMID:40205054
  title: Multimodal cell maps as a foundation for structural and functional genomics.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      Multimodal cell-mapping study; source of a bare protein-binding IPI annotation. Uninformative for
      molecular function.
- id: PMID:24522192
  title: CKMT1 regulates the mitochondrial permeability transition pore in a process that provides evidence for alternative forms of the complex.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Reports CKMT1 as a necessary gatekeeper of the mitochondrial permeability transition pore;
      supports a role in permeability-transition regulation beyond the core kinase function. Not cached.
- id: Reactome:R-HSA-200326
  title: creatine + ATP => phosphocreatine + ADP [CK octamer]
  findings: []
suggested_questions:
- question: >-
    Is the regulation of the mitochondrial permeability transition by CKMT1 a direct structural/gating
    role of the octamer at contact sites, or an indirect consequence of altered local ATP/ADP and
    cardiolipin organization?
- question: >-
    Do CKMT1A and CKMT1B (the duplicated genes encoding identical protein) show any tissue- or
    context-specific differences in expression or regulation despite producing the same enzyme?
suggested_experiments:
- hypothesis: >-
    Octamer formation and cardiolipin binding are required for functional coupling of CKMT1 to the
    adenine nucleotide translocator and for efficient phosphocreatine export.
  description: >-
    Reconstitute purified CKMT1 octamers vs. dimer-stabilized mutants with cardiolipin-containing
    proteoliposomes containing ANT/VDAC and measure ATP/ADP channeling and phosphocreatine production
    relative to bulk substrate.
  experiment_type: biochemical reconstitution