SUCLG1

SUCLG1 encodes the alpha subunit of mitochondrial succinate-CoA ligase (SUCL), a heterodimeric TCA cycle enzyme that catalyzes substrate-level phosphorylation and is essential for mtDNA maintenance, mitochondrial respiration, and mitochondrial morphology. SUCLG1 is required to stabilize both beta subunit isoforms (SUCLA2 and SUCLG2); its absence leads to degradation or mislocalization of these partners, loss of SCS activity, mtDNA depletion, and impaired respiration, all of which are rescued by ectopic SUCLG1 expression (PMID:20693550, PMID:27484306, PMID:30470562). Beyond its catalytic role, SUCLG1 controls succinyl-CoA levels to suppress succinylation of mitochondrial RNA polymerase (POLRMT) at K622, thereby maintaining POLRMT–mtDNA interactions, mtDNA transcription, and mitochondrial biogenesis; this axis is co-opted by FLT3-mutant leukemias to enhance mitobiogenesis and disease progression (PMID:38649537). Biallelic loss-of-function mutations in SUCLG1 cause encephalomyopathic mtDNA depletion syndrome with methylmalonic aciduria (PMID:19526370, PMID:27484306).

1. 2009 Medium

   Establishing that SUCLG1 is a disease gene: before this, the genetic basis of some encephalomyopathic mtDNA depletion cases was unknown; identification of biallelic SUCLG1 mutations linked the alpha subunit to a Mendelian mtDNA depletion syndrome with methylmalonic aciduria, paralleling SUCLA2-associated disease.

   Evidence Genetic sequencing and biochemical profiling (methylmalonic aciduria, lactic acidosis, mtDNA depletion) in affected patients

   PMID:19526370

   Open questions at the time

   • Single case report; functional consequences of mutation not directly tested in cell-based assays
   • No enzyme activity measurement performed
2. 2010 High

   Demonstrating that SUCLG1 is required to stabilize the SUCLA2 beta subunit: it was unclear whether the alpha subunit was structurally necessary for beta subunit integrity; western blotting of SUCLG1-null fibroblasts showed complete loss of SUCLA2, establishing obligate heterodimer stabilization.

   Evidence Western blot of patient fibroblasts lacking SUCLG1 protein; minigene expression system confirming splicing mutation

   PMID:20693550

   Open questions at the time

   • Did not test SUCLG2 stability independently
   • Mechanism of beta subunit degradation (proteasomal vs. other) not identified
3. 2016 High

   Proving SUCLG1 is necessary and sufficient for SCS activity, mtDNA maintenance, and respiration: rescue of all three phenotypes by ectopic wild-type SUCLG1 in patient cells established a direct causal chain from SUCLG1 loss to TCA cycle dysfunction and mtDNA depletion.

   Evidence SCS enzyme activity assay, mtDNA quantification, cellular respiration assay, and rescue by ectopic wild-type SUCLG1 in patient fibroblasts

   PMID:27484306

   Open questions at the time

   • Mechanism linking SCS activity loss to mtDNA depletion (nucleotide pool vs. other) not resolved
   • In vivo rescue not performed
4. 2018 High

   Extending the alpha subunit's stabilizing role to SUCLG2 and linking SUCLG1 loss to mitochondrial fragmentation: prior work addressed SUCLA2 stability; here, a pathogenic SUCLG1 missense mutation was shown to also deplete SUCLG2 protein and cause its mislocalization, while inducing mitochondrial fragmentation independent of ETC activity or mtDNA copy number changes.

   Evidence Enzyme activity assay for substrate-level phosphorylation, immunoblot, confocal immunocytochemistry, and respirometry in patient fibroblasts

   PMID:30470562

   Open questions at the time

   • Mechanism by which SUCLG1 loss causes mitochondrial fragmentation is unknown
   • Whether SUCLG2 mislocalization is cause or consequence of fragmentation is unresolved
5. 2024 High

   Revealing a non-catalytic signaling role for SUCLG1 through metabolite-mediated post-translational modification: it was unknown how TCA cycle enzymes could regulate mitochondrial gene expression; SUCLG1 was shown to restrict succinyl-CoA levels, preventing succinylation of POLRMT at K622, thereby preserving POLRMT–mtDNA interactions and mtDNA transcription, with FLT3-mutant leukemias exploiting SUCLG1 upregulation to enhance mitochondrial biogenesis.

   Evidence Succinylation proteomics, K622 site-directed mutagenesis, Co-IP of POLRMT with mtDNA/transcription factors, SUCLG1/POLRMT genetic depletion in mouse and humanized leukemia models, succinyl-CoA measurement in clinical samples

   PMID:38649537

   Open questions at the time

   • Whether POLRMT K622 succinylation is regulated in non-leukemic contexts is unknown
   • Other succinylation targets controlled by SUCLG1-dependent succinyl-CoA flux not systematically defined
6. 2025 Medium

   Demonstrating SUCLG1 promotes mitochondrial fusion and aerobic respiration in a tumor context: loss- and gain-of-function experiments in plexiform neurofibroma cells showed SUCLG1 drives mitochondrial fusion, enhances oxidative phosphorylation, and promotes cell proliferation and migration.

   Evidence SUCLG1 knockdown and overexpression, Seahorse metabolic flux assay, electron microscopy, flow cytometry, qPCR and western blot in neurofibroma cell lines

   PMID:39749698

   Open questions at the time

   • Whether the pro-fusion role is direct or secondary to TCA cycle metabolite changes is unknown
   • Mechanism linking SUCLG1 to SLC25A1 upregulation not established
   • Single tumor type studied
7. 2026 Medium

   Identifying post-translational regulation of SUCLG1 enzymatic activity by butyrylation: it was unknown how SUCLG1 activity is modulated beyond substrate availability; proteomic screening revealed butyrylation at K90 impairs SUCLG1 catalytic function, and SIRT4 was identified as the debutyrylase responsible for removing this modification.

   Evidence Butyrylome proteomic screening, SIRT4 inhibitor and overexpression experiments, in vivo mouse HFpEF model and phenylephrine-treated cellular model

   PMID:41991142

   Open questions at the time

   • Whether K90 butyrylation affects SUCLG1 interactions with beta subunits is untested
   • In vitro reconstitution of SIRT4-mediated debutyrylation of SUCLG1 not shown

• The mechanism by which SUCLG1/SCS activity loss leads to mtDNA depletion remains unresolved — whether this occurs via disrupted mitochondrial nucleotide salvage (NDPK interaction), altered succinyl-CoA signaling, or another pathway is not definitively established.
• No direct demonstration of mechanism linking SCS catalysis to mtDNA copy number
• Structural basis for beta subunit stabilization by SUCLG1 not determined
• Tissue-specific roles of SUCLG1 (brain vs. muscle vs. hematopoietic) not systematically compared