ANALYSIS OF THE SOLUTION STRUCTURE OF THE HOMEODOMAIN OF RAT-THYROID TRANSCRIPTION FACTOR-1 BY H-1-NMR SPECTROSCOPY AND RESTRAINED MOLECULAR MECHANICS

The solution structure of the rat thyroid transcription factor 1 (TTF- 1) homeodomain has been elucidated by H-1-NMR and restrained modeling. The TTF-1 homeodomain folds in the same manner as classical homeodoma ins, with three helices, a loose loop between the first two helices, a nd a tight turn between helix II and helix III. The typical assembly o f the hydrophobic core is maintained and N-capping motifs are identifi ed in helix I and helix III. The N-terminal stretch of helix LI exhibi ts some mobility, similar to the preceding loop region, which may be r elated to its anomalous capping. The N-terminal decapeptide and the C- terminal octapeptide of the molecule (68 residues long) are disordered . All the previous characteristics are shared by all known isolated ho meodomain structures. An important difference among these structures o ccurs at the C-terminal extension of helix III, which is either disord ered or helically folded. In the TTF-1 homeodomain, the C-terminal ext ension of helix III (residues 51-59) appears structured, albeit not as rigidly as the preceding portion. Analysis of the NOEs and hydrogen-d euterium exchange of backbone amides provides evidence for discontinui ty between the two moieties of helix III, which is introduced by a tig htening or a kink of residues 51-53.