Affinage

APOO

MICOS complex subunit MIC26 · UniProt Q9BUR5

Audit flag: ungrounded claim
Length
198 aa
Mass
22.3 kDa
Annotated
2026-06-09
13 papers in source corpus 10 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 7/7 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

APOO (MIC26) encodes a 22 kDa integral inner mitochondrial membrane protein that functions as a subunit of the MICOS complex governing crista junction formation and mitochondrial ultrastructure (PMID:25764979, PMID:26217776). It is incorporated into MICOS through physical interactions with MIC60, MIC27, and MIC10, and its depletion reduces crista junction number and distorts cristae architecture (PMID:25764979, PMID:26217776). Within the complex MIC26 acts antagonistically to its paralog MIC27 (APOOL): it destabilizes the MIC10 oligomeric scaffold that MIC27 stabilizes, and the two proteins reciprocally regulate each other's levels (PMID:25764979, PMID:26217776, PMID:29733859). MIC26 and MIC27 assemble late into MICOS and are dispensable for integration of the remaining subunits, but together they are required for cardiolipin homeostasis and for the integrity of respiratory chain supercomplexes and F1Fo-ATP synthase, with cardiolipin synthase overexpression rescuing supercomplex stability in double-knockout cells (PMID:32788226). Through this role MIC26 shapes mitochondrial bioenergetics and physiology: adipocyte-specific loss impairs oxidative phosphorylation and shifts metabolism toward glycolysis (PMID:37088120), and macrophage-specific loss enhances efferocytosis by lowering OPA1 levels to drive mitochondrial fission (PMID:37995600). Pathogenic APOO mutations—a missense I117T that impairs import and membrane insertion, and a C-terminal-truncating nonsense E178* that destabilizes the protein—disrupt MICOS assembly and cristae architecture, causing X-linked mitochondrial disease (PMID:32439808, PMID:37649161). An earlier model of APOO as a secreted glycoprotein acting in cholesterol efflux (PMID:16956892) was shown to reflect non-specific signal rather than a genuine protein isoform; MIC26 is exclusively mitochondrial (PMID:37279200).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2006 Medium

    The first functional assignment placed APOO outside the mitochondrion, proposing it as a secreted chondroitin-sulfate proteoglycan acting in macrophage cholesterol efflux.

    Evidence Confocal immunofluorescence, enzymatic deglycosylation, MTP pharmacological inhibition, and cholesterol efflux assays in a single lab

    PMID:16956892

    Open questions at the time
    • The secreted glycoprotein assignment was later attributed to non-specific signal
    • No mitochondrial role identified at this stage
  2. 2015 High

    Identification of MIC26 as a MICOS subunit redefined APOO as an inner-membrane protein required for crista junction formation, answering where it acts and which complex it belongs to.

    Evidence Reciprocal co-immunoprecipitation with MIC60/MIC27/MIC10, miRNA knockdown, and electron microscopy of cristae, with companion data paper

    PMID:25764979 PMID:26217776

    Open questions at the time
    • Molecular mechanism of crista junction formation not resolved
    • Stoichiometry within MICOS not defined
  3. 2015 Medium

    Antagonistic regulation of protein levels between MIC26 and MIC27 and correlation with tafazzin established a regulatory relationship linking MIC26 to cardiolipin remodeling.

    Evidence miRNA knockdown and overexpression with miRNA-resistant rescue constructs and immunoblotting of MICOS subunits

    PMID:25764979 PMID:26217776

    Open questions at the time
    • Mechanism of reciprocal level regulation unknown
    • Direct effect on cardiolipin not yet demonstrated
  4. 2018 High

    Biochemical reconstitution defined the molecular action of MIC26 as destabilizing MIC10 oligomers, opposite to the stabilizing role of MIC27, clarifying how the two paralogs tune the MICOS scaffold.

    Evidence Blue-native PAGE, in vitro oligomerization assays, and genetic manipulation in yeast

    PMID:29733859

    Open questions at the time
    • Structural basis of MIC10 oligomer destabilization unresolved
    • Quantitative balance of MIC26/MIC27 in vivo not established
  5. 2020 High

    Double-knockout genetics linked MIC26/MIC27 to cardiolipin homeostasis and respiratory chain supercomplex and ATP synthase integrity, and showed they assemble late into MICOS, connecting MICOS architecture to bioenergetic assembly.

    Evidence Single/double knockout cells, complexome profiling, STED nanoscopy, BN-PAGE, and cardiolipin synthase overexpression rescue

    PMID:32788226

    Open questions at the time
    • Direct mechanism by which cardiolipin loss destabilizes supercomplexes not detailed
    • Individual contribution of MIC26 versus MIC27 partly redundant
  6. 2020 High

    A patient missense mutation establishing APOO as a cause of X-linked mitochondrial disease, by showing impaired import and MICOS assembly defects validated across model organisms.

    Evidence Whole exome sequencing, patient fibroblast import assays, MICOS co-IP, and yeast and Drosophila models with EM

    PMID:32439808

    Open questions at the time
    • Genotype-phenotype spectrum across patients not defined
    • How import failure translates to organ-level disease unclear
  7. 2023 Medium

    A second pathogenic allele (E178* truncation) showed that loss of the C-terminus destabilizes MIC26 while preserving its MICOS interactions, distinguishing protein stability from complex binding in disease.

    Evidence Exome sequencing, KO complementation with mutant protein, co-IP of MICOS subunits, and morphology analysis

    PMID:37649161

    Open questions at the time
    • Why residual interacting mutant fails to restore cristae unexplained
    • Single patient context
  8. 2023 High

    Rigorous knockout and antibody controls overturned the original secreted-glycoprotein model, establishing MIC26 as an exclusively mitochondrial protein.

    Evidence CRISPR/siRNA knockouts in four cell lines, four antibodies, tagged constructs, glycosylation-site mutagenesis, and mass spectrometry of gel bands

    PMID:37279200

    Open questions at the time
    • Identity of the misattributed 55 kDa band not fully resolved
  9. 2023 Medium

    Tissue-specific knockout mice extended MIC26 function to physiology, showing roles in adipocyte oxidative metabolism and in macrophage efferocytosis via OPA1.

    Evidence Adipocyte- and macrophage-specific knockout mice with metabolic phenotyping, EM, PPARalpha analysis, and OPA1 epistasis (knockdown and overexpression)

    PMID:37088120 PMID:37995600

    Open questions at the time
    • Mechanism connecting MIC26 loss to OPA1 reduction unknown
    • Generalizability of metabolic shift across tissues untested

Open questions

Synthesis pass · forward-looking unresolved questions
  • How MIC26 mechanistically links MICOS architecture and cardiolipin to downstream regulators such as OPA1 and to tissue-specific metabolic outcomes remains unresolved.
  • No structure of MIC26 within MICOS
  • Mechanism of OPA1 regulation by MIC26 undefined
  • Full clinical spectrum of APOO mutations not characterized

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 2
Localization
GO:0005739 mitochondrion 2
Pathway
R-HSA-1430728 Metabolism 2 R-HSA-1852241 Organelle biogenesis and maintenance 2
Partners
MIC10MIC27MIC60
Complex memberships
MICOS complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 ApoO (APOO) is a secreted glycoprotein that belongs to the proteoglycan family (contains chondroitin sulfate chains), co-localizes with perilipins at lipid droplets, promotes cholesterol efflux from macrophages, and requires microsomal triglyceride transfer protein (MTP) activity for secretion. Confocal immunofluorescence microscopy, chondroitinase ABC deglycosylation, xyloside treatment, naringenin/CP-346086 pharmacological inhibition of MTP, cholesterol efflux assay The Journal of biological chemistry Medium 16956892
2015 The non-glycosylated 22 kDa mitochondrial isoform of MIC26 (APOO) spans the mitochondrial inner membrane, physically interacts with MICOS complex subunits MIC60, MIC27, and MIC10, and is required for crista junction formation; its depletion reduces crista junction number and alters mitochondrial ultrastructure. Co-immunoprecipitation, miRNA-mediated knockdown, electron microscopy, mitochondrial fractionation, immunofluorescence Biochimica et biophysica acta High 25764979 26217776
2015 MIC26 and MIC27 (APOOL) regulate each other's protein levels in an antagonistic manner; overexpression of MIC26 increases MIC10 levels while depletion of MIC26 increases MIC27 levels. Both proteins are positively correlated with tafazzin levels, linking MIC26 to cardiolipin remodeling. miRNA-mediated knockdown, overexpression, immunoblotting with miRNA-resistant rescue constructs Biochimica et biophysica acta Medium 25764979 26217776
2015 Overexpression of MIC26 induces mitochondrial fragmentation, promotes ROS formation, and impairs mitochondrial respiration; knockdown of MIC26 decreases mitochondrial oxygen consumption. Overexpression and miRNA-mediated knockdown, live-cell ROS measurement, oxygen consumption assay, mitochondrial network imaging Biochimica et biophysica acta Medium 25764979
2018 MIC26 destabilizes Mic10 oligomers in an antagonistic manner to MIC27 (which stabilizes Mic10 oligomers), demonstrating that MIC26 negatively regulates the oligomeric scaffold formed by the core MICOS subunit Mic10. Blue-native PAGE, biochemical fractionation, in vitro reconstitution/oligomerization assays, genetic manipulation in yeast Journal of molecular biology High 29733859
2020 MIC26 and MIC27 together are required for integrity of respiratory chain (super)complexes and F1Fo-ATP synthase (including integration of F1 subunits), and cooperatively regulate cardiolipin levels; restoring cardiolipin by overexpression of cardiolipin synthase in double knockout cells rescues respiratory chain supercomplex stability. Single and double knockout cell lines, complexome profiling, STED nanoscopy, blue-native gel electrophoresis, cardiolipin measurement, cardiolipin synthase overexpression rescue Life science alliance High 32788226
2020 MIC26 and MIC27 are dispensable for stability and integration of the remaining MICOS subunits into the complex, indicating they assemble late into MICOS. Double knockout human cells, complexome profiling, blue-native gel electrophoresis Life science alliance Medium 32788226
2020 A missense mutation (I117T) in APOO impairs MIC26 import processing and insertion into the inner mitochondrial membrane, causing altered MICOS assembly and crista junction disruption; corresponding mutations in yeast and Drosophila models confirmed MIC26 involvement in MICOS assembly and mitochondrial function. Whole exome sequencing, patient-derived fibroblasts, protein import assay, MICOS co-immunoprecipitation, Drosophila and yeast knockout models, electron microscopy Journal of medical genetics High 32439808
2023 A nonsense mutation (E178*) in APOO/MIC26 producing a truncated protein lacking 20 C-terminal amino acids results in a highly unstable protein; remaining mutant MIC26 correctly localizes to mitochondria and physically interacts with other MICOS subunits, but cannot restore normal cristae architecture in MIC26 KO cells. Exome sequencing, MIC26 KO cell complementation with mutant protein, immunoprecipitation of MICOS subunits, immunofluorescence localization, mitochondrial morphology analysis Clinical genetics Medium 37649161
2023 MIC26 and MIC27 are exclusively mitochondrial proteins (22 kDa and 30 kDa respectively); the previously reported 55 kDa glycosylated secreted MIC26 isoform is a non-specific signal, not a genuine MIC26 protein form, as confirmed by knockout lines, four independent antibodies, epitope-tagged constructs, glycosylation site mutagenesis, and mass spectrometry. CRISPR/siRNA knockouts in four human cell lines, four anti-MIC26 antibodies, GFP/myc-tagged constructs, glycosylation site mutagenesis, mass spectrometry of excised gel bands PloS one High 37279200
2023 Adipocyte-specific APOO knockout mice show disrupted mitochondrial structure in brown adipocytes, impaired oxidative phosphorylation, shift from oxidative to glycolytic metabolism, increased lipogenic enzyme levels, reduced long-chain fatty acid oxidation, and disturbed peroxisomal biogenesis and very-long-chain fatty acid oxidation via PPARα. Adipocyte-specific APOO knockout mice (ApooACKO), electron microscopy, oxygen consumption assays, metabolic phenotyping, PPARα pathway analysis Metabolism: clinical and experimental Medium 37088120
2023 Macrophage-specific MIC26 (APOO) deletion increases efferocytosis by reducing mitochondrial OPA1 protein levels (causing increased mitochondrial fission and reduced membrane potential); OPA1 silencing phenocopied the efferocytosis increase, and OPA1 overexpression abolished the efferocytosis enhancement caused by MIC26 deficiency. Macrophage-specific MIC26 knockout mice (MIC26LysM), in vitro efferocytosis assay, in vivo thymus efferocytosis assay, OPA1 siRNA knockdown and overexpression epistasis, mitochondrial membrane potential measurement, electron microscopy Atherosclerosis Medium 37995600

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2015 The non-glycosylated isoform of MIC26 is a constituent of the mammalian MICOS complex and promotes formation of crista junctions. Biochimica et biophysica acta 72 25764979
2006 ApoO, a novel apolipoprotein, is an original glycoprotein up-regulated by diabetes in human heart. The Journal of biological chemistry 62 16956892
2018 Assembly of the Mitochondrial Cristae Organizer Mic10 Is Regulated by Mic26-Mic27 Antagonism and Cardiolipin. Journal of molecular biology 46 29733859
2020 MIC26 and MIC27 cooperate to regulate cardiolipin levels and the landscape of OXPHOS complexes. Life science alliance 42 32788226
2020 Mutation in the MICOS subunit gene APOO (MIC26) associated with an X-linked recessive mitochondrial myopathy, lactic acidosis, cognitive impairment and autistic features. Journal of medical genetics 40 32439808
2014 Novel intracellular functions of apolipoproteins: the ApoO protein family as constituents of the Mitofilin/MINOS complex determines cristae morphology in mitochondria. Biological chemistry 30 24391192
2023 Loss of APOO (MIC26) aggravates obesity-related whitening of brown adipose tissue via PPARα-mediated functional interplay between mitochondria and peroxisomes. Metabolism: clinical and experimental 16 37088120
2023 Macrophage-specific deletion of MIC26 (APOO) mitigates advanced atherosclerosis by increasing efferocytosis. Atherosclerosis 12 37995600
2023 A X-linked nonsense APOO/MIC26 variant causes a lethal mitochondrial disease with progeria-like phenotypes. Clinical genetics 11 37649161
2018 An APOO Pseudogene on Chromosome 5q Is Associated With Low-Density Lipoprotein Cholesterol Levels. Circulation 10 29593015
2023 MIC26 and MIC27 are bona fide subunits of the MICOS complex in mitochondria and do not exist as glycosylated apolipoproteins. PloS one 4 37279200
2015 Data supporting the role of the non-glycosylated isoform of MIC26 in determining cristae morphology. Data in brief 3 26217777
2011 Construction and expression of recombinant fusion protein of thioredoxin-ApoO. Zhong nan da xue xue bao. Yi xue ban = Journal of Central South University. Medical sciences 1 21368419

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