Mapping and functional role of phosphorylation sites in the thyroid transcription factor-1 (TTF-I)

Abstract

The phosphorylation of thyroid transcription factor-1 (TTF-I), a homeodomain-containing transcription factor that is required for thyroid-specific expression of the thyroglobulin and thyroperoxidase gene promoters, has been studied. Phosphorylation occurs on a maximum of seven serine residues that are distributed in three tryptic peptides. Mutant derivatives of TTF-I, with alanine residues replacing the serines in the phosphorylation sites, have been constructed and used to assess the functional relevance of TTF-I phosphorylation. The DNA binding activity of TTF-I appears to be phosphorylation-independent, as indicated also by the performance of TTF-I purified from an overexpressing Escherichia coli strain. Transcriptional activation by TTF-I could require phosphorylation only in specific cell types since in a co-transfection assay in heterologous cells both wild-type and mutant proteins show a similar transcriptional activity.