Thermal unfolding of the N-terminal RNA binding domain of the human U1A protein studied by differential scanning calorimetry

Thermal unfolding of the N-terminal RNA binding domain of human U1A protein (RBD1) and several variants has been observed by differential scanning calorimetry. Unfolding of the 95 amino acid domain is reversible and cooperative between pH 2.0 and 2.8 in 30 mM glycine, with a heat capacity for the transition of 1.2 kcal mol(-1)K(-1), and an unfolding free energy of 4.0 kcal mol(-1) at pH 2.3 and 25 degrees C. At higher pH, thermal unfolding is irreversible. In contrast, unfolding of the protein by guanidine hydrochloride denaturation at pH 2.3 and pH 7.0 is reversible, with unfolding free energies of 6.6 and 9.0 kcal mol(-1), respectively. DSC experiments show that RBD variants in which the N-terminal tail is truncated, or in which a functional loop is substituted, have altered unfolding free energies but little variation in their heat capacities of transition.