Didier Monté

The Mediator complex is a central regulator of RNA polymerase II transcription, integrating signals from transcription factors and coordinating pre-initiation complex assembly. Beyond this canonical role, numerous studies have implicated Mediator complex in diverse cellular processes, including RNA processing, DNA repair, and translational control. These observations raise a fundamental question: do such functions reflect an expansion of Mediator activity as a complex, or the emergence of specialized functions at the level of individual subunits? In this review, we examine the ability of Mediator subunits to function independently of their canonical context and discuss the mechanistic principles that enable subunit specialization within multiprotein assemblies.

One function of Mediator complex subunit MED23 is to mediate transcriptional activation by the phosphorylated transcription factor Elk-1, in response to the Ras-MAPK signaling pathway. Using cryogenic electron microscopy, we solve a 3.0 Å structure of human MED23 complexed with the phosphorylated activation domain of Elk-1. Elk-1 binds to MED23 via a hydrophobic sequence PSIHFWSTLS^PP containing one phosphorylated residue (S383^p), which forms a tight turn around the central Phenylalanine. Binding of Elk-1 induces allosteric changes in MED23 that propagate to the opposite face of the subunit, resulting in the dynamic behavior of a 19-residue segment, which alters the molecular surface of MED23. We design a specific MED23 mutation (G382F) that disrupts Elk­-1 binding and consequently impairs Elk-1-dependent serum-induced activation of target genes in the Ras-Raf-MEK-ERK signaling pathway. The structure provides molecular details and insights into a Mediator subunit-transcription factor interface.

Human Mediator complex subunit MED25 binds transactivation domains (TADs) present in various cellular and viral proteins using two binding interfaces found on opposite sides of its ACID domain, and referenced as H1 and H2. Here, we use and compare deep learning methods to characterize Human MED25-TADs interfaces and assess the predicted models to published experimental data. For the H1 interface, AlphaFold produces predictions with high reliability scores that agree well with experimental data, while the H2 interface predictions appear inconsistent, preventing reliable binding modes. Despite these limitations, we experimentally assess the validity of Lana-1 and IE62 MED25 interface predictions. AlphaFold predictions also suggest the existence of a unique hydrophobic pocket for Arabidopsis MED25 ACID domain.

The mechanisms whereby the Hippo pathway effector YAP regulates cancer cell stemness, plasticity, and chemoresistance are not fully understood. We previously showed that in 5-fluorouracil (5-FU)–resistant colorectal cancer cells, the transcriptional coactivator YAP is differentially regulated at critical transitions connected with reversible quiescence/dormancy to promote metastasis. Here, we found that experimental YAP activation in 5-FU–sensitive and 5-FU–resistant HT29 colorectal cancer cells enhanced nuclear YAP localization and the transcript levels of the retinoic acid (RA) receptors RARα/γ and RAR target genes CYP26A1, ALDH1A3, and LGR5 through RA Response Elements (RARE). In these two cell models, constitutive YAP activation reinforced the expression of the stemness biomarkers and regulators ALDH1A3, LGR5, and OCT4. Conversely, YAP silencing, RAR/RXR inhibition by the pan-RAR antagonist BMS493, and vitamin A depletion downregulated stemness traits and self-renewal. Regarding the mechanisms engaged, proximity-dependent labeling, nuclear YAP pulldown coupled with mass spectrometry, and chromatin immunoprecipitation (ChIP)/re-ChIP experiments revealed: (i) the nuclear colocalization/interaction of YAP with RARγ and RXRs; and (ii) combined genomic co-occupancy of YAP, RARα/γ, and RXRα interactomes at proximal RAREs of LGR5 and ALDH1A3 promoters. Moreover, activation of the YAP/RAR-RXR cross-talk in colorectal cancer cells promoted RAR self-activation loops via vitamin A metabolism, RA, and active RAR ligands generated by ALDH1A3. Together, our data identify YAP as a bona fide RAR-RXR transcriptional coactivator that acts through RARE-activated stemness genes.Implications:Targeting the newly identified YAP/RAR-RXR cross-talk implicated in cancer cell stemness maintenance may lead to multitarget combination therapies for patients with colorectal cancer.

Supplementary Figure S3 shows the validation of the YAP/RAR-RXR crosstalk in control and YAPdc-RKO colorectal cancer cells.

Supplementary Figure S1 identifies YAP/RXRalpha and YAP/RARgamma molecular complexes in HT29-YAPdc cells expressing activated nuclear YAP

Supplementary Table S2 presents the detailed proteomics of nuclear YAP-interactome identified in HT29-YAPdc cells