TMEM126B

TMEM126B is a mitochondrial inner membrane protein that functions as a core subunit of the mitochondrial complex I intermediate assembly (MCIA) complex, together with ACAD9, ECSIT, NDUFAF1, TMEM186, and COA1, where it specifically mediates assembly of the ND2-module of the membrane arm of NADH:ubiquinone oxidoreductase (PMID:24191001, PMID:32320651, PMID:33879611). This role is non-redundant with its paralogue TMEM126A, which instead assembles the ND4 distal membrane module (PMID:33879611). Under chronic hypoxia, TMEM126B is selectively targeted for ubiquitin-proteasomal degradation via HIF-1α-induced β-TrCP1, thereby remodeling the mitochondrial respiratory chain; loss of TMEM126B reduces mitochondrial ROS production, impairing oxidative modification of SDHA and downstream HIF-1α stabilization and IL-1β expression in macrophages (PMID:29464284, PMID:30368040). Biallelic loss-of-function mutations in TMEM126B cause isolated mitochondrial complex I deficiency in humans (PMID:27374774, PMID:29093663, PMID:36482121).

1. 2013 High

   The identity of factors that build the membrane arm of complex I was incomplete; co-fractionation of TMEM126B with 550 and 815 kDa assembly intermediates established it as a membrane-arm assembly factor.

   Evidence Blue-native PAGE and complexome profiling after NDUFA11 knockdown in human cells

   PMID:24191001

   Open questions at the time

   • Precise submodule specificity (ND2 vs ND4 vs ND5) was not yet resolved
   • No information on which other factors form a stable complex with TMEM126B
   • No disease-causing mutations yet reported
2. 2016 High

   Whether TMEM126B mutations cause human disease was unknown; biallelic loss-of-function variants were shown to cause isolated complex I deficiency, and viral complementation rescued assembly, confirming causality and placing TMEM126B as a bona fide MCIA complex member alongside ACAD9, ECSIT, NDUFAF1, and TIMMDC1.

   Evidence Whole-exome sequencing across multiple families, lentiviral rescue in patient fibroblasts, complexome profiling

   PMID:27374774

   Open questions at the time

   • Complete subunit composition of the MCIA complex was not yet defined
   • The hierarchy of stability among MCIA subunits was unknown
   • No regulatory mechanism controlling TMEM126B levels had been identified
3. 2017 Medium

   The functional consequence of the recurrent p.G212V missense mutation was undefined; complementation in patient fibroblasts demonstrated it causes incomplete peripheral arm assembly, and fatty acid supplementation partially rescued OXPHOS capacity.

   Evidence Complementation study in patient fibroblasts, Seahorse respirometry with palmitic acid

   PMID:29093663

   Open questions at the time

   • Mechanism by which fatty acids bypass the assembly defect was not elucidated
   • Whether the metabolic rescue generalizes to other MCIA factor mutations was untested
4. 2018 Medium

   How cells remodel complex I under hypoxia was unclear; HIF-1α was shown to induce β-TrCP1-mediated ubiquitin-proteasomal degradation of TMEM126B selectively among MCIA members, linking oxygen sensing to respiratory chain remodeling. Downstream, TMEM126B loss reduced mitochondrial ROS, diminished oxidative modification of SDHA, and attenuated HIF-1α stabilization and IL-1β induction in macrophages.

   Evidence Complexome profiling under chronic hypoxia, siRNA knockdown of HIF-1α and β-TrCP1 in THP-1 cells, BIAM switch assay with LC-MS, Seahorse respirometry, pharmacological SDH inhibition with atpenin A5

   PMID:29464284 PMID:30368040

   Open questions at the time

   • Direct ubiquitination sites on TMEM126B were not mapped
   • Whether β-TrCP1-mediated degradation of TMEM126B operates in cell types beyond THP-1 macrophages was not tested
   • Contribution of TMEM126B-dependent ROS signaling in vivo remains uncharacterized
5. 2020 High

   The complete composition and internal hierarchy of the MCIA complex were unresolved; CRISPR knockout and quantitative proteomics demonstrated that TMEM126B is a core MCIA subunit with a stability hierarchy centered on ACAD9, and identified TMEM186 and COA1 as additional bona fide MCIA components with roles distinct from TMEM126B.

   Evidence CRISPR/Cas9 knockouts, co-immunoprecipitation, quantitative proteomics, complexome profiling, pulse-labeling in HEK293T cells

   PMID:32320651

   Open questions at the time

   • Structural basis for TMEM126B interactions within the MCIA complex was not determined
   • Sequential order of MCIA subunit recruitment to the ND2-module was not fully resolved
6. 2021 High

   Whether TMEM126B and its paralogue TMEM126A have overlapping or distinct functions was unknown; comparative knockouts established that TMEM126B acts specifically on the ND2-module while TMEM126A assembles the ND4 distal membrane module, demonstrating non-redundant, module-specific roles.

   Evidence Genome editing (knockout of each paralogue), interaction studies, quantitative proteomics, pulse-labeling

   PMID:33879611

   Open questions at the time

   • Molecular determinants that confer module specificity between the two paralogues were not identified
   • No structural model of either paralogue in complex with its cognate module exists
7. 2022 Medium

   The molecular consequences of intronic TMEM126B mutations were undefined; minigene and patient RNA analyses demonstrated that a splice-site mutation (c.82-2A>G) and an exonic insertion (c.290dupT) each cause aberrant exon skipping, frameshifts, and premature termination, resulting in severe complex I deficiency.

   Evidence Minigene splicing assay, patient-derived lymphocyte RNA analysis, complex I assembly assessment

   PMID:36482121

   Open questions at the time

   • Whether any truncated TMEM126B protein is produced and retains partial function was not assessed
   • Genotype-phenotype correlations across the growing mutation spectrum remain incomplete

• Key unresolved questions include the structural basis of TMEM126B within the MCIA complex, the identity of its ubiquitination sites mediating hypoxic degradation, and the in vivo physiological significance of TMEM126B-dependent mitochondrial ROS signaling in immunity and disease.
• No high-resolution structure of TMEM126B or the MCIA complex has been determined
• Direct ubiquitination sites on TMEM126B have not been mapped
• In vivo animal models of TMEM126B deficiency have not been reported