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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004111 creatine kinase activity | IBA GO_REF:0000033 | ACCEPT | 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. Reason: Directly supported core catalytic activity of the gene product (EC 2.7.3.2), consistent across IBA, EC/RHEA, and TAS evidence. Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | 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. Reason: Correct localization but more general than the precise compartment; useful context rather than the defining molecular function. Supporting Evidence: UniProtKB:P17540 Mitochondrion inner membrane; Peripheral membrane |
| GO:0046314 phosphocreatine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: The creatine kinase reaction catalyzed by CKMT2 produces phosphocreatine; this biological process directly reflects the enzyme's core function. Reason: Directly supported core process corresponding to the creatine kinase reaction (the phosphate-loading limb of the phosphocreatine shuttle). Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MODIFY | Summary: Catalytic activity is correct but far too generic for a well-characterized creatine kinase. Reason: The specific activity (creatine kinase activity, GO:0004111) is known and strictly more informative. Proposed replacements: creatine kinase activity Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0004111 creatine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Creatine kinase activity inferred from EC 2.7.3.2 / RHEA:17157; matches the core catalytic function. Reason: Correct specific molecular function, consistent with the curated EC and reaction mapping. Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: CKMT2 is a peripheral inner-membrane protein on the intermembrane-space side; the inner-membrane localization is well supported. Reason: Matches the curated subcellular location (mitochondrion inner membrane, peripheral, intermembrane side) and Reactome TAS evidence. Supporting Evidence: UniProtKB:P17540 Mitochondrion inner membrane; Peripheral membrane |
| GO:0016301 kinase activity | IEA GO_REF:0000002 | MODIFY | Summary: Kinase activity is correct but too generic given the known creatine kinase activity. Reason: The specific term creatine kinase activity is supported and more informative. Proposed replacements: creatine kinase activity Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0016772 transferase activity, transferring phosphorus-containing groups | IEA GO_REF:0000002 | MODIFY | Summary: A high-level transferase grouping term; correct but far less informative than the specific creatine kinase activity. Reason: Too general; the specific catalytic activity is known. Proposed replacements: creatine kinase activity Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0016775 phosphotransferase activity, nitrogenous group as acceptor | IEA GO_REF:0000002 | MODIFY | 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. Reason: Directionally correct but too general; creatine kinase activity is the precise, supported term. Proposed replacements: creatine kinase activity Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0046314 phosphocreatine biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: Phosphocreatine biosynthesis is the process directly carried out by the creatine kinase reaction; correctly annotated. Reason: Directly supported core process, consistent with the IBA annotation of the same term. Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | 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. 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. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | 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. 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. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | 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. Reason: Correct but general localization, supporting context rather than the core molecular function. |
| GO:0005739 mitochondrion | HDA PMID:20833797 Phosphoproteome analysis of functional mitochondria isolated... | KEEP AS NON CORE | Summary: CKMT2 was detected in a phosphoproteome of mitochondria isolated from human skeletal muscle, confirming mitochondrial localization in its native tissue. Reason: Correct localization in the relevant tissue but more general than the precise inner-membrane/intermembrane-space compartment. Supporting Evidence: PMID:20833797 functional mitochondria isolated from resting human muscle |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-200326 | ACCEPT | Summary: Inner-membrane localization asserted by Reactome for the CK octamer; consistent with the curated UniProt subcellular location. Reason: Correct, specific localization for the catalytically active octamer (peripheral inner-membrane protein on the intermembrane side). Supporting Evidence: UniProtKB:P17540 Mitochondrion inner membrane; Peripheral membrane |
| GO:0004111 creatine kinase activity | TAS PMID:2324105 Separate nuclear genes encode sarcomere-specific and ubiquit... | ACCEPT | Summary: Creatine kinase activity attributed by an author statement; CKMT2 is the sarcomeric mitochondrial creatine kinase isoenzyme. Reason: Directly supported core catalytic activity; the cited paper establishes CKMT2 as a sarcomeric mitochondrial creatine kinase isoenzyme. Supporting Evidence: PMID:2324105 separate nuclear genes encode two closely related, tissue-specific isoenzymes of MtCK |
| GO:0005739 mitochondrion | TAS PMID:2324105 Separate nuclear genes encode sarcomere-specific and ubiquit... | KEEP AS NON CORE | Summary: Mitochondrial localization from author statement; CKMT2 encodes a mitochondrial CK with a transit peptide essential for mitochondrial import. Reason: Correct localization but general; the paper notes a transit peptide essential for mitochondrial import. Supporting Evidence: PMID:2324105 contains a 39-residue transit peptide essential for mitochondrial import |
| GO:0006936 muscle contraction | TAS PMID:2324105 Separate nuclear genes encode sarcomere-specific and ubiquit... | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: PMID:2324105 Creatine kinase (EC 2.7.3.2) isoenzymes play a central role in energy transduction |
| GO:0005758 mitochondrial intermembrane space | IC UniProtKB:P17540 | NEW | 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. Reason: More specific localization for the active octamer, inferred from the curated UniProt subcellular location (inner membrane, peripheral, intermembrane side). Supporting Evidence: UniProtKB:P17540 Mitochondrion inner membrane; Peripheral membrane |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Q: 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?
Experiment: 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.
Experiment: 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.
Experiment: 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.
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