Reversible conformational changes in a leucine-binding protein from Escherichia coli.

A leucine-binding protein, earlier characterized and implicated in branched chain amino acid transport, undergoes large reversible conformational changes in the presence of urea or guanidine. These changes appear to represent loss of organized structure, as estimated by loss of binding activity, reactivity with iodine, and optical rotatory dispersion. Upon removal of the denaturing agent by dialysis, full binding activity is restored. In addition, the electrophoretic and antigenic properties of the urea-treated and dialyzed protein are indistinguishable from those of the native protein. The binding of the amino acid does not result in detectable changes in the conformation of the protein, as judged by studies involving the Stokes radius, sedimentation rate, fluorescence, binding of fluorescent probes, and optical rotatory dispersion and circular dichroism.