The LEF-1 high-mobility group box adopts residual structure in its DNA-free form

Intrinsically disordered regions (IDRs) lack stable tertiary structure and instead rapidly interconvert between different conformations. This structural plasticity enables IDRs to act as key players in cellular signaling pathways. Transcription factors are enriched in IDRs, many of which are stabilized by or acquire tertiary structure in the presence of DNA or other binding partners. Using the T-cell factor/lymphoid enhancer binding factor 1 (TCF/LEF-1) transcription factor as a model system, we characterized the structure and dynamics of the high-mobility group (HMG) domain in the absence of DNA. Inclusion of the IDRs that flank the HMG domain led to enhanced solubility of the protein. Secondary ¹³Cα chemical shifts, ¹H nuclear Overhauser effects, ¹⁵N spin relaxation, and ¹H^N solvent paramagnetic relaxation enhancements indicate that the three helices in the HMG domain are oriented similarly to the DNA-bound form of the protein. By contrast, the flanking IDRs do not show evidence of structure. Helix 1 and helix 3 appear to be less stable in the DNA-free conformation, indicating some form of conformational exchange or local motion in the absence of DNA. Given the high degree of sequence conservation in the TCF/LEF family of transcription factors, our results should apply to other members of the family.