SLC25A39

SLC25A39 is a mitochondrial inner membrane carrier that imports glutathione (GSH) from the cytosol into the mitochondrial matrix, thereby sustaining iron-sulfur cluster biogenesis, oxidative phosphorylation, and cellular redox defense. Loss of SLC25A39 selectively depletes mitochondrial GSH without affecting cytosolic pools, and combined deletion with its paralog SLC25A40 impairs iron-sulfur cluster protein activity, cell proliferation, and red blood cell development in mice (PMID:34707288, PMID:35513392). SLC25A39 is a short-lived protein whose abundance is controlled by degradation through the m-AAA protease AFG3L2 via matrix loop 1, with stabilization occurring when four matrix cysteine residues sense iron-sulfur cluster status, directly coupling mitochondrial iron homeostasis to GSH import capacity (PMID:38157846). Additional stabilizing inputs include DLAT binding independent of GSH levels and FGF21 signaling that inhibits AFG3L2-mediated turnover, linking SLC25A39 regulation to metabolic and neuroprotective pathways (PMID:42009144, PMID:41039428).

1. 2001 Medium

   Establishing where SLC25A39 resides: before any function was known, demonstrating that the CGI-69 gene product localizes to mitochondria placed it within the mitochondrial carrier family and framed subsequent transport hypotheses.

   Evidence FLAG-tagged CGI-69 overexpression with fluorescence microscopy in MCF7 cells

   PMID:11139402

   Open questions at the time

   • No transport substrate identified
   • Localization based on overexpression only, not endogenous protein
   • No functional characterization performed
2. 2021 High

   Identifying the transported substrate: organellar metabolomics and genetic rescue revealed that SLC25A39 is the long-sought mitochondrial GSH importer, resolving how GSH enters the matrix and showing this is essential for iron-sulfur cluster protein function, proliferation, and erythropoiesis.

   Evidence SLC25A39 KO and SLC25A39/SLC25A40 double KO in cell lines with mitochondrial metabolomics; bacterial GshF rescue; mouse genetic models

   PMID:34707288

   Open questions at the time

   • Direct reconstitution of GSH transport by purified SLC25A39 not shown
   • Counter-substrate for the exchange cycle not identified
   • Structural basis for GSH recognition unknown
3. 2022 High

   Confirming transport function and revealing genetic buffering: organelle transport assays and structure-guided mutagenesis independently validated SLC25A39 as a mitochondrial GSH transporter and uncovered a functional interaction with the iron importer SLC25A37, linking GSH and iron uptake to joint OXPHOS support.

   Evidence Dual CRISPR screens in four metabolic states, mitochondrial metabolite profiling, organelle transport assays, mutagenesis

   PMID:35513392

   Open questions at the time

   • Molecular basis of the SLC25A39–SLC25A37 genetic interaction unresolved
   • No reconstituted liposomal transport with purified protein
   • Selectivity for GSH versus GSSG or other thiol substrates not rigorously defined
4. 2023 High

   Decoding the degradation switch: identifying AFG3L2 as the protease that degrades SLC25A39 via matrix loop 1, and showing that four matrix cysteines sense iron-sulfur cluster status to block this degradation, revealed how cells couple iron homeostasis to mitochondrial GSH supply at the post-translational level.

   Evidence Co-IP/MS, CRISPR KO of AFG3L2, cysteine-to-alanine mutagenesis, protein stability assays in mammalian cells

   PMID:38157846

   Open questions at the time

   • Whether cysteines sense Fe-S clusters directly or via a cofactor/adaptor is unresolved
   • Structural model of AFG3L2 recognition of matrix loop 1 lacking
   • Relative contributions of GSH-dependent versus Fe-S-dependent regulation not quantified
5. 2024 Medium

   Extending SLC25A39 function to copper-mediated cell death: placing SLC25A39 downstream of the NRF2-GSH axis showed that mitochondrial GSH import protects cells from cuproptosis by chelating copper, and that blocking this axis enhances cuproptosis-mediated tumor suppression.

   Evidence Genetic KD/KO of NFE2L2, GCLM, GCLC, SLC25A39 in cell lines and xenograft mouse models; copper chelation assays

   PMID:39609608

   Open questions at the time

   • Direct copper binding by mitochondrial GSH not biochemically demonstrated
   • Whether SLC25A39 regulation changes under copper stress is unknown
   • Single-laboratory finding
6. 2025 Medium

   Identifying additional stability regulators and downstream effectors: FGF21 stabilizes SLC25A39 by inhibiting AFG3L2-mediated degradation for neuroprotection; DLAT directly binds and stabilizes SLC25A39 independently of GSH to maintain mitochondrial GSH and resist ferroptosis; CRABP2 modulates the AFG3L2-SLC25A39 axis in colorectal cancer; and SLC25A39 stabilizes PRDX1 to suppress necroptosis in hepatocellular carcinoma.

   Evidence Neuron-specific Slc25a39 KO mice with FGF21 rescue; Co-IP identifying DLAT and PRDX1 as direct binding partners; Crabp2 conditional KO mouse model; protein stability and cell death assays

   PMID:40305785 PMID:41039428 PMID:41558298 PMID:42009144

   Open questions at the time

   • DLAT and PRDX1 interactions each reported by single labs without independent replication
   • Whether DLAT binding and Fe-S cluster sensing act through the same or distinct stabilization mechanisms is unclear
   • How SLC25A39 stabilizes PRDX1 mechanistically is unknown

• Key open questions include the atomic structure of SLC25A39, direct reconstitution of GSH transport in proteoliposomes, the identity of the counter-substrate, and the precise mechanism by which matrix cysteines sense iron-sulfur cluster occupancy.
• No high-resolution structure of SLC25A39 available
• Reconstituted transport with purified protein not demonstrated
• Counter-substrate identity unknown
• Molecular mechanism of cysteine-mediated Fe-S cluster sensing unresolved