Studies on the physicochemical structure and stability of an active fragment (T2A) of colicin E3

The physicochemical nature of an active fragment (T2A) of colicin E3 was examined together with those of its specific inhibitor, B, and the T2A-B complex under various conditions by means of CD and fluorescence spectroscopy. 1. T2A was mainly composed of unordered structure, although the molecule was folded into a rather compact structure, probably by beta-turns and ionic interactions. 2. The structure of T2A was not stable, and unfolding of the molecule was observed on treatments with acid, alkali, heat, guanidine hydrochloride, and detergents. However, once denaturing conditions were removed, the unfolded T2A molecule quickly regained the native conformation, recovering its full activity. 3. Formation of a one-to-one compex of T2A and B was confirmed. No gross conformational changes in T2A and B were found upon formation of the T2A-B complex. 4. A mechanism of dissociation of colicin E3 and A and B, or of T2A-B into T2A and B, during passage through the membrane of colicin E3-infected cells is proposed, based mainly on the results obtained here.