Disorder-order transition of λ CII promoted by low concentrations of guanidine hydrochloride suggests a stable core and a flexible C-terminus

The CII protein of bacteriophage lambda, which activates the synthesis of the lambda repressor, plays a key role in the lysis-lysogeny switch. CII has a small in vivo half-life due to its proteolytic susceptibility, and this instability is a key component for its regulatory role. The structural basis of this instability is not known. While studying guanidine hydrochloride-assisted unfolding of CII, we found that low concentrations of the chaotrope (50-500 mM) have a considerable effect on the structure of this protein. This effect is manifest in an increase in molar ellipticity, an enhancement of intrinsic tryptophan fluorescence intensity and a reduction in ANS binding. At low concentrations of guanidine hydrochloride CII is stabilized, as reflected in a significant decrease in the rate of proteolysis by trypsin and resistance to thermal aggregation, while the tetrameric nature of the protein is retained. Thus low concentrations of guanidine hydrochloride promote a more structured conformation of the CII protein. On the basis of these observations, a model has been proposed for the structure of CII wherein the protein equilibrates between a compact form and a proteolytically accessible form, in which the C-terminal region assumes different structures.