Alternative titles; symbols
HGNC Approved Gene Symbol: MED14
Cytogenetic location: Xp11.4 Genomic coordinates (GRCh38) : X:40,648,305-40,736,159 (from NCBI)
MED14 is a subunit of the Mediator complex, a critical coactivator for RNA polymerase II (Pol II; see 180660)-mediated transcription. The Mediator complex is organized into head, middle, tail, and kinase subcomplexes, and MED14 is the essential backbone that bridges the core head, middle, and tail subcomplexes (Cevher et al., 2014).
Studies of coactivators in yeast have identified a holoenzyme that contains core RNA polymerase II and a reversibly bound complex of factors called 'mediator.' This holoenzyme has been implicated in both basal and TAF-II-independent activated transcription. The mediator complex contains MED (RNA polymerase II transcriptional regulation mediator) proteins as well as a subset of SRB (suppressor of RNA polymerase B) proteins. Gu et al. (1999) identified SMCC (SRB/MED cofactor complex), a novel human cofactor complex that contains homologs of a subset of yeast mediator/holoenzyme components, homologs of other yeast transcriptional regulatory factors, and some components of a human thyroid hormone receptor-associated coactivator complex (TRAP220, 604311; TRAP170; and TRAP100, 607000). Based on amino acid sequences derived from cognate polypeptides in the immunopurified TR/TRAP complex, Gu et al. (1999) isolated a cDNA encoding human TRAP170. The predicted 1,454-amino acid protein is 22% identical to yeast RGR1, a component of the mediator complex.
Independently, Yoshikawa et al. (1998) identified TRAP170, which they designated EXLM1, as an X-chromosome gene that escapes X inactivation. They found that the EXLM1 protein contains a cytochrome C family heme-binding motif and a bipartite nuclear localization signal. Northern blot analysis revealed that EXLM1 is expressed as 6.5- and 8-kb mRNAs predominantly in skeletal muscle.
Gene transcription requires factors that recognize transcriptional enhancer sites in DNA. These factors work with coactivators to direct transcriptional initiation by the RNA polymerase II apparatus. Transcriptional activation by enhancer-binding factors such as SP1 (189906) requires interaction with the TFIID complex (see TAF2A, 313650). To identify other potential SP1 cofactors, Ryu et al. (1999) developed an in vitro transcription assay consisting of TFIIA (GTF2A1; 600520), RNA polII, and the basal transcription factors GTF2B (189963), GTF2E (189962), GTF2F (189968), and GTF2H (189972), supplemented with TFIID or TBP (600075). By sequential chromatography, they excluded PC4 (600503) as an SP1 cofactor and identified a multisubunit cofactor, CRSP (cofactor required for SP1 activation), which, along with TFIID, is required for efficient activation by SP1. CRSP behaves as a single complex of approximately 700 kD. Ryu et al. (1999) tentatively identified 9 polypeptides as CRSP subunits (see also PPARBP, 604311). Using microsequence peptide analysis, they cloned a CRSP cDNA encoding a 150-kD protein, CRSP2, which they termed CRSP150.
Transcriptional regulation by nuclear hormone receptors is thought to involve interactions with putative cofactors that may potentiate receptor function. Fondell et al. (1996) showed that human thyroid hormone receptor (TR)-alpha (190120) purified from HeLa cells grown in the presence of thyroid hormone (T3) is associated with a group of distinct nuclear proteins termed thyroid hormone receptor-associated proteins (TRAPs). The 'liganded' TR/TRAP complex markedly activated transcription from a promoter template containing T3-response elements. The authors suggested that TRAPs play a role as positive coactivators for gene-specific transcriptional activation.
By knockdown analysis in mouse embryonic fibroblasts (MEFs), Grontved et al. (2010) showed that Med14 was necessary for full transcriptional activity of Ppar-gamma (PPARG; 601487) and that the Mediator complex was required for transcriptional activation of target genes by Ppar-gamma. Med14 interacted directly with the Ppar-gamma N-terminal A/B domain to tether Ppar-gamma to Mediator and was required for Ppar-gamma transcriptional activity of a subset of target genes. More specifically, Med14 was required for recruitment of the Mediator complex to a Ppar-gamma-responsive promoter. Furthermore, Med14 was required for adipogenesis, as its knockdown impaired adipose conversion of 3T3-L1 cells.
By immunoprecipitation in HEK293 cells, Galbraith et al. (2013) identified human MED14 as an ERK (see 601795) phosphorylation target in response to mitogen stimulation. In vitro and in vivo analyses confirmed that ERK phosphorylated MED14 in its serine-proline (SP)-rich region, specifically at S986. Phosphorylation of MED14 by ERK was required for activation of mitogen-responsive immediate early gene (IEG) transcription by ELK1 (311040), as MED14 knockdown impaired IEG induction in Hela cells and mutation of S986 impaired oncogene- and mitogen-induced transcription. Mitogens induced phosphorylation of MED14 by ERK on S986 at IEG promoters, and ELK1 interacted with MED14 independently of MED23 (605042) in the Mediator complex to facilitate phosphorylation of MED14 by ERK.
Cevher et al. (2014) reconstituted the head and middle modules of the human Mediator complex and found that they were stably associated with each other in a bimodular subcomplex. The bimodular complex was inactive in transcriptional assays unless MED14 was also incorporated. MED14 was critical for basal and activated transcription, because it enhanced interaction of the Mediator complex with Pol II to stimulate Pol II-mediated transcription. However, the subcomplex with MED14 was still unable to support transcription unless complemented with MED26 (605043), as MED26 was required for Mediator function in nucleus to overcome a restriction for Pol II recruitment to promoters.
By fluorescence in situ hybridization, Yoshikawa et al. (1998) mapped the MED14 gene to chromosome Xp11.4-p11.2.
Cevher, M. A., Shi, Y., Li, D., Chait, B. T., Malik, S., Roeder, R. G. Reconstitution of active human core Mediator complex reveals a critical role of the MED14 subunit. Nature Struct. Molec. Biol. 21: 1028-1034, 2014. [PubMed: 25383669] [Full Text: https://doi.org/10.1038/nsmb.2914]
Fondell, J. D., Ge, H., Roeder, R. G. Ligand induction of a transcriptionally active thyroid hormone receptor coactivator complex. Proc. Nat. Acad. Sci. 93: 8329-8333, 1996. [PubMed: 8710870] [Full Text: https://doi.org/10.1073/pnas.93.16.8329]
Galbraith, M. D., Saxton, J., Li, L., Shelton, S. J., Zhang, H., Espinosa, J. M., Shaw, P. E. ERK phosphorylation of MED14 in promoter complexes during mitogen-induced gene activation by Elk-1. Nucleic Acids Res. 41: 10241-10253, 2013. [PubMed: 24049075] [Full Text: https://doi.org/10.1093/nar/gkt837]
Grontved, L., Madsen, M. S., Boergesen, M., Roeder, R. G., Mandrup, S. MED14 tethers mediator to the N-terminal domain of peroxisome proliferator-activated receptor gamma and is required for full transcriptional activity and adipogenesis. Molec. Cell. Biol. 30: 2155-2169, 2010. [PubMed: 20194623] [Full Text: https://doi.org/10.1128/MCB.01238-09]
Gu, W., Malik, S., Ito, M., Yuan, C.-X., Fondell, J. D., Zhang, X., Martinez, E., Qin, J., Roeder, R. G. A novel human SRB/MED-containing cofactor complex, SMCC, involved in transcription regulation. Molec. Cell 3: 97-108, 1999. Note: Erratum: Molec. Cell 3: page following 541, 1999. [PubMed: 10024883] [Full Text: https://doi.org/10.1016/s1097-2765(00)80178-1]
Ryu, S., Zhou, S., Ladurner, A. G., Tjian, R. The transcriptional cofactor complex CRSP is required for activity of the enhancer-binding protein Sp1. Nature 397: 446-450, 1999. [PubMed: 9989412] [Full Text: https://doi.org/10.1038/17141]
Yoshikawa, H., Fujiyama, A., Nakai, K., Inazawa, J., Matsubara, K. Detection and isolation of a novel human gene located on Xp11.2-p11.4 that escapes X-inactivation using a two-dimensional DNA mapping method. Genomics 49: 237-246, 1998. [PubMed: 9598311] [Full Text: https://doi.org/10.1006/geno.1998.5246]