Ras Induces Mediator Complex Exchange on C/EBPβ

Abstract

C/EBPbeta is an intrinsically repressed transcription factor that regulates genes involved in differentiation, proliferation, tumorigenesis, and apoptosis. C/EBPbeta acts as a repressor that is turned into an activator by the Ras oncoprotein through phosphorylation of a MAPK site. C/EBPbeta activation is accompanied by a conformational change. Active and repressive C/EBPbeta interacts with multisubunit Mediator complexes through the CRSP130/Sur2 subunit. The CRSP130/Sur2 subunit is common to two distinct types of Mediator complexes, characterized by CRSP70 and CDK8 proteins as transcriptionally active and inactive Mediator, respectively. Knockdown of CRSP130/Sur2 prevents Mediator binding and transactivation through C/EBPbeta. Oncogenic Ras signaling or activating mutations in C/EBPbeta selects the transcriptionally active Mediator complex that also associates with RNA polymerase II. These results show that a Ras-induced structural alteration of C/EBPbeta determines differential gene activation through selective interaction with distinct Mediator complexes.

Interaction and Cooperation of the CCAAT-box Enhancer-binding Protein β (C/EBPβ) with the Homeodomain-interacting Protein Kinase 2 (Hipk2)

“…Recent work has shown that phosphorylation of Thr-220 triggers a cascade of events involving abrogation of the binding of the lysine methylase G9a and arginine methyl transferase PRMT4, thereby altering the pattern of covalent post-translational modifications, in particular the PRMT4-dependent methylation of arginine 3 of C/EBP␤. This, in turn, facilitates the recruitment of SWI/SNF chromatin remodeling and Mediator complexes by C/EBP␤ to ultimately increase the transcription of C/EBP␤ target genes (26,27,49). Our work suggests increased binding and phosphorylation of p300 as an alternative or additional mechanism by which the phosphorylation of C/EBP␤ at Thr-220 stimulates the transactivation potential of C/EBP␤.…”

“…Functional differences between the LAP* and LAP isoforms have been implicated in senescence (50) and the differentiation of specific cell types (51). On the molecular level, LAP* but not LAP interacts with the SWI/SNF chromatin remodeling proteins and the mediator complex (49,52), explaining differential effects of the isoforms on the expression of specific target genes, for example in myeloid cells. Another interesting difference between the isoforms is the ability of LAP* but not of LAP to be sumoylated (22).…”

Pygopus activates Wingless target gene transcription through the mediator complex subunits Med12 and Med13

“…The kinase module of the mediator complex is commonly thought to have a repressive function; it has been shown to sterically hinder recruitment of Pol II (67), and Ras signaling promotes transcriptional elongation by inducing loss of this module from the mediator complex bound to C/EBP-regulated promoters (68). However, recent results suggest that the kinase module can play a role in transcriptional activation as well as repression (20,21).…”

Specific erythroid-lineage defect in mice conditionally deficient for Mediator subunit Med1

“…Several laboratories provided evidence for mammalian Mediator complexes that differ in composition (37)(38)(39). Med1 deficiency clearly does not lead to the disintegration of the Mediator complex (38).…”