Med8, Med18, and Med20 subunits of the Mediator head domain are interdependent upon each other for folding and complex formation

We have studied folding and complex formation of the yeast Mediator head-module protein subunits Med8, Med18, and Med20. Using a combination of immunoprecipitation, far-UV circular dichroism, and fluorescence measurements on recombinantly expressed and denatured proteins that were allowed to renature separately or in different combinations, we found that Med8, Med18, and Med20 can fold in different ways to form both soluble monomeric proteins and different distinct subcomplexes. However, the concurrent presence of all three protein subunits during the renaturation process is required for proper folding and trimer complex formation.