Integral protein of the mitochondrial inner membrane, enriched in cristae, and the principal characterised route by which calcium is exported from the mitochondrial matrix into the cytosol. NCLX belongs to the Ca2+/cation antiporter (CaCA) superfamily and, within it, to the cation/calcium exchanger (CCX) branch rather than to the canonical plasma-membrane Na+/Ca2+ exchangers; it has roughly thirteen transmembrane helices, self-associates, and both published cryo-EM structures resolve it as a trimer. Its transport is blocked by CGP-37157 and, unusually among calcium exchangers, it accepts Li+ as well as Na+ as the counterion. By limiting the duration of matrix calcium elevations, NCLX couples calcium signalling to NAD(P)H production and matrix redox state, and its activity shapes cytosolic calcium transients; loss of NCLX causes matrix calcium overload, and in excitable tissue this drives cell death and organ failure. Downstream, NCLX activity influences glucose-stimulated insulin secretion in pancreatic beta cells, sustained store-operated calcium entry through Orai1, lymphocyte chemotaxis and cardiomyocyte physiology. The identity of the counterion is currently disputed: NCLX lacks several of the serine residues that coordinate Na+ in canonical Na+/Ca2+ exchangers, and recent structural studies variously conclude that it is a H+/Ca2+ exchanger, or a non-selective exchanger that can use Na+, K+, Li+ or protons, whereas earlier mutagenesis identified separate Na+- and Li+-selective residues in its transport site. Whether NCLX is also the protein responsible for sodium-dependent mitochondrial calcium efflux, or whether that activity belongs to TMEM65, is likewise unsettled.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Membrane residence, asserted from the phylogenetic tree or the InterPro signature. Reason: Correct but uninformative; the mitochondrial inner membrane and crista annotations this gene also carries are far more specific about where NCLX sits. |
| GO:0099093 calcium export from the mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: NCLX mediates export of calcium from the mitochondrial matrix. Reason: Core biological process. That NCLX exports Ca2+ from the mitochondrial matrix is supported by loss- and gain-of-function work from several independent laboratories and is not challenged by the recent structures, which dispute the counterion rather than the direction or the substrate. |
| GO:0006874 intracellular calcium ion homeostasis | IBA GO_REF:0000033 | ACCEPT | Summary: NCLX shapes intracellular calcium homeostasis. Reason: Correct; the general parent of the mitochondrial calcium homeostasis role that NCLX demonstrably has. |
| GO:0005432 calcium:sodium antiporter activity | IBA GO_REF:0000033 | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000262860 · PTN000262860 SUPPORTS TRANSFER The node placement is not in question - every extant member tested exchanges Ca2+ for a monovalent cation, so the ancestral exchange function transfers correctly to SLC8B1. What does not follow is the Na+ specificity: the PANTHER family (PTHR12266) that contains SLC8B1 is dominated by plant cation/calcium exchangers CCX1-CCX5 and by fungal members (ECM27, YDL206W), which are not Na+/Ca2+ exchangers, so a counterion-neutral term is the better fit at this node. MGI:MGI:2180781 · Slc8b1 (mouse) SUPPORTS TRANSFER True orthologue with the same mitochondrial role; carries the same over-specific counterion assignment, so generalising the term applies equally to the donor. UniProtKB:Q6J4K2 · SLC8B1 (human, the target itself) SUPPORTS TRANSFER The target appearing among its own sources is expected and correct - SLC8B1's own experimental annotations are part of the descendant evidence the PAINT curator used. Not circular. WB:WBGene00003574 · C. elegans donor UNRESOLVED Could not resolve this WormBase identifier to a gene record from here, so its contribution was not assessed. PTHR12266 contains three C. elegans members (ncx-6, ncx-7, ncx-9). Proposed replacements: calcium:monoatomic cation antiporter activity |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: NCLX resides in the mitochondrial inner membrane. Reason: Well-supported core location; NCLX is an integral protein of the mitochondrial inner membrane, confirmed by fractionation, imaging and two cryo-EM studies. |
| GO:0016020 membrane | IEA GO_REF:0000002 | ACCEPT | Summary: Membrane residence, asserted from the phylogenetic tree or the InterPro signature. Reason: Correct but uninformative; the mitochondrial inner membrane and crista annotations this gene also carries are far more specific about where NCLX sits. |
| GO:0035725 sodium ion transmembrane transport | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Sodium transport, inferred logically from the calcium:sodium antiporter annotation. Reason: Derived automatically from GO:0005432 by inter-ontology logical inference (GO_REF:0000108), so it simply inherits the counterion claim that is currently disputed. There is no independent measurement of NCLX-mediated Na+ flux, and Fan et al. report that imposed Na+ gradients do not drive NCLX Ca2+ transport at all. Since the parent MF annotation is generalised here to a counterion-neutral term, this derived process annotation no longer follows. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | ACCEPT | Summary: Generic transmembrane transport from the InterPro cation/calcium exchanger signature. Reason: Correct but very general; retained because the InterPro signature assignment is sound and the more specific transport terms on this gene carry the information. |
| GO:0005432 calcium:sodium antiporter activity | IEA GO_REF:0000107 | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | ACCEPT | Summary: Mitochondrial residence. Reason: Correct, though the mitochondrial inner membrane and crista annotations are more informative about where NCLX sits. |
| GO:0005886 plasma membrane | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Plasma membrane localisation. Reason: A residue of the pre-2010 literature, when the protein (then NCKX6/FLJ22233) was characterised as a plasma-membrane Li+/Ca2+ exchanger, and still modelled that way by Reactome (R-HSA-425822). Everything since PMID:20018762 places NCLX in the mitochondrial inner membrane and cristae, and the mitochondrial role is what all subsequent work addresses. |
| GO:0030061 mitochondrial crista | IEA GO_REF:0000107 | ACCEPT | Summary: NCLX is enriched in mitochondrial cristae. Reason: Experimentally determined sub-mitochondrial location; consistent with the inner-membrane annotations and more specific than them. |
| GO:0042383 sarcolemma | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Sarcolemmal localisation projected from rat. Reason: Orthology projection of a sarcolemmal location that follows from the obsolete plasma-membrane assignment. NCLX acts in the mitochondrial inner membrane, including in cardiomyocytes. |
| GO:0042802 identical protein binding | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: NCLX binds itself. Reason: NCLX does self-associate - co-immunoprecipitation shows it, and both cryo-EM studies resolve a homo-oligomer - so the assertion is correct. It is kept as non-core because self-association is an assembly property rather than the gene's characterised activity. |
| GO:0042803 protein homodimerization activity | IEA GO_REF:0000120 | MODIFY | Summary: NCLX is asserted to form a homodimer. Reason: Self-association is real, but the dimeric stoichiometry is contradicted by both independent cryo-EM studies, which resolve NCLX as a trimer (PMID:40931067, PMID:42431881). The underlying co-immunoprecipitation evidence supports self-association without establishing a dimer, so the annotation is generalised to its direct is_a parent GO:0042802 identical protein binding rather than removed. Proposed replacements: identical protein binding |
| GO:0051480 regulation of cytosolic calcium ion concentration | IEA GO_REF:0000107 | ACCEPT | Summary: NCLX activity shapes cytosolic calcium levels. Reason: A direct consequence of exporting matrix Ca2+ into the cytosol, demonstrated by NCLX knockdown and overexpression; part of the gene's core calcium-handling role. |
| GO:0051560 mitochondrial calcium ion homeostasis | IEA GO_REF:0000107 | ACCEPT | Summary: NCLX sets matrix calcium levels. Reason: Core biological process; matrix Ca2+ levels track NCLX activity in every system tested. |
| GO:0086038 calcium:sodium antiporter activity involved in regulation of cardiac muscle cell membrane potential | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Calcium:sodium antiport specifically in the context of cardiomyocyte membrane potential. Reason: The term's definition ties calcium:sodium antiport specifically to regulating the membrane potential of a cardiac muscle cell - a sarcolemmal NCX1/SLC8A1 role. A crista-resident mitochondrial exchanger does not set the cardiomyocyte membrane potential directly. The annotation is IEA/ISS only, and it compounds the contested Na+ counterion claim with a membrane NCLX does not occupy. |
| GO:0099093 calcium export from the mitochondrion | IEA GO_REF:0000120 | ACCEPT | Summary: NCLX mediates export of calcium from the mitochondrial matrix. Reason: Core biological process. That NCLX exports Ca2+ from the mitochondrial matrix is supported by loss- and gain-of-function work from several independent laboratories and is not challenged by the recent structures, which dispute the counterion rather than the direction or the substrate. |
| GO:1901623 regulation of lymphocyte chemotaxis | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Regulation of lymphocyte chemotaxis, projected from mouse Slc8b1. Reason: A tissue-specific downstream consequence of mitochondrial calcium handling in lymphocytes, projected from mouse; real but far from the molecular function. |
| GO:0006851 mitochondrial calcium ion transmembrane transport | TAS Reactome:R-HSA-8949215 | ACCEPT | Summary: Calcium movement across the mitochondrial inner membrane. Reason: Core biological process; NCLX is the principal characterised route of calcium movement across the mitochondrial inner membrane in the efflux direction. |
| GO:0070588 calcium ion transmembrane transport | TAS Reactome:R-HSA-425561 | ACCEPT | Summary: Calcium ion transmembrane transport. Reason: Correct general parent of the mitochondrial calcium transport role. |
| GO:0005743 mitochondrial inner membrane | EXP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | ACCEPT | Summary: NCLX resides in the mitochondrial inner membrane. Reason: Well-supported core location; NCLX is an integral protein of the mitochondrial inner membrane, confirmed by fractionation, imaging and two cryo-EM studies. Supporting Evidence: PMID:20018762 we show that the Na(+)/Ca(2+) exchanger NCLX is enriched in mitochondria, where it is localized to the cristae. |
| GO:0005743 mitochondrial inner membrane | EXP PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | ACCEPT | Summary: NCLX resides in the mitochondrial inner membrane. Reason: Well-supported core location; NCLX is an integral protein of the mitochondrial inner membrane, confirmed by fractionation, imaging and two cryo-EM studies. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: Mitochondrial residence. Reason: Correct, though the mitochondrial inner membrane and crista annotations are more informative about where NCLX sits. |
| GO:0005432 calcium:sodium antiporter activity | IMP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:20018762 our results converge to the conclusion that NCLX is the long-sought mitochondrial Na(+)/Ca(2+) exchanger. PMID:20018762 NCLX-mediated mitochondrial Ca(2+) transport was inhibited, moreover, by CGP-37157 and exhibited Li(+) dependence, both hallmarks of mitochondrial Na(+)-dependent Ca(2+) efflux. PMID:40931067 Imposing inward Na+ or K+ gradients across the oocyte membrane did not affect NCLX Ca2+ transport (Fig. 5f) PMID:42431881 our assays indicate that NCLX operates as a non-selective cation/Ca²⁺ exchanger, capable of coupling Ca²⁺ transport to Na⁺, K⁺, Li⁺, and potentially H⁺. |
| GO:0086036 regulation of cardiac muscle cell membrane potential | IMP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | KEEP AS NON CORE | Summary: NCLX loss alters cardiomyocyte membrane potential. Reason: Cardiomyocyte membrane potential is affected downstream of mitochondrial calcium handling, but this is an organ-physiology consequence rather than the molecular role, and NCLX does not act at the sarcolemma. |
| GO:0015368 calcium:monoatomic cation antiporter activity | TAS Reactome:R-HSA-425822 | ACCEPT | Summary: Calcium exchange against an unspecified monovalent cation. Reason: The counterion-neutral antiporter term, and the term this review proposes as the replacement for the contested GO:0005432. It is compatible with Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+ exchange, i.e. with every reading currently on offer, and it is supported by the fact that NCLX undisputedly transports Ca2+ across a membrane in exchange for a monovalent cation. |
| GO:0005432 calcium:sodium antiporter activity | IDA PMID:15060069 Lithium-calcium exchange is mediated by a distinct potassium... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:15060069 Surprisingly, NCLX catalyzes active Li(+)/Ca(2+) exchange, thereby explaining the exchange of these ions in mammalian tissues. |
| GO:0005432 calcium:sodium antiporter activity | IDA PMID:28130126 Identification of residues that control Li(+) versus Na(+) d... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:28130126 We identified distinct Na+ and Li+ selective residues in the NCLX transport site. PMID:28130126 In permeabilized cells, N149A, P152A, D153A, N467Q, S468T and G494S demonstrated normal Li+/Ca2+ exchange activity but a reduced Na+/Ca2+ exchange activity. |
| GO:0005432 calcium:sodium antiporter activity | TAS Reactome:R-HSA-8949688 | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity |
| GO:0005432 calcium:sodium antiporter activity | IDA PMID:24898248 NCLX protein, but not LETM1, mediates mitochondrial Ca2+ ext... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:24898248 We conclude that NCLX, but not LETM1, mediates Ca(2+) extrusion from mitochondria. |
| GO:0005432 calcium:sodium antiporter activity | IDA PMID:28219928 Mitochondria control store-operated Ca(2+) entry through Na(... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:28219928 we reveal that SOCE is accompanied by a rise in cytosolic Na+ that is critical in activating the mitochondrial Na+/Ca2+ exchanger (NCLX) causing enhanced mitochondrial Na+ uptake and Ca2+ efflux. |
| GO:0006851 mitochondrial calcium ion transmembrane transport | IDA PMID:24898248 NCLX protein, but not LETM1, mediates mitochondrial Ca2+ ext... | ACCEPT | Summary: Calcium movement across the mitochondrial inner membrane. Reason: Core biological process; NCLX is the principal characterised route of calcium movement across the mitochondrial inner membrane in the efflux direction. |
| GO:0051560 mitochondrial calcium ion homeostasis | IDA PMID:24898248 NCLX protein, but not LETM1, mediates mitochondrial Ca2+ ext... | ACCEPT | Summary: NCLX sets matrix calcium levels. Reason: Core biological process; matrix Ca2+ levels track NCLX activity in every system tested. Supporting Evidence: PMID:24898248 By controlling the duration of matrix Ca(2+) elevations, NCLX contributes to the regulation of NAD(P)H production and to the conversion of Ca(2+) signals into redox changes. |
| GO:0086038 calcium:sodium antiporter activity involved in regulation of cardiac muscle cell membrane potential | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Calcium:sodium antiport specifically in the context of cardiomyocyte membrane potential. Reason: The term's definition ties calcium:sodium antiport specifically to regulating the membrane potential of a cardiac muscle cell - a sarcolemmal NCX1/SLC8A1 role. A crista-resident mitochondrial exchanger does not set the cardiomyocyte membrane potential directly. The annotation is IEA/ISS only, and it compounds the contested Na+ counterion claim with a membrane NCLX does not occupy. |
| GO:0099093 calcium export from the mitochondrion | IDA PMID:24898248 NCLX protein, but not LETM1, mediates mitochondrial Ca2+ ext... | ACCEPT | Summary: NCLX mediates export of calcium from the mitochondrial matrix. Reason: Core biological process. That NCLX exports Ca2+ from the mitochondrial matrix is supported by loss- and gain-of-function work from several independent laboratories and is not challenged by the recent structures, which dispute the counterion rather than the direction or the substrate. Supporting Evidence: PMID:24898248 We conclude that NCLX, but not LETM1, mediates Ca(2+) extrusion from mitochondria. |
| GO:1901623 regulation of lymphocyte chemotaxis | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Regulation of lymphocyte chemotaxis, projected from mouse Slc8b1. Reason: A tissue-specific downstream consequence of mitochondrial calcium handling in lymphocytes, projected from mouse; real but far from the molecular function. |
| GO:2001256 regulation of store-operated calcium entry | IDA PMID:28219928 Mitochondria control store-operated Ca(2+) entry through Na(... | KEEP AS NON CORE | Summary: NCLX is required for sustained store-operated calcium entry. Reason: NCLX links mitochondrial Na+ uptake to a redox signal that protects Orai1 from inactivation, so the annotation is well supported, but store-operated calcium entry is a downstream process that NCLX modulates rather than executes. Supporting Evidence: PMID:28219928 mitochondrial targeting of catalase is sufficient to rescue redox transients, SOCE, and Orai1 currents in NCLX-deficient cells |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-8949688 | ACCEPT | Summary: NCLX resides in the mitochondrial inner membrane. Reason: Well-supported core location; NCLX is an integral protein of the mitochondrial inner membrane, confirmed by fractionation, imaging and two cryo-EM studies. |
| GO:0030061 mitochondrial crista | IDA PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | ACCEPT | Summary: NCLX is enriched in mitochondrial cristae. Reason: Experimentally determined sub-mitochondrial location; consistent with the inner-membrane annotations and more specific than them. Supporting Evidence: PMID:20018762 we show that the Na(+)/Ca(2+) exchanger NCLX is enriched in mitochondria, where it is localized to the cristae. |
| GO:0042803 protein homodimerization activity | IPI PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | MODIFY | Summary: NCLX is asserted to form a homodimer. Reason: Self-association is real, but the dimeric stoichiometry is contradicted by both independent cryo-EM studies, which resolve NCLX as a trimer (PMID:40931067, PMID:42431881). The underlying co-immunoprecipitation evidence supports self-association without establishing a dimer, so the annotation is generalised to its direct is_a parent GO:0042802 identical protein binding rather than removed. Proposed replacements: identical protein binding Supporting Evidence: PMID:40931067 adopts a trimeric assembly in both Ca2+-bound and -free conditions PMID:42431881 The larger SEC fraction contained trimeric NCLX, whose structure was determined at a resolution of 3.15 Å, with all three protomers adopting identical conformations |
| GO:0051480 regulation of cytosolic calcium ion concentration | IMP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | ACCEPT | Summary: NCLX activity shapes cytosolic calcium levels. Reason: A direct consequence of exporting matrix Ca2+ into the cytosol, demonstrated by NCLX knockdown and overexpression; part of the gene's core calcium-handling role. Supporting Evidence: PMID:23056385 Suppression either of NCLX expression, using a siRNA construct (siNCLX) or of its activity, by a dominant negative construct (dnNCLX), enhanced mitochondrial Ca(2+) influx and blocked efflux induced by glucose or by cell depolarization. |
| GO:0051560 mitochondrial calcium ion homeostasis | IMP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | ACCEPT | Summary: NCLX sets matrix calcium levels. Reason: Core biological process; matrix Ca2+ levels track NCLX activity in every system tested. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-425822 | MARK AS OVER ANNOTATED | Summary: Plasma membrane localisation. Reason: A residue of the pre-2010 literature, when the protein (then NCKX6/FLJ22233) was characterised as a plasma-membrane Li+/Ca2+ exchanger, and still modelled that way by Reactome (R-HSA-425822). Everything since PMID:20018762 places NCLX in the mitochondrial inner membrane and cristae, and the mitochondrial role is what all subsequent work addresses. |
| GO:0005432 calcium:sodium antiporter activity | IDA PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | MODIFY | Summary: Calcium:sodium antiport, the classical assignment for NCLX. Reason: The specific counterion in this term is what is now disputed, and the dispute is not resolved. Two 2025/2026 cryo-EM studies agree that NCLX lacks several of the serines that coordinate Na+ in canonical NCX proteins, but reach opposite functional conclusions: Fan et al. (PMID:40931067) report that imposed Na+ or K+ gradients do not drive NCLX Ca2+ transport and reassign it as a H+/Ca2+ exchanger, while Zhang et al. (PMID:42431881) report that Na+, K+ and Li+ all serve as counterions and call it a non-selective cation/Ca2+ exchanger. Against both, earlier mutagenesis mapped distinct Na+- and Li+-selective residues in the NCLX transport site (PMID:28130126), and the gene's founding characterisation turned on its anomalous Li+/Ca2+ exchange (PMID:15060069). MODIFY rather than REMOVE or ACCEPT: none of the experimental annotations is discarded, and no single structure paper is allowed to overturn fifteen years of loss-of-function work. What changes is only the granularity - the annotation is moved up one step to its direct is_a parent GO:0015368 calcium:monoatomic cation antiporter activity, which is compatible with every reading on offer (Na+/Ca2+, Li+/Ca2+, K+/Ca2+ and H+/Ca2+) and which this gene already carries from Reactome. The over-specific part of GO:0005432 is precisely the part under challenge. Note also that Fan et al. share senior authors with PMID:40691517, which assigns mito-NCX to TMEM65; the two form one model rather than two independent challenges. Proposed replacements: calcium:monoatomic cation antiporter activity Supporting Evidence: PMID:23056385 These findings suggest that the mitochondrial Na(+)/Ca(2+) exchanger, NCLX, shapes glucose-dependent mitochondrial and cytosolic Ca(2+) signals thereby regulating the temporal pattern of insulin secretion in |
| GO:0006851 mitochondrial calcium ion transmembrane transport | IDA PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | ACCEPT | Summary: Calcium movement across the mitochondrial inner membrane. Reason: Core biological process; NCLX is the principal characterised route of calcium movement across the mitochondrial inner membrane in the efflux direction. |
| GO:0031966 mitochondrial membrane | IDA PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | ACCEPT | Summary: Mitochondrial membrane localisation. Reason: Correct, though less specific than the mitochondrial inner membrane annotations this gene also carries. |
| GO:0042593 glucose homeostasis | IDA PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | KEEP AS NON CORE | Summary: NCLX knockdown perturbs glucose handling in beta cells. Reason: A tissue-specific physiological consequence of NCLX activity in pancreatic beta cells; real, but several steps downstream of the molecular function. |
| GO:0050796 regulation of insulin secretion | IDA PMID:23056385 The mitochondrial Na+/Ca2+ exchanger upregulates glucose dep... | KEEP AS NON CORE | Summary: NCLX shapes the timing of glucose-evoked insulin secretion. Reason: A tissue-specific physiological consequence of NCLX shaping glucose-evoked mitochondrial and cytosolic calcium transients in beta cells; downstream of the core function. Supporting Evidence: PMID:23056385 These findings suggest that the mitochondrial Na(+)/Ca(2+) exchanger, NCLX, shapes glucose-dependent mitochondrial and cytosolic Ca(2+) signals thereby regulating the temporal pattern of insulin secretion in |
| GO:0006851 mitochondrial calcium ion transmembrane transport | IMP PMID:20018762 NCLX is an essential component of mitochondrial Na+/Ca2+ exc... | ACCEPT | Summary: Calcium movement across the mitochondrial inner membrane. Reason: Core biological process; NCLX is the principal characterised route of calcium movement across the mitochondrial inner membrane in the efflux direction. |
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Download this section (compressed HTML)Q: What is the physiological counterion for NCLX-mediated calcium efflux in intact mitochondria - Na+, Li+, K+, H+, or several of them? Fan et al. (PMID:40931067) find imposed Na+ and K+ gradients do not drive NCLX calcium transport, Zhang et al. (PMID:42431881) find that they do, and Ben-Kasus Nissim et al. (PMID:28130126) previously mapped separate Na+- and Li+-selective residues in the transport site. The three results cannot all be right.
Q: Why does sodium-dependent mitochondrial calcium efflux survive NCLX knockout in HeLa, CHO, HEK293 and HCT116 cells (PMID:40931067), when NCLX knockdown or knockout reduces mitochondrial calcium efflux in so many other systems? Is the residual activity TMEM65, a compensating transporter, or an assay difference?
Q: What is the physical and functional relationship between NCLX and TMEM65? TMEM65 co-purifies with NCLX and its effect on calcium efflux is lost in NCLX-null fibroblasts (PMID:40200126), yet purified TMEM65 alone is reported to reconstitute exchange without NCLX present (PMID:40691517).
Q: Both cryo-EM studies resolve NCLX as a trimer, contradicting the homodimer in the GO record. Is the trimer the functional unit in the inner membrane, and does it imply cooperativity or a shared ion pathway?
Q: Does NCLX contribute to the classical Na+-independent H+/Ca2+ exchange (NICE) of mitochondria, and if so is NICE the activity that its many loss-of-function phenotypes actually report?
Experiment: Reconstitute purified NCLX into proteoliposomes with controlled internal and external ionic composition and measure Ca2+ flux against imposed Na+, Li+, K+ and pH gradients in the same preparation, so that the Fan and Zhang results are compared under identical conditions rather than across two heterologous expression systems.
Experiment: Record NCLX currents electrophysiologically in mitoplasts from wild-type and NCLX-knockout cells under defined Na+ and pH gradients, which would establish the counterion and the stoichiometry in the native membrane rather than in oocytes or HEK cells.
Experiment: Re-test the Na+- and Li+-selective residues identified in PMID:28130126 (N149, P152, N467, S468, G494) on the background of the new structures, mapping them onto the ion-binding pocket to determine whether they line a genuine monovalent site or act allosterically.
Experiment: Measure mitochondrial Na+-dependent calcium efflux in cells lacking NCLX, lacking TMEM65, and lacking both, in a tissue where both are expressed (cardiomyocytes or neurons), to determine whether the two proteins act in one pathway or two.
Experiment: Determine whether the trimeric assembly seen by cryo-EM exists in the native inner membrane, using in-cell crosslinking or native mass spectrometry of mitochondria, and test whether interface mutations that break the trimer abolish exchange.
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