The effect of pH on the unfolding pathway and stability of ribosome‐inactivating protein abrin‐II

The effect of pH on the unfolding pathway and the stability of the toxic protein abrin‐II have been studied by increasing denaturant concentrations of guanidine hydrochloride and by monitoring the change in 8,1‐anilino naphthalene sulfonic acid (ANS) fluorescence upon binding to the hydrophobic sites of the protein. Intrinsic protein fluorescence, far and near UV‐circular dichroism (CD) spectroscopy and ANS binding studies reveal that the unfolding of abrin‐II occurs through two intermediates at pH 7.2 and one intermediate at pH 4.5. At pH 7.2, the two subunits A and B of abrin‐II unfold sequentially. The native protein is more stable at pH 4.5 than at pH 7.2. However, the stability of the abrin‐II A‐subunit is not affected by a change in pH. These observations may assist in an understanding of the physiologically relevant transmembrane translocation of the toxin.