title: Proteasome-mediated turnover of Arabidopsis MED25 is coupled to the activation of FLOWERING LOCUS T transcription.
abstract: The Mediator complex is a greater than 1-megadalton complex, composed of about 30 subunits and found in most eukaryotes, whose main role is to transmit signals from DNA-bound transcription factors to RNA Polymerase II. The proteasome is emerging as an important regulator of transcription during both initiation and elongation. It is increasing the number of cases where the proteolysis of transcriptional activators by the proteasome activates their function. This counterintuitive phenomenon was called "activation by destruction." Here, we show that, in Arabidopsis (Arabidopsis thaliana), PHYTOCHROME AND FLOWERING TIME1 (PFT1), the MEDIATOR25 (MED25) subunit of the plant Mediator complex, is degraded by the proteasome and that proteasome-mediated PFT1 turnover is coupled to its role in stimulating the transcription of FLOWERING LOCUS T, the plant florigen, which is involved in the process of flowering induction. We further identify two novel RING-H2 proteins that target PFT1 for degradation. We show that MED25-BINDING RING-H2 PROTEIN1 (MBR1) and MBR2 bind to PFT1 in yeast (Saccharomyces cerevisiae) and in vitro, and they promote PFT1 degradation in vivo, in a RING-H2-dependent way, typical of E3 ubiquitin ligases. We further show that both MBR1 and MBR2 also promote flowering by PFT1-dependent and -independent mechanisms. Our findings extend the phenomenon of activation by destruction to a Mediator subunit, adding a new mechanism by which Mediator subunits may regulate downstream genes in specific pathways. Furthermore, we show that two novel RING-H2 proteins are involved in the destruction of PFT1, adding new players to this process in plants.
authors.affiliation: Fundación Instituto Leloir, IIBBA-Consejo Nacional de Investigaciones Científicas y Técnicas, C1405BWE Buenos Aires, Argentina.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Iñigo
authors.forename: Adrián N
authors.initials: AN
authors.lastname: Giraldez
authors.forename: Joanne
authors.initials: J
authors.lastname: Chory
authors.forename: Pablo D
authors.initials: PD
authors.lastname: Cerdán
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D004268:DNA-Binding Proteins; D035264:Flowers / Q000235:genetics / Q000502:physiology; D018506:Gene Expression Regulation, Plant; D055785:Gene Knockdown Techniques; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011485:Protein Binding; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D059748:Proteolysis; D014158:Transcription, Genetic; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D004268:DNA-Binding Proteins; C418905:FT protein, Arabidopsis; D009687:Nuclear Proteins; C475686:PFT1 protein, Arabidopsis; D044767:Ubiquitin-Protein Ligases; D046988:Proteasome Endopeptidase Complex