TMEM126B

TMEM126B is a core subunit of the mitochondrial complex I intermediate assembly (MCIA) complex—together with ACAD9, ECSIT, NDUFAF1, and TIMMDC1—that drives biogenesis of the membrane arm of respiratory complex I (PMID:27374774, PMID:32320651). Within the MCIA complex, which is organized around a stability hierarchy centered on ACAD9, loss of TMEM126B destabilizes the entire assembly module and specifically blocks construction of the ND2-module, a role distinct from its paralogue TMEM126A, which instead supports assembly of the ND4 distal membrane module (PMID:32320651, PMID:33879611). Consistent with this assembly-factor function, biallelic loss-of-function, missense, and splice-disrupting variants in TMEM126B cause severe complex I deficiency, and re-expression of wild-type TMEM126B in patient cells restores complex I assembly (PMID:27374774, PMID:29093663, PMID:36482121). Beyond its constitutive assembly role, TMEM126B is a regulatory node coupling complex I biogenesis to redox and hypoxic signaling: its loss attenuates LPS-induced mitochondrial ROS production, reducing SDHA oxidation and thereby limiting HIF-1α stabilization and IL-1β expression in macrophages (PMID:30368040), while under chronic hypoxia HIF-1α–induced β-TrCP1 targets TMEM126B for proteolytic degradation, lowering MCIA abundance and complex I assembly (PMID:29464284).

1. 2013 Medium

   Established that TMEM126B physically associates with complex I biogenesis, situating an uncharacterized membrane protein within respiratory chain assembly.

   Evidence NDUFA11 suppression with subcomplex isolation and mass spectrometry identifying TMEM126B in 550/815 kDa membrane-arm subcomplexes

   PMID:24191001

   Open questions at the time

   • Did not define which assembly step or module TMEM126B serves
   • Association inferred from subcomplex accumulation rather than direct functional assay
2. 2016 High

   Defined TMEM126B as a bona fide MCIA complex component and established disease causation, answering whether its loss is pathogenic.

   Evidence Complexome profiling plus viral rescue in patient cell lines and whole-exome sequencing across families

   PMID:27374774

   Open questions at the time

   • Did not resolve the specific module within complex I that requires TMEM126B
   • Stoichiometry and architecture within MCIA not defined
3. 2017 Medium

   Confirmed the assembly-factor role through a specific missense allele, linking peripheral arm assembly defects to reduced mature complex I.

   Evidence Whole-exome sequencing and complementation of patient fibroblasts with OXPHOS and BN-PAGE readouts for the p.G212V mutation

   PMID:29093663

   Open questions at the time

   • Single-family/single-lab functional validation
   • Mechanism by which the missense change disrupts assembly not resolved
4. 2018 Medium

   Connected TMEM126B-dependent complex I assembly to inflammatory redox signaling, showing it is not merely a housekeeping assembly factor.

   Evidence shRNA knockdown in THP-1 macrophages with BIAM-switch LC-MS oxidation profiling, SDH activity, HIF-1α/IL-1β measurement, and pharmacological SDH inhibition rescue

   PMID:30368040

   Open questions at the time

   • Knockdown rather than clean knockout
   • Causal chain from mtROS to SDHA oxidation inferred via pharmacology in a single lab
   • In vivo relevance not tested
5. 2018 Medium

   Revealed that TMEM126B abundance is dynamically regulated, identifying HIF-1α/β-TrCP1–mediated degradation as a hypoxic control point on complex I assembly.

   Evidence Complexome profiling of mitochondria under hypoxia with HIF-1α and β-TrCP1 modulation and protein stability assays

   PMID:29464284

   Open questions at the time

   • Direct ubiquitination site/E3 engagement not mapped
   • Single-lab evidence for the degradation axis
6. 2020 High

   Resolved the module specificity and intra-complex hierarchy, showing TMEM126B is required to build the ND2-module and that its loss collapses the whole MCIA complex.

   Evidence CRISPR/Cas9 knockout of each MCIA component with quantitative proteomics, complexome profiling, and co-immunoprecipitation

   PMID:32320651

   Open questions at the time

   • Atomic/structural basis of ND2-module handover not defined
   • Order of subunit recruitment within ND2 assembly not fully resolved
7. 2021 High

   Distinguished TMEM126B from its paralogue TMEM126A, assigning each to a separate membrane module and clarifying non-redundant functions.

   Evidence Knockout, pulse-labeling of newly synthesized mtDNA-encoded subunits, quantitative proteomics, and co-immunoprecipitation

   PMID:33879611

   Open questions at the time

   • How the two paralogues partition substrate modules mechanistically is unresolved
   • Whether they ever cooperate at module interfaces unknown
8. 2022 Medium

   Extended the mutational spectrum to splice-disrupting alleles, showing loss of protein via exon skipping recapitulates the severe complex I defect.

   Evidence Minigene splicing assays, patient RNA analysis, and functional complex I assessment in patient lymphocytes for intronic and insertion variants

   PMID:36482121

   Open questions at the time

   • Genotype–phenotype correlation across allele types not systematized
   • Single-lab functional validation

• How TMEM126B mechanistically templates ND2-module assembly and how its degradation is structurally triggered remain unresolved.
• No high-resolution structure of TMEM126B in the MCIA complex
• Ubiquitination determinants for β-TrCP1 not mapped
• In vivo significance of the redox/hypoxia regulatory axis untested