Mediator is a large complex of up to 33 proteins that is conserved from plants to fungi to humans - the number and representation of individual subunits varying with species [1-2]. It is arranged into four different sections, a core, a head, a tail and a kinase-activity part, and the number of subunits within each of these is what varies with species. Overall, Mediator regulates the transcriptional activity of RNA polymerase II but it would appear that each of the four different sections has a slightly different function [3]. Subunit Med28 of the Mediator may function as a scaffolding protein within Mediator by maintaining the stability of a submodule within the head module, and components of this submodule act together in a gene-regulatory programme to suppress smooth muscle cell differentiation. Thus, mammalian Mediator subunit Med28 functions as a repressor of smooth muscle-cell differentiation, which could have implications for disorders associated with abnormalities in smooth muscle cell growth and differentiation, including atherosclerosis, asthma, hypertension, and smooth muscle tumours [4].

1: A unified nomenclature for protein subunits of mediator complexes linking transcriptional regulators to RNA polymerase II. Bourbon HM, Aguilera A, Ansari AZ, Asturias FJ, Berk AJ, Bjorklund S, Blackwell TK, Borggrefe T, Carey M, Carlson M, Conaway JW, Conaway RC, Emmons SW, Fondell JD, Freedman LP, Fukasawa T, Gustafsson CM, Han M, He X, Herman PK, Hinnebusch AG, Holmberg S, Mol Cell. 2004;14:553-557. PMID:15175151

2: A set of consensus mammalian mediator subunits identified by multidimensional protein identification technology. Sato S, Tomomori-Sato C, Parmely TJ, Florens L, Zybailov B, Swanson SK, Banks CA, Jin J, Cai Y, Washburn MP, Conaway JW, Conaway RC; Mol Cell. 2004;14:685-691. PMID:15175163

3: Transcription mechanisms. Blackwell TK, Walker AK; WormBook. 2006;5:1-16. PMID:18050436

4: Mediator subunit MED28 (Magicin) is a repressor of smooth muscle cell differentiation. Beyer KS, Beauchamp RL, Lee MF, Gusella JF, Naar AM, Ramesh V; J Biol Chem. 2007;282:32152-32157. PMID:17848560

5: The nematode homologue of Mediator complex subunit 28, F28F8.5, is a critical regulator of C. elegans development. Kostrouchova M, Kostrouch D, Chughtai AA, Kassak F, Novotny JP, Kostrouchova V, Benda A, Krause MW, Saudek V, Kostrouchova M, Kostrouch Z; PeerJ. 2017;5:e3390. PMID:28603670