APOOL

APOOL (MIC27) is a non-glycosylated inner mitochondrial membrane protein of the MICOS complex that shapes cristae architecture by coupling cardiolipin binding to assembly of the membrane-sculpting MICOS scaffold (PMID:23704930, PMID:25918844, PMID:37279200). It faces the intermembrane space, specifically binds cardiolipin but not its precursor phosphatidylglycerol, and physically associates with MICOS/MIB subunits including Mitofilin/MIC60, MINOS1/MIC10, and SAMM50 (PMID:23704930). Within MICOS it belongs to the MIC10/MIC12/MIC27 subcomplex that assembles independently of the MIC60/MIC19 subcomplex and whose formation depends on cardiolipin and respiratory complexes (PMID:25918844); its integration requires MIC13/QIL1, whose loss triggers degradation of MIC27 together with MIC10 and MIC26 (PMID:25997101, PMID:27479602). Mechanistically, MIC27 stabilizes MIC10 oligomers and promotes oligomerization of the F1FO-ATP synthase, thereby supporting crista junction formation and cristae membrane curvature (PMID:26968360, PMID:28845423). MIC27 and MIC26 reciprocally regulate each other's levels and act antagonistically on MIC10 oligomer stability, and their combined loss cooperatively depletes cardiolipin and destabilizes respiratory chain supercomplexes and ATP synthase, a defect reversed by cardiolipin synthase overexpression (PMID:25764979, PMID:29733859, PMID:32788226). Loss-of-function mutation in QIL1/MIC13 eliminates the MIC10–MIC26–MIC27–QIL1 subcomplex and produces aberrant cristae and respiratory chain deficiency in patient tissue (PMID:29618761).

1. 2013 High

   Established APOOL/MIC27 as a cardiolipin-binding inner membrane protein integral to the MICOS/MINOS complex, answering whether this protein had a defined mitochondrial role.

   Evidence Complexome profiling, in vitro lipid-binding assay, Co-IP, and knockdown/overexpression with morphology and respiration readouts in human cells

   PMID:23704930

   Open questions at the time

   • Did not resolve subcomplex membership within MICOS
   • Did not define the structural basis of cardiolipin selectivity
2. 2015 High

   Placed MIC27 architecturally within a distinct MIC10/MIC12/MIC27 subcomplex separate from the MIC60/MIC19 module, and showed its assembly depends on cardiolipin and respiratory complexes.

   Evidence Yeast genetic deletions, complexome profiling, and respiratory growth assays; complemented by human knockdown studies of MICOS/MIB subunits

   PMID:25781180 PMID:25918844

   Open questions at the time

   • Conflicting reports on whether MIC27 depletion alone alters cristae morphology
   • Did not establish the assembly trigger linking respiratory complexes to subcomplex formation
3. 2015 High

   Defined the antagonistic, reciprocal regulation between MIC27 and MIC26 and linked both to MIC10 and the cardiolipin remodeling enzyme tafazzin.

   Evidence Bidirectional miRNA knockdown, overexpression, Co-IP, and western blot for protein-level changes

   PMID:25764979

   Open questions at the time

   • Mechanism of reciprocal protein-level control not defined
   • Direct tafazzin interaction not established
4. 2016 High

   Determined the MICOS assembly hierarchy, showing MIC13/QIL1 is required for integration of MIC27 and that MIC27 functions to stabilize MIC10 oligomers.

   Evidence CRISPR/Cas9 MIC13 knockout, yeast genetic deletion, complexome profiling, blue-native PAGE, and electron microscopy

   PMID:26968360 PMID:27479602

   Open questions at the time

   • Did not resolve how MIC27 physically stabilizes MIC10 oligomers
   • Structural model of the subcomplex absent
5. 2017 High

   Connected MIC27 to bioenergetic membrane shaping by showing it promotes F1FO-ATP synthase oligomerization and crista junction formation, partially compensating for MIC60 loss.

   Evidence Yeast genetic deletion, complexome profiling, chemical crosslinking of MIC10 to MIC27, blue-native PAGE, and electron microscopy

   PMID:28845423

   Open questions at the time

   • Mechanism coupling MICOS to ATP synthase oligomers not fully defined
   • Crosslink demonstrates proximity but not interface architecture
6. 2018 High

   Demonstrated mechanistically that MIC27 and cardiolipin stabilize MIC10 oligomers while MIC26 destabilizes them, linking MIC27 lipid binding to scaffold regulation.

   Evidence Yeast single and combined MIC26/MIC27 deletions, blue-native PAGE, and cardiolipin manipulation

   PMID:29733859

   Open questions at the time

   • Did not establish whether MIC27 acts via direct cardiolipin presentation to MIC10
7. 2018 Medium

   Provided human disease validation that the MIC10–MIC26–MIC27–QIL1 subcomplex is lost in QIL1/MIC13 deficiency, causing cristae and respiratory chain defects.

   Evidence Patient genetic analysis, western blot of MICOS subunits, tissue electron microscopy, and OXPHOS enzyme activity assays

   PMID:29618761

   Open questions at the time

   • Single case without in vitro reconstitution
   • MIC27-specific contribution to phenotype not isolated from QIL1 loss
8. 2019 Medium

   Established the mitochondrial import route of MIC27, showing it uses the presequence pathway distinct from the MIA pathway used by Mic19.

   Evidence In vitro mitochondrial import assays with radiolabeled precursors and inhibitors in yeast

   PMID:30718713

   Open questions at the time

   • Single-lab in vitro study
   • Human MIC27 import not directly tested
9. 2020 High

   Showed MIC26 and MIC27 act cooperatively as late-assembling MICOS subunits whose combined loss depletes cardiolipin and destabilizes OXPHOS supercomplexes, reversible by cardiolipin synthase.

   Evidence CRISPR/Cas9 single and double knockouts in human cells with complexome profiling, STED nanoscopy, blue-native PAGE, lipidomics, and cardiolipin synthase rescue; supported by a Drosophila ortholog knockdown model

   PMID:32788226 PMID:33268479

   Open questions at the time

   • Did not separate direct cardiolipin-binding from cardiolipin-level regulation
   • Tissue-specific consequences in mammals not addressed
10. 2023 High

    Definitively established MIC27 as an exclusively mitochondrial, non-glycosylated MICOS subunit, ruling out a glycosylated extramitochondrial isoform.

    Evidence CRISPR/Cas9 knockout across four human cell lines, glycosylation-site mutagenesis, multiple antibodies, and mass spectrometry of gel bands

    PMID:37279200

    Open questions at the time

    • Did not address whether other post-translational modifications regulate MIC27
11. 2025 Medium

    Mapped the molecular neighbourhood of MIC27 within MICOS, identifying proximal OXPHOS subunits, translocases, ribosomal proteins, and solute carriers.

    Evidence APEX2 proximity biotinylation in MIC27 knockout human cells with electron microscopy and STED validation (preprint)

    PMID:bio_10.1101_2025.05.20.655052

    Open questions at the time

    • Preprint without functional follow-up for most proximal proteins
    • Proximity does not establish direct physical interaction

• How MIC27 cardiolipin binding is structurally coupled to MIC10 oligomer stabilization and ATP synthase oligomerization at the atomic level remains unresolved.
• No high-resolution structure of the MIC10/MIC12/MIC27 subcomplex
• Direct lipid-presentation mechanism untested
• Mammalian tissue-level physiological roles incompletely defined