MED31

MED31 is an ancient, stably incorporated subunit of the RNA polymerase II Mediator complex that functions as a context-specific transcriptional co-activator linking sequence-specific transcription factors to gene-expression programs governing proliferation, differentiation, and development (PMID:15356001, PMID:31280994). Biochemical isolation of Mediator from two yeast species established MED31 as a bona fide complex component conserved among the oldest Mediator subunits (PMID:15356001), and affinity purification in Tetrahymena recovered a Mediator-like interactome together with a genome-wide chromatin-binding role demonstrated by ChIP-seq (PMID:31280994). Rather than regulating Mediator targets globally, MED31 acts on distinct gene sets: in fission yeast it coactivates Sep1/Ace2-dependent cell-separation genes (PMID:17922236), and in mouse it is required for embryonic cell proliferation and chondrogenesis, with loss reducing Cdc2 and Sox9/Col2a1 expression while other Mediator targets remain unaffected (PMID:20347762). MED31 is predominantly nuclear, but the cytoplasmic engulfment protein Elmo1 binds MED31, drives its cytoplasmic redistribution and monoubiquitination, and together with MED31 shapes Il10 and Il33 cytokine expression during macrophage infection (PMID:23273896). Its proliferative function extends to disease contexts, where MED31 is a direct miR-1 target that supports osteosarcoma cell proliferation via MET signaling (PMID:24969180). Beyond these contexts, the structural basis of MED31's selective target specificity within Mediator has not been characterized in the available corpus.

1. 2004 Medium

   Establishing whether MED31 is a genuine Mediator subunit was prerequisite to any functional interpretation; biochemical isolation defined it as a stable, deeply conserved component of the complex.

   Evidence Biochemical isolation of Mediator from S. pombe and S. cerevisiae plus bioinformatic conservation analysis

   PMID:15356001

   Open questions at the time

   • Did not define which transcription factors or genes MED31 regulates
   • No structural placement of MED31 within the complex
2. 2007 Medium

   Whether MED31 has selective rather than global transcriptional roles was unknown; expression profiling showed it coactivates a specific Sep1/Ace2-dependent cell-separation program, linking Mediator to a defined phase of cell division.

   Evidence Genome-wide expression profiling of med31 (sep10) mutants with sep1/ace2 epistasis in S. pombe

   PMID:17922236

   Open questions at the time

   • Did not show direct physical contact between MED31 and Sep1/Ace2
   • Conservation of this selectivity to metazoans untested
3. 2010 Medium

   Whether MED31's selective regulation matters in a vertebrate was open; a mouse loss-of-function mutant showed it is required for embryonic proliferation and chondrogenesis through a distinct set of targets, not the whole Mediator program.

   Evidence ENU mutant mouse analysis, fibroblast proliferation assays, Cdc2 Western blot, Sox9/Col2a1 expression

   PMID:20347762

   Open questions at the time

   • Direct vs indirect regulation of Cdc2 and Sox9 not resolved
   • Transcription factors recruiting MED31 to these targets unidentified
4. 2012 Medium

   Whether MED31 has functions outside the nuclear Mediator complex was unexplored; Elmo1 was shown to bind MED31, drive its cytoplasmic relocation and monoubiquitination, and tune specific cytokine genes during infection.

   Evidence Reciprocal Co-IP, subcellular fractionation, ubiquitination assay, cytokine expression in primary macrophages

   PMID:23273896

   Open questions at the time

   • Function of cytoplasmic monoubiquitinated MED31 not defined
   • Whether cytoplasmic redistribution is regulatory or degradative unclear
5. 2014 Medium

   Whether MED31's proliferative role is exploited in cancer was unknown; it was identified as a direct miR-1 target whose levels drive osteosarcoma proliferation through MET signaling.

   Evidence Luciferase reporter validation of miR-1 targeting, siRNA knockdown, overexpression rescue, MET signaling readouts

   PMID:24969180

   Open questions at the time

   • Mechanism by which MED31 controls MET expression not established
   • Whether MET regulation is Mediator-dependent untested
6. 2018 Low

   Whether MED31 contributes to adult stem-cell differentiation was untested; knockdown impaired hMSC self-renewal and adipogenesis.

   Evidence siRNA knockdown with self-renewal and adipogenesis (lipid staining, marker expression) assays in hMSCs

   PMID:30006772

   Open questions at the time

   • Single knockdown approach with no rescue or pathway placement
   • Direct adipogenic targets of MED31 unidentified
7. 2019 Medium

   Whether MED31's Mediator-associated and genome-wide regulatory roles are conserved in a distant eukaryote was open; AP-MS, ChIP-seq, and knockdown in Tetrahymena demonstrated a Mediator-like interactome, global chromatin binding, and control of developmental DNA-rearrangement genes.

   Evidence AP-MS interactome, ChIP-seq, immunofluorescence localization to meiotic micronuclei, knockdown gene expression analysis

   PMID:31280994

   Open questions at the time

   • Direct vs indirect repression of developmental genes not resolved
   • Functional significance of micronuclear localization unclear
8. 2026 Low

   Whether the MED31-ELMO1 axis operates in human cancer pathophysiology was untested; a methylation-dependent ELMO1-MED31 interaction was linked to M2 macrophage polarization and EMT in H. pylori-infected gastric cancer cells.

   Evidence Co-IP, methylation-specific PCR, Western blot, co-culture, viability/migration assays

   PMID:41535333

   Open questions at the time

   • Single Co-IP-based interaction with limited mechanistic pathway placement
   • No replication and causal role of MED31 versus ELMO1 not separated

• The structural and molecular basis for how MED31 confers selectivity for distinct target-gene programs within an otherwise general Mediator complex remains unresolved.
• No structural model of MED31 within Mediator
• Transcription-factor interfaces that direct MED31 to specific genes unidentified
• Relationship between nuclear Mediator role and cytoplasmic Elmo1-dependent function unclear