CKM encodes the muscle-type (M-type) cytosolic creatine kinase (EC 2.7.3.2), a member of the ATP:guanido phosphotransferase (phosphagen kinase) family. It reversibly catalyzes the transfer of a phosphoryl group between ATP and creatine (creatine + ATP <-> N-phosphocreatine + ADP + H+). The enzyme is a dimer: CK-MM homodimers are the dominant form in skeletal muscle and myocardium, and CK-MB heterodimers with the brain-type subunit CKB occur in myocardium. Together with mitochondrial creatine kinase, cytosolic CKM forms the cytosolic limb of the phosphocreatine (PCr) energy-shuttle and ATP-buffering system, regenerating ATP from phosphocreatine at sites of high ATP turnover such as the myofibrillar myosin ATPase and membrane ion pumps. A functionally important pool of CK-MM is bound at the sarcomeric M-band through M-specific N-terminal lysine residues that interact with the central domains of the M-band proteins myomesin and M-protein, positioning it as an intramyofibrillar ATP regenerator. CKM is highly and selectively expressed in skeletal and cardiac muscle; its serum activity (CK-MM, CK-MB) is a clinical marker of skeletal-muscle and myocardial injury.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IBA GO_REF:0000033 | REMOVE | Summary: CKM is a cytosolic enzyme; this IBA "is_active_in extracellular region" is an over-propagation through the phylogenetic tree. Reason: CKM/CK-MM is a cytosolic creatine kinase (UniProt SUBCELLULAR LOCATION: Cytoplasm) that functions intracellularly in the phosphocreatine shuttle and at the myofibrillar M-band. Serum/extracellular CK is enzyme leakage from damaged muscle, not a secreted, extracellularly-active form. An "is_active_in extracellular region" assertion is biologically incorrect for the cytosolic CK reaction and is an electronic (IBA) over-propagation, so it is removed. |
| GO:0004111 creatine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Creatine kinase activity is the defining molecular function of CKM; strongly supported across evidence types. Reason: CKM catalyzes the reversible creatine + ATP <-> phosphocreatine + ADP reaction (EC 2.7.3.2). This phylogenetic (IBA) annotation is concordant with the experimental/structural and sequence evidence and represents the core molecular function. Supporting Evidence: PMID:3778496 A full length cDNA for human M creatine kinase has been isolated and sequenced. |
| GO:0046314 phosphocreatine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: CKM catalyzes the reaction that produces phosphocreatine; this is the direct biological-process correlate of its molecular function. Reason: The creatine kinase reaction is reversible; in the phosphocreatine-synthesizing direction CKM produces N-phosphocreatine from creatine and ATP, justifying involvement in phosphocreatine biosynthetic process. The physiologically dominant cytosolic role is the reverse (ATP regeneration), but this BP term is the established correlate of creatine kinase activity and is accepted as core. |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Correct but uninformatively general; a high-level parent of creatine kinase activity. Reason: "catalytic activity" (GO:0003824) is a root-level MF parent that is true but adds no information beyond the specific creatine kinase activity (GO:0004111) already annotated. It is an InterPro2GO electronic generalization subsumed by the specific term. |
| GO:0004111 creatine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Correct core molecular function (electronic call from Rhea/EC mapping). Reason: This IEA annotation derives from the Rhea/EC 2.7.3.2 mapping to creatine kinase activity and agrees with the experimental and phylogenetic evidence for the same term. Retained as a correct statement of the core function. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | MODIFY | Summary: Correct localization, but the more specific cytosol term is preferable. Reason: CKM is a soluble cytosolic enzyme. UniProt records SUBCELLULAR LOCATION as Cytoplasm, and Reactome annotates the more specific cytosol (GO:0005829). The broad "cytoplasm" should be refined to cytosol, which is also where the phosphocreatine-shuttle reaction occurs. Proposed replacements: cytosol |
| GO:0016301 kinase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Correct but over-general parent of creatine kinase activity. Reason: "kinase activity" (GO:0016301) is a general parent subsumed by the specific creatine kinase activity (GO:0004111). It is an InterPro2GO electronic generalization adding no specificity. |
| GO:0016772 transferase activity, transferring phosphorus-containing groups | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Correct but over-general parent of creatine kinase activity. Reason: A high-level transferase parent term subsumed by the specific creatine kinase activity (GO:0004111); InterPro2GO electronic generalization with no added information. |
| GO:0016775 phosphotransferase activity, nitrogenous group as acceptor | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Accurate intermediate parent (creatine's guanidino nitrogen is the acceptor) but still less informative than creatine kinase activity. Reason: Creatine kinase is indeed a phosphotransferase with a nitrogenous group as acceptor, so this term is biologically accurate, but it is a parent of the specific creatine kinase activity (GO:0004111) and is an electronic generalization that does not add information over the specific term. |
| GO:0046314 phosphocreatine biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: Correct biological process (electronic call), duplicating the accepted IBA annotation. Reason: InterPro2GO electronic annotation to the same biological process supported by the IBA annotation; consistent with the creatine kinase reaction and accepted. |
| GO:0005515 protein binding | IPI PMID:21516116 Next-generation sequencing to generate interactome datasets. | KEEP AS NON CORE | Summary: Uninformative bare "protein binding"; supports a real interaction (ASB9) but not a core function. Reason: Bare "protein binding" (GO:0005515) is uninformative per curation guidelines. The underlying IntAct interaction (with ASB9, Q96DX5) is real, so the annotation is kept but marked non-core; it does not describe CKM's catalytic function. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" from a proteome-scale interactome (ASB9). Reason: Real binary interaction evidence (CKM-ASB9, Q96DX5) but the term itself is uninformative; kept as non-core supporting evidence rather than a core function. |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" from an interactome map (CKB, ASB9). Reason: Supports interactions with CKB (P12277; consistent with CK-MB heterodimer formation) and ASB9 (Q96DX5). The CKB partnership is biologically meaningful but "protein binding" is uninformative; kept as non-core. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" from the HuRI binary interactome (CKMT1B, ASB9). Reason: Supports interactions with mitochondrial CK (CKMT1B, P12532) and ASB9 (Q96DX5). Real partner evidence but uninformative term; kept as non-core. |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" (huntingtin interaction). Reason: Supports a CKM-huntingtin (HTT, P42858) interaction reported in a neurodegeneration interactome. Real partner evidence but uninformative term; kept as non-core. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" from BioPlex (CKB, ASB9). Reason: Supports interactions with CKB (P12277) and ASB9 (Q96DX5). Real partner evidence but uninformative term; kept as non-core. |
| GO:0005515 protein binding | IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce... | KEEP AS NON CORE | Summary: Uninformative bare "protein binding" from OpenCell (CKB). Reason: Supports a CKM-CKB (P12277) interaction consistent with CK-MB heterodimer formation. Real partner evidence but uninformative term; kept as non-core. |
| GO:0005576 extracellular region | IEA GO_REF:0000107 | REMOVE | Summary: CKM is cytosolic; extracellular localization is an electronic ortholog-transfer error. Reason: This Ensembl-Compara electronic transfer asserts extracellular localization, but CKM is a cytosolic enzyme (UniProt: Cytoplasm). Extracellular CK is leakage from damaged cells rather than a functional secreted location. Removed as an incorrect electronic (IEA) over-propagation. |
| GO:0004111 creatine kinase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Core molecular function, supported by sequence similarity to a well-characterized ortholog. Reason: ISS transfer from UniProtKB:P00563 (rabbit M-CK) to the same creatine kinase activity term; concordant with all other evidence for the core function. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-200318 | ACCEPT | Summary: Correct, specific subcellular location where the creatine kinase reaction occurs. Reason: Reactome (TAS) places CKM in the cytosol for the creatine + ATP => phosphocreatine + ADP reaction. This matches UniProt (Cytoplasm) and is the appropriate specific location for the cytosolic limb of the phosphocreatine shuttle. |
| GO:0005515 protein binding | IPI PMID:12972258 Muscle-type creatine kinase interacts with central domains o... | KEEP AS NON CORE | Summary: Uninformative bare "protein binding", but underlies a functionally important M-band interaction (myomesin/M-protein). Reason: The underlying interaction is biologically significant - MM-CK binds the central domains of the M-band proteins myomesin and M-protein, anchoring it at the sarcomeric M-band where it regenerates ATP for the myosin ATPase. However, the bare "protein binding" term is uninformative; the functional content is captured in core_functions (M band location) rather than as a core MF here. Supporting Evidence: PMID:12972258 A yeast two-hybrid screen led to the identification of MM-CK as a binding partner of a central portion of myomesin (My7-8). An interaction was observed with domains six to eight of the closely related M-protein |
| GO:0004111 creatine kinase activity | TAS PMID:3778496 Isolation and sequence analysis of a full-length cDNA for hu... | ACCEPT | Summary: Core molecular function asserted by the original human M-CK characterization. Reason: Traceable author statement (TAS) for creatine kinase activity from the human M-CK cDNA/sequence paper. Confirms the core molecular function. Supporting Evidence: PMID:3778496 A full length cDNA for human M creatine kinase has been isolated and sequenced. |
| GO:0031430 M band | IPI PMID:12972258 Muscle-type creatine kinase interacts with central domains o... | NEW | Summary: A functionally important pool of CK-MM localizes to the sarcomeric M-band, where it binds myomesin and M-protein and regenerates ATP for the myosin ATPase. Reason: Not present in the seeded GOA, but well supported experimentally: MM-CK binds the M-band of sarcomeric muscle in an isoform-specific manner via its M-specific N-terminal lysines, interacting with central domains of myomesin and M-protein, and serves there as an intramyofibrillar ATP regenerator. This is a key structural/functional localization for the muscle isoform and is proposed as a new localization annotation. Supporting Evidence: PMID:12972258 MM-CK interacts in an isoform-specific manner with the M-band of sarcomeric muscle, where it serves as an efficient intramyofibrillar ATP-regenerating system for the actin-activated myosin ATPase located nearby on both sides of the M-band. |
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Download this section (compressed HTML)Q: Beyond the sarcomeric M-band, how much of cellular CK-MM function depends on its association with sarcoplasmic reticulum and sarcolemmal/T-tubule membranes for local ATP/ADP regulation of ion pumps?
Experiment: Express wild-type CKM and an M-band-binding-deficient lysine mutant (K8/K24/K104/K115) in muscle cells or Ckm-null myofibers, confirm equal intrinsic creatine kinase activity in vitro, then compare M-band localization (immuno-EM), contractile ATP/ADP buffering, and fatigue resistance during burst activity.
Hypothesis: M-band binding of CK-MM, mediated by the four M-specific N-terminal lysines (K8, K24, K104, K115), is required for efficient ATP regeneration at the myofibril rather than for catalysis per se.
Type: structure-function / localization
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