Role of the Ada adaptor complex in gene activation by the glucocorticoid receptor

We have shown that the Ada adaptor complex is important for the gene activation capacity of the glucocorticoid receptor in yeast. The recently isolated human Ada2 protein also increases the potency of the receptor protein in mammalian cells. The Ada pathway is of key significance for the τ1 core transactivation domain (τ1c) of the receptor, which requires Ada for activity in vivo and in vitro. Ada2 can be precipitated from nuclear extracts by a glutathione S-transferase-τ1 fusion protein coupled to agarose heads, and a direct interaction between Ada2 and τ1c can be shown by using purified proteins. This interaction is strongly reduced by a mutation in τ1c that reduces transactivation activity. Mutations affecting the Ada complex do not reverse transcriptional squelching by the τ1 domain, as they do for the VP16 transactivation domain, and thus these powerful acidic activatory differ in at least some important aspects of gene activation. Mutations that reduce the activity of the τ1c domain in wild-type yeast strains cause similar reductions in ada mutants that contain little or no Ada activity. Thus, gene activation mechanisms, in addition to the Ada pathway, are involved in the activity of the τ1c domain.