Compact molten globule-like state of hUBF HMG Box1 at extremely low pH

Abstract

Using far and near-UV CD, ANS fluorescence and 2D NMR spectroscopy, an acid-induced partly folded state (A state) at extremely low pH for hUBF HMG Box1 was identified and characterized. As compared to the native state (N), the A state has similar secondary structure, less compact pack with larger amounts of exposed hydrophobic surface, and narrower chemical shift dispersion in 1H– 15N HSQC spectrum, which implies that it is a molten globule (MG)-like species. On the other hand, substantial tertiary contacts and cooperative thermal denaturing transition indicate that the A state is closer–relative to the classic MG–to the native folded state. In addition, when the solution pH is adjusted to neutrality, the protein in the A state refolds to the native state easily. All these data suggest that the A state of hUBF HMG Box1 could represent a potential folding intermediate on protein folding pathway.