Over-expression and purification of isotopically labeled recombinant ligand-binding domain of orphan nuclear receptor human B1-binding factor/human liver receptor homologue 1 for NMR studies

The human hepatitis B virus enhancer II B1 binding factor (hB1F), which regulates the expression of hepatitis B virus genes, is identified as a nuclear receptor. It regulates several liver-specific genes and plays an important role in the bile acid biosynthesis pathway. A significantly optimized protocol has been worked out to prepare 15N and/or 13C-labeled hB1F ligand-binding domain in minimal medium with high yields for NMR studies. Under the various conditions optimized for the purification of His6-hB1F ligand-binding domain, the yield of the purified protein is estimated to be 25–30 mg from 0.5 L of M9 minimal media. Electrospray ionization mass spectrometry data confirm the correctness of the primary sequence. Dynamic light scattering experiment proves that the protein exists as a monomeric form. In addition, the circular dichroism results show that the protein has a well-regulated secondary structure and a high α-helical content in ammonium bicarbonate buffer at 20 °C and pH 7.4. Finally, uniformly 15N-labeled protein is characterized by a TROSY–HSQC spectrum, and the dispersion of 15N–1H cross-peaks in the spectrum indicates the presence of well-ordered and properly folded protein as a monomer.