Folding and Association of an Extremely Stable Dimeric Protein from Sulfolobus islandicus

ORF56 is a plasmid-encoded protein from Sulfolobus islandicus, which probably controls the copy number of the pRN1 plasmid by binding to its own promotor. The protein showed an extremely high stability in denaturant, heat, and pH-induced unfolding transitions, which can be well described by a two-state reaction between native dimers and unfolded monomers. The homodimeric character of native ORF56 was confirmed by analytical ultracentrifugation. Far-UV circular dichroism and fluorescence spectroscopy gave superimposable denaturant-induced unfolding transitions and the midpoints of both heat as well as denaturant-induced unfolding depend on the protein concentration supporting the two-state model. This model was confirmed by GdmSCN-induced unfolding monitored by heteronuclear 2D NMR spectroscopy. Chemical denaturation was accomplished by GdmCl and GdmSCN, revealing a Gibbs free energy of stabilization of −85.1 kJ/mol at 25 °C. Thermal unfolding was possible only above 1 M GdmCl, which shifted the melting temperature (tm) below the boiling point of water. Linear extrapolation of tm to 0 M GdmCl yielded a tm of 107.5 °C (5 μM monomer concentration). Additionally, ORF56 remains natively structured over a remarkable pH range from pH 2 to pH 12. Folding kinetics were followed by far-UV CD and fluorescence after either stopped-flow or manual mixing. All kinetic traces showed only a single phase and the two probes revealed coincident folding rates (kf, ku), indicating the absence of intermediates. Apparent first-order refolding rates depend linearly on the protein concentration, whereas the unfolding rates do not. Both ln kf and ln ku depend linearly on the GdmCl concentration. Together, folding and association of homodimeric ORF56 are concurrent events. In the absence of denaturant ORF56 refolds fast (7.0×107 M−1 s−1) and unfolds extremely slowly (5.7 year−1). Therefore, high stability is coupled to a slow unfolding rate, which is often observed for proteins of extremophilic organisms.