Proteasome-Mediated Turnover of Arabidopsis MED25 Is Coupled to the Activation of FLOWERING LOCUS T Transcription

SABRINA INIGO; ADRIÁN NICOLÁS GIRALDEZ; JOANNE CHORY; PABLO D. CERDÁN

PLANT PHYSIOLOGY.

AMER SOC PLANT BIOLOGISTS

Lugar: Rockville; Año: 2012

0032-0889

The Mediator complex is a >1 MDa complex, composed of about 30 subunits 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 thaliana, PHYTOCHROME AND FLOWERING TIME 1 (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 FT, the plant florigen, which is involved in the process of flowering induction. We further identify two novel RING-H2 proteins, which target PFT1 for degradation. We show that MED25 BINDING RING-H2 PROTEIN 1 and 2 (MBR1 and MBR2) bind to PFT1 in yeast 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. Further, we show that two novel RING-H2 proteins are involved in the destruction of PFT1, adding new players to this process in plants.