MED28

MED28 is a dual-compartment protein that operates both as a subunit of the mammalian Mediator transcriptional co-activator complex in the nucleus and as a membrane-associated cytoskeletal adaptor in the cytoplasm (PMID:15467741, PMID:17848560). As a Mediator head-module component, MED28 acts as a scaffolding protein that stabilizes a subcomplex with Med6, Med8, and Med18 and represses smooth muscle cell differentiation (PMID:17848560), and it is essential for pluripotency: loss of Med28 in mice causes peri-implantation lethality with collapse of the inner cell mass and reduced Oct4 and Nanog expression, while its overexpression enhances iPSC reprogramming (PMID:26445504). MED28 expression peaks at the G1-S transition and during mitosis, and its level controls cell cycle duration and genomic stability, with overexpression driving micronucleus formation and aneuploidy (PMID:30970566). In the cytoplasm MED28 binds the NF2 tumor suppressor merlin and the adaptor Grb2, forming a ternary complex beneath the plasma membrane in association with the actin cytoskeleton (PMID:15467741); it interacts with Src-family kinases and is phosphorylated at Y64 by Lck upon T-cell receptor (CD3) stimulation, generating a Grb2-SH2 binding motif (PMID:16899217). Across cancer contexts MED28 promotes proliferation, migration, and invasion through several transcriptional and signaling axes, including Wnt/β-catenin (repressing the negative regulator HBP1) (PMID:26660958), MEK1-MMP2 (PMID:22495818), and FOXM1-MMP2 (PMID:30499104), and it directly binds nuclear partners ZNF224 and RCOR1 to modulate cell survival and cancer stem cell-like properties (PMID:29435049, PMID:32306431).

1. 2004 High

   Established MED28 (magicin) as a cytoplasmic, membrane-proximal scaffold by identifying its direct physical partners, answering what this protein binds and where it resides.

   Evidence Co-IP, blot overlay, immunofluorescence, EM, and subcellular fractionation showing merlin and Grb2 binding and actin association

   PMID:15467741

   Open questions at the time

   • Functional consequence of the merlin-MED28-Grb2 ternary complex not defined
   • No link yet to a signaling output or transcriptional role
2. 2005 Medium

   Connected MED28 overexpression to proliferative and oncogenic signaling, showing it associates with c-Src and correlates with MAPK activation.

   Evidence Overexpression/knockdown, proliferation assays, xenograft model, and Co-IP for c-Src in cancer cells

   PMID:16024617

   Open questions at the time

   • Whether MED28 directly activates Src or acts downstream not resolved
   • MAPK activation correlative rather than mechanistic
3. 2006 High

   Defined MED28 as a substrate and partner of Src-family kinases and identified the exact tyrosine phosphorylation site coupling it to TCR signaling.

   Evidence Yeast two-hybrid, in vitro kinase assay, site-directed mutagenesis, and CD3 stimulation in Lck-deficient versus wild-type Jurkat cells

   PMID:16899217 PMID:16964398

   Open questions at the time

   • Downstream signaling consequence of Y64 phosphorylation and Grb2 recruitment not traced
   • Conflicting reports on whether MED28 is a direct Src substrate
4. 2007 Medium

   Placed MED28 in the nucleus as a Mediator head-module subunit and assigned it a scaffolding role repressing smooth muscle differentiation.

   Evidence Mediator subunit identification with siRNA knockdown/overexpression and transdifferentiation assays in NIH3T3 and mesenchymal precursors

   PMID:17848560

   Open questions at the time

   • Direct target genes of the Med6/8/18/28 subcomplex not mapped
   • Relationship between nuclear and cytoplasmic pools unclear
5. 2015 High

   Demonstrated an essential developmental requirement for MED28 in maintaining pluripotency, linking the Mediator subunit to Oct4/Nanog-driven inner cell mass identity.

   Evidence Full, conditional, and heterozygous Med28 knockout mice plus iPSC reprogramming assays

   PMID:26445504

   Open questions at the time

   • Mechanism by which MED28 sustains Oct4/Nanog expression not defined
   • Whether the effect is via Mediator or a separable function untested
6. 2015 Medium

   Identified MED28 as a repressor of the Wnt/β-catenin negative regulator HBP1, providing a transcriptional mechanism for its pro-tumorigenic activity in colorectal cancer.

   Evidence siRNA, overexpression, luciferase reporter of the HBP1 promoter, and immunoblotting for β-catenin targets

   PMID:26660958

   Open questions at the time

   • Whether MED28 acts on the HBP1 promoter directly or through Mediator not established
   • Generalizability beyond colorectal cells untested
7. 2012 Medium

   Established a MEK1-MMP2 axis as the mechanism for MED28-driven migration and invasion through ordered epistasis.

   Evidence Knockdown/overexpression, dominant-negative MEK1, MEK1 inhibitors, and MMP2 rescue in breast cancer migration/invasion assays

   PMID:21942447 PMID:22495818

   Open questions at the time

   • How MED28 elevates MEK1 expression not defined
   • Direct versus indirect control of MMP2 unresolved
8. 2018 Medium

   Showed MED28 partners with FOXM1 to drive MMP2-dependent invasion, and directly binds ZNF224 to stabilize it against degradation, expanding its nuclear interaction network.

   Evidence Co-IP, SPR, BiFC for ZNF224; Co-IP plus inducible constitutively active FOXM1 rescue in NSCLC and breast cancer cells

   PMID:29435049 PMID:30499104

   Open questions at the time

   • Whether MED28-FOXM1 and MED28-ZNF224 functions involve Mediator recruitment unknown
   • Mechanism of ZNF224 stabilization not detailed
9. 2019 Medium

   Defined cell-cycle-coupled regulation of MED28 itself and its dose-dependent control of mitotic timing and genomic stability.

   Evidence Luciferase reporters for E2F1/NRF1/ETS1/C/EBPβ, cell cycle synchronization, live-cell imaging, FACS with knockdown/overexpression in HeLa cells

   PMID:30970566

   Open questions at the time

   • Molecular basis of MED28-driven aneuploidy not identified
   • Link to its Mediator or cytoplasmic functions not made
10. 2020 Low

    Identified RCOR1 as a direct binder that antagonizes MED28-induced cancer stem cell-like properties.

    Evidence Co-IP, overexpression, colony/sphere formation, and CSC marker immunoblotting in oral squamous carcinoma cells

    PMID:32306431

    Open questions at the time

    • Single Co-IP without reciprocal/orthogonal binding validation
    • Transcriptional targets of the MED28-RCOR1 interplay unknown
11. 2024 Low

    Linked MED28 to AKT/mTOR signaling and lipogenic SREBP1 regulation in liver cancer.

    Evidence siRNA/overexpression with cell cycle analysis, AKT/mTOR immunoblotting, and SREBP1 nuclear fractionation in HepG2/Huh7 cells

    PMID:38619972

    Open questions at the time

    • Pathway placement based on expression changes without direct mechanistic link
    • Whether effect is transcriptional or post-translational unresolved

• How MED28 is partitioned between its nuclear Mediator role and its cytoplasmic cytoskeletal/signaling role, and whether post-translational modifications such as Y64 phosphorylation or acetylation coordinate this switch, remains unresolved.
• No mechanism reconciling nuclear and cytoplasmic pools in mammalian cells
• Acetylation control of nuclear localization shown only for the C. elegans orthologue MDT-28 (PMID 29871732)
• No structural model of MED28 within the Mediator head module