MISO1

MISO1 (C3orf33/AC3-33) encodes a conserved integral inner mitochondrial membrane protein that governs stress-induced mitochondrial fission and quality control (PMID:41398051). Under basal conditions MISO is rapidly turned over, but inner-membrane stresses—mtDNA damage, OXPHOS dysfunction, or cristae disruption—stabilize the protein and trigger assembly of small MTFP1-enriched mitochondria (SMEM) subdomains through its C-terminal domain (PMID:41398051). Mechanistically, MISO recruits MTFP1 to activate the FIS1–DRP1(DNM1L) fission pathway while excluding OPA1 to suppress fusion, and the resulting peripheral SMEM undergo fission and are routed to lysosomes for mitophagic clearance of damaged mtDNA; genetic ablation in Drosophila and mammalian cells abolishes SMEM formation (PMID:41398051, PMID:41568773). An earlier line of overexpression work characterized AC3-33 as a cytoplasmic, secretory protein that suppresses AP-1 transcriptional activity by reducing ERK1/2 phosphorylation, thereby inhibiting Elk1 and c-Jun but not c-Fos (PMID:20680465, PMID:20515784), with a splice variant additionally restraining MCF-7 proliferation and upregulating p21 (PMID:31853289). The relationship between this reported cytoplasmic/secretory AP-1 activity and the mitochondrial fission role has not been reconciled in the available corpus.

1. 2008 Low

   Initial functional screening assigned a phenotype to an otherwise uncharacterized ORF, establishing AC3-33 as a suppressor of AP-1 transcriptional activation.

   Evidence High-throughput cell-based reporter screen of function-known genes plus molecular cloning and subcellular localization in cells

   PMID:18487146

   Open questions at the time

   • No mechanistic dissection beyond AP-1 suppression
   • Overexpression only; no loss-of-function validation
   • Localization not linked to a defined activity
2. 2010 Medium

   The AP-1 suppression was traced to a specific signaling node, showing AC3-33 acts upstream by reducing ERK1/2 phosphorylation rather than affecting JNK or p38.

   Evidence Overexpression with dual-luciferase AP-1 reporter, phospho-ERK1/2 Western blots, AP-1 DNA-binding assay, and recombinant GST-fusion protein functional testing

   PMID:20515784 PMID:20680465

   Open questions at the time

   • Relies on overexpression without rescue or mutagenesis
   • Mechanism by which the protein modulates ERK1/2 unresolved
   • Secretory/signal-sequence role not mechanistically linked to intracellular ERK suppression
3. 2019 Medium

   A splice variant extended the AP-1 axis to a growth-control output, linking AC3-33 to suppression of breast cancer cell proliferation via c-Jun downregulation and p21 induction.

   Evidence RT-PCR cloning from MCF-7 cells, proliferation assays (CCK-8, EdU), RNAi knockdown, luciferase reporter, and Western blotting

   PMID:31853289

   Open questions at the time

   • Single cell line and single lab
   • Variant-specific versus canonical-isoform contributions not separated
   • No in vivo validation
4. 2025 High

   A comprehensive mitochondrial study redefined the protein as an integral inner-membrane factor that nucleates stress-induced fission, establishing the MTFP1/FIS1-DRP1 recruitment and OPA1-exclusion mechanism and its role in clearing damaged mtDNA.

   Evidence Drosophila miso mutants, mammalian CRISPR knockout, live-cell imaging, reciprocal co-immunoprecipitation of MTFP1/FIS1/DRP1/OPA1, mitochondrial fractionation, mtDNA damage and OXPHOS assays, and C-terminal domain mutagenesis

   PMID:41398051

   Open questions at the time

   • Molecular basis of stress-triggered stabilization not defined
   • Direct evidence for C-terminal oligomerization incomplete
   • Relationship to the earlier cytoplasmic/secretory AP-1 role unaddressed
5. 2026 High

   Independent corroboration reinforced the fission and mitophagy model, adding detail on OPA1 exclusion and lysosomal routing of damaged mtDNA via mitophagy.

   Evidence Genetic studies in Drosophila and mammalian cells, co-immunoprecipitation, mitochondrial subfractionation, live imaging, and mtDNA damage/mitophagy assays

   PMID:41568773

   Open questions at the time

   • Mitophagy receptor and machinery engaged downstream of SMEM not identified
   • Sensor distinguishing the different IMM stresses unknown

• Whether the cytoplasmic/secretory AP-1-suppressing activity and the inner-membrane fission role represent two functions of one protein, isoform-specific behaviors, or an unresolved discrepancy remains open.
• No study reconciles cytoplasmic secretory localization with integral IMM localization
• ERK/AP-1 modulation has not been re-examined in the mitochondrial framework