Folding of an antibody variable domain in two functional conformations in vitro: calorimetric and spectroscopic study of the anti-ferritin antibody VL domain.

Understanding refolding pathways of recombinant antibody fragments is essential for efficient production of these proteins of high biomedical significance. The recombinant VL domain of mouse anti-human ferritin antibody F11 formed two distinct functional conformations obtained by refolding from bacterial inclusion bodies using two different procedures. Involvement of a dialysis step at pH 2-3 resulted in the VL-1 conformation with fluorescence of the highly conserved Trp-35 residue quenched by the spatially proximal disulfide bond. This conformation was identical to the 'native' VL domain folded in host cells and purified from the cytoplasm. In the absence of the acidic dialysis step, the VL domain adopted a previously unreported conformation, VL-2, that demonstrated prominent fluorescence due to a local structural disorder around Trp-35. Furthermore, VL-2 showed changes in secondary structure and significantly lower stability as determined by differential scanning calorimetry and denaturant-induced unfolding. While more flexible VL-2 binds human ferritin both in solution and after surface adsorption of the antibody domain, the VL-1 conformer needs an adsorption-induced conformational change to allow the access of ferritin to the antigen-binding site. Noteworthy, the two macroscopic conformations constitute kinetically trapped dimers and do not interconvert at elevated temperatures (3 weeks at 37 degrees C or 15 min at 60 degrees C), which indicates a high energetic barrier between them. As a major finding, this paper provides the first description for two stable and functional conformations of an antibody domain.