Covalent and Non-Covalent Associations Mediate MED28 Homo-Oligomerization

The Mediator is a multi-protein complex that plays a key role in modulating gene expression. Our previous studies suggested that the MED10a, MED28, MED32 complex subunits could be subject to redox regulation. In this study we tested the capacity of different thioredoxins (TRXs) from poplar (TRX-H3 and TRX-H5) and Arabidopsis thaliana (TPR repeat-containing thioredoxin, TDX) as well as glutaredoxins (GRXs) from poplar (GRX-C3 and GRX-C4) to reduce MED28 oligomers in vitro and found that these proteins were less efficient than the the previously tested poplar TRX-H1 and Arabidopsis GRX-C1. Concerning the susceptibility of MED28 to oxidation, both hydrogen peroxide (H2O2) and glutathione disulfide (GSSG) are efficiently mediating the formation of intermolecular disulfides. In fact, MED28 forms homo- oligomers in vivo as assessed by yeast two-hybrid experiments but also in vitro in solution as shown by size-exclusion chromatography, the latter also demonstrated the formation of noncovalent homo-oligomers. These findings suggest that both the redox-dependent and - independent MED28 oligomerization could regulate its biological activities, could it be linked or not to the Mediator. In particular, it would be important to assess MED28 oligomerization state during senescence considering the previously observed phenotype of med28 plants.