MAPPING AND FUNCTIONAL-ROLE OF PHOSPHORYLATION SITES IN THE THYROID TRANSCRIPTION FACTOR-I (TTF-1)

The phosphorylation of thyroid transcription factor-1 (TTF-1), a homeo domain-containing transcription factor that is required for thyroid-sp ecific expression of the thyroglobulin and thyroperoxidase gene promot ers, has been studied. Phosphorylation occurs on a maximum of seven se rine residues that are distributed in three tryptic peptides. Mutant d erivatives of TTF-1, with alanine residues replacing the serines in th e phosphorylation sites, have been constructed and used to assess the functional relevance of TTF-1 phosphorylation. The DNA binding activit y of TTF-1 appears to be phosphorylation-independent, as indicated als o by the performance of TTF-1 purified from an overexpressing Escheric hia coli strain. Transcriptional activation by TTF-1 could require pho sphorylation only in specific cell types since in a co-transfection as say in heterologous cells both wild-type and mutant proteins show a si milar transcriptional activity.