STRUCTURAL AND FUNCTIONAL-PROPERTIES OF THE N TRANSCRIPTIONAL ACTIVATION DOMAIN OF THYROID TRANSCRIPTION FACTOR-I - SIMILARITIES WITH THE ACIDIC ACTIVATION DOMAINS

The thyroid transcription factor 1 (TTF-1) is a tissue-specific transc ription factor involved in the development of thyroid and lung. TTF-1 contains two transcriptional activation domains (N and C domain). The primary amino acid sequence of the N domain does not show any typical characteristic of known transcriptional activation domains. In aqueous solution the N domain exists in a random-coil conformation. The incre ase of the milieu hydrophobicity, by the addition of trifluoroethanol, induces a considerable gain of alpha-helical structure. Acidic transc riptional activation domains are largely unstructured in solution, but , under hydrophobic conditions, folding into alpha-helices or beta-str ands can be induced. Therefore our data indicate that the inducibility of alpha-helix by hydrophobic conditions is a property not restricted to acidic domains. Co-transfections experiments indicate that the aci dic domain of herpes simplex virus protein VP16 (VP16) and the TTF-1 N domain are interchangeable and that a chimaeric protein, which combin es VP16 linked to the DNA-binding domain of TTF-1, undergoes the same regulatory constraints that operate for the wild-type TTF-1. In additi on, we demonstrate that the TTF-1 N domain possesses two typical prope rties of acidic activation domains: TBP (TATA-binding protein) binding and ability to activate transcription in yeast. Accordingly, the TTF- 1 N domain is able to squelch the activity of the p65 acidic domain. A ltogether, these structural and functional data suggest that a non-aci dic transcriptional activation domain (TTF-1 N domain) activates trans cription by using molecular mechanisms similar to those used by acidic domains. TTF-1 N domain and acidic domains define a family of protein s whose common property is to activate transcription through the use o f mechanisms largely conserved during evolutionary development.