id: P17540
gene_symbol: CKMT2
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  CKMT2 encodes the sarcomeric mitochondrial creatine kinase (S-MtCK, also
  Mib-CK), a member of the ATP guanido phosphotransferase (phosphagen kinase)
  family expressed selectively in heart and skeletal muscle. The protein is
  synthesized with an N-terminal mitochondrial transit peptide and matures into a
  419-residue enzyme that assembles into a homo-octamer of four homodimers located
  on the intermembrane-space face of the mitochondrial inner membrane, to which it
  is peripherally anchored largely through electrostatic binding to cardiolipin.
  S-MtCK reversibly catalyzes the transfer of the gamma-phosphate of ATP to
  creatine, producing phosphocreatine and ADP (EC 2.7.3.2). Positioned at
  inner/outer-membrane contact sites and functionally coupled to the adenine
  nucleotide translocator, it uses ATP exported from oxidative phosphorylation to
  generate diffusible phosphocreatine, constituting the phosphate-loading limb of
  the phosphocreatine/creatine shuttle that buffers and transports high-energy
  phosphate in tissues with large, fluctuating energy demands. Through its
  octameric, cardiolipin- and VDAC-associated state at contact sites it also
  contributes to the structural organization of these sites and can influence
  mitochondrial permeability transition.
existing_annotations:
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      CKMT2 is a mitochondrial creatine kinase that reversibly transfers phosphate
      between ATP and creatine; creatine kinase activity is the core molecular
      function and is directly supported.
    action: ACCEPT
    reason: >-
      Directly supported core catalytic activity of the gene product (EC 2.7.3.2),
      consistent across IBA, EC/RHEA, and TAS evidence.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      CKMT2 acts within the mitochondrion; this localization is correct but less
      specific than the intermembrane-space/inner-membrane compartment where the
      active octamer resides.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct localization but more general than the precise compartment; useful
      context rather than the defining molecular function.
    supported_by:
    - reference_id: UniProtKB:P17540
      supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      The creatine kinase reaction catalyzed by CKMT2 produces phosphocreatine; this
      biological process directly reflects the enzyme's core function.
    action: ACCEPT
    reason: >-
      Directly supported core process corresponding to the creatine kinase reaction
      (the phosphate-loading limb of the phosphocreatine shuttle).
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0003824
    label: catalytic activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Catalytic activity is correct but far too generic for a well-characterized
      creatine kinase.
    action: MODIFY
    reason: >-
      The specific activity (creatine kinase activity, GO:0004111) is known and
      strictly more informative.
    proposed_replacement_terms:
    - id: GO:0004111
      label: creatine kinase activity
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- 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 from EC 2.7.3.2 / RHEA:17157; matches the
      core catalytic function.
    action: ACCEPT
    reason: >-
      Correct specific molecular function, consistent with the curated EC and
      reaction mapping.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      CKMT2 is a peripheral inner-membrane protein on the intermembrane-space side;
      the inner-membrane localization is well supported.
    action: ACCEPT
    reason: >-
      Matches the curated subcellular location (mitochondrion inner membrane,
      peripheral, intermembrane side) and Reactome TAS evidence.
    supported_by:
    - reference_id: UniProtKB:P17540
      supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
- term:
    id: GO:0016301
    label: kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Kinase activity is correct but too generic given the known creatine kinase
      activity.
    action: MODIFY
    reason: >-
      The specific term creatine kinase activity is supported and more informative.
    proposed_replacement_terms:
    - id: GO:0004111
      label: creatine kinase activity
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0016772
    label: transferase activity, transferring phosphorus-containing groups
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      A high-level transferase grouping term; correct but far less informative than
      the specific creatine kinase activity.
    action: MODIFY
    reason: "Too general; the specific catalytic activity is known."
    proposed_replacement_terms:
    - id: GO:0004111
      label: creatine kinase activity
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0016775
    label: phosphotransferase activity, nitrogenous group as acceptor
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      This grouping term correctly describes phosphate transfer to a nitrogenous
      acceptor (the guanidino group of creatine) but is less specific than creatine
      kinase activity.
    action: MODIFY
    reason: >-
      Directionally correct but too general; creatine kinase activity is the precise,
      supported term.
    proposed_replacement_terms:
    - id: GO:0004111
      label: creatine kinase activity
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      Phosphocreatine biosynthesis is the process directly carried out by the
      creatine kinase reaction; correctly annotated.
    action: ACCEPT
    reason: >-
      Directly supported core process, consistent with the IBA annotation of the
      same term.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: >-
      Generic protein-binding annotation from a high-throughput binary interactome
      screen (HuRI; partners include CETN3, NRF1, P4HA3, KLHL42). The term conveys no
      specific molecular function and several partners are of unclear physiological
      relevance.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Bare protein binding is uninformative as a molecular function and is derived
      from a large-scale yeast-two-hybrid interactome rather than a gene-specific
      functional study; not adopted as a core function.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: >-
      Generic protein-binding annotation from a high-throughput affinity-MS
      interactome (BioPlex); the reported partner CKMT1B (the ubiquitous MtCK paralog)
      is biologically plausible, but the term itself is uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Bare protein binding carries no functional content; a more informative
      molecular-function term would be required to be useful, so it is not treated as
      a core function.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      Mitochondrial localization from a high-confidence human mitochondrial proteome
      dataset; consistent with the known localization but less specific than the
      inner-membrane/intermembrane-space compartment.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct but general localization, supporting context rather than the core
      molecular function.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HDA
  original_reference_id: PMID:20833797
  qualifier: located_in
  review:
    summary: >-
      CKMT2 was detected in a phosphoproteome of mitochondria isolated from human
      skeletal muscle, confirming mitochondrial localization in its native tissue.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct localization in the relevant tissue but more general than the precise
      inner-membrane/intermembrane-space compartment.
    supported_by:
    - reference_id: PMID:20833797
      supporting_text: "functional mitochondria isolated from resting human muscle"
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-200326
  qualifier: located_in
  review:
    summary: >-
      Inner-membrane localization asserted by Reactome for the CK octamer; consistent
      with the curated UniProt subcellular location.
    action: ACCEPT
    reason: >-
      Correct, specific localization for the catalytically active octamer (peripheral
      inner-membrane protein on the intermembrane side).
    supported_by:
    - reference_id: UniProtKB:P17540
      supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: TAS
  original_reference_id: PMID:2324105
  qualifier: enables
  review:
    summary: >-
      Creatine kinase activity attributed by an author statement; CKMT2 is the
      sarcomeric mitochondrial creatine kinase isoenzyme.
    action: ACCEPT
    reason: >-
      Directly supported core catalytic activity; the cited paper establishes CKMT2
      as a sarcomeric mitochondrial creatine kinase isoenzyme.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "separate nuclear genes encode two closely related, tissue-specific isoenzymes of MtCK"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: TAS
  original_reference_id: PMID:2324105
  qualifier: located_in
  review:
    summary: >-
      Mitochondrial localization from author statement; CKMT2 encodes a mitochondrial
      CK with a transit peptide essential for mitochondrial import.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct localization but general; the paper notes a transit peptide essential
      for mitochondrial import.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "contains a 39-residue transit peptide essential for mitochondrial import"
- term:
    id: GO:0006936
    label: muscle contraction
  evidence_type: TAS
  original_reference_id: PMID:2324105
  qualifier: involved_in
  review:
    summary: >-
      CKMT2 supplies high-energy phosphate (phosphocreatine) to contracting striated
      muscle, but it is an energy-metabolism enzyme, not a component of the
      contractile apparatus or a direct participant in the contraction process.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The link to muscle contraction is indirect (energetic support of contracting
      tissue); the gene product's role is energy transduction (phosphocreatine
      biosynthesis), not the contraction process itself.
    supported_by:
    - reference_id: PMID:2324105
      supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
- term:
    id: GO:0005758
    label: mitochondrial intermembrane space
  evidence_type: IC
  original_reference_id: UniProtKB:P17540
  qualifier: located_in
  review:
    summary: >-
      The catalytically active CKMT2 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.
    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:P17540
      supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
core_functions:
- description: >-
    Reversibly catalyzes transfer of the gamma-phosphate of ATP to creatine,
    producing phosphocreatine and ADP (EC 2.7.3.2); the defining catalytic activity
    of the sarcomeric mitochondrial creatine kinase.
  supported_by:
  - reference_id: PMID:2324105
    supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
  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
- description: >-
    As a membrane-associated homo-octamer in the mitochondrial intermembrane space
    coupled to the adenine nucleotide translocator, CKMT2 uses ATP from oxidative
    phosphorylation to generate diffusible phosphocreatine, constituting the
    phosphate-loading limb of the phosphocreatine/creatine shuttle in heart and
    skeletal muscle.
  supported_by:
  - reference_id: PMID:2324105
    supporting_text: "Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction"
  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
references:
- 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: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PAN-GO/GO_Central phylogenetic inference correctly assigns the conserved
      creatine kinase activity and phosphocreatine biosynthetic process to CKMT2.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:20833797
  title: Phosphoproteome analysis of functional mitochondria isolated from resting human muscle reveals extensive phosphorylation of inner membrane protein complexes and enzymes.
  findings:
  - statement: >-
      CKMT2 was identified among phosphoproteins of mitochondria isolated from human
      skeletal muscle, confirming its native mitochondrial localization.
    supporting_text: "functional mitochondria isolated from resting human muscle"
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Large-scale phosphoproteomics of human muscle mitochondria; supports
      mitochondrial localization (HDA) but is not a function-specific study.
- id: PMID:2324105
  title: Separate nuclear genes encode sarcomere-specific and ubiquitous human mitochondrial creatine kinase isoenzymes.
  findings:
  - statement: >-
      CKMT2 (sarcomeric MtCK) and CKMT1 (ubiquitous MtCK) are encoded by separate
      nuclear genes; the sarcomeric isoenzyme is heart/skeletal-muscle specific and
      contains a transit peptide for mitochondrial import.
    supporting_text: "separate nuclear genes encode two closely related, tissue-specific isoenzymes of MtCK"
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Foundational paper cloning the human sarcomeric mitochondrial creatine kinase
      cDNA and distinguishing it from the ubiquitous isoenzyme.
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      HuRI binary interactome screen; source of generic protein-binding IPI
      annotations of unclear functional significance for CKMT2.
- 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 affinity-MS interactome; reports a CKMT2-CKMT1B paralog association
      (plausible) but only supports a generic protein-binding term.
- 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-confidence human mitochondrial proteome; supports mitochondrial
      localization (HTP).
- id: Reactome:R-HSA-200326
  title: creatine + ATP => phosphocreatine + ADP [CK octamer]
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Reactome reaction for the mitochondrial CK octamer; supports both the creatine
      kinase reaction and inner-membrane localization.
- id: UniProtKB:P17540
  title: Creatine kinase S-type, mitochondrial (human)
  findings:
  - statement: >-
      CKMT2 is a peripheral mitochondrial inner-membrane protein on the intermembrane
      side that exists as an octamer of four homodimers and binds cardiolipin; it
      reversibly catalyzes creatine + ATP = N-phosphocreatine + ADP.
    supporting_text: "Mitochondrion inner membrane; Peripheral membrane"
    reference_section_type: OTHER
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Curated UniProt record; primary source for catalytic activity, octamer
      assembly, cardiolipin binding, localization, and tissue specificity.
proposed_new_terms: []
suggested_questions:
- question: >-
    What distinguishes the sarcomeric S-MtCK (CKMT2) from the ubiquitous U-MtCK (CKMT1A) beyond
    tissue expression - are there differences in octamer stability, cardiolipin affinity, or
    coupling to the adenine nucleotide translocator that matter functionally in heart and
    skeletal muscle?
- question: >-
    How does the CKMT2 octamer/dimer equilibrium regulate its contribution to mitochondrial
    permeability transition, and is this a driver or a bystander in cardiac ischemia-reperfusion
    injury?
- question: >-
    Is CKMT2 expression or activity dynamically regulated (e.g. by cardiac workload, hypoxia, or
    metabolic state), and does altered CKMT2 contribute to the energy-starvation phenotype of
    heart failure?
suggested_experiments:
- description: >-
    Compare octamer stability and cardiolipin-binding affinity of purified CKMT2 vs CKMT1A under
    matched conditions to test whether the sarcomeric isoform is intrinsically more membrane-
    associated.
- description: >-
    Use a CKMT2-specific knockout/knockdown in cardiomyocytes with controlled ischemia-
    reperfusion to dissect CKMT2's role in permeability-transition-pore opening from its role in
    the phosphocreatine shuttle.
- description: >-
    Quantify CKMT2 protein and the phosphocreatine/ATP ratio across a graded cardiac-workload or
    heart-failure model to test whether CKMT2 abundance tracks energetic reserve.