Stability of the lac repressor headpiece against thermal denaturation and tryptic hydrolysis

The stability of the conformation of the lac repressor headpiece against thermal denaturation and tryptic hydrolysis has been studied by circular dichroism measurements. In both cases the stability depends strongly on the concentration of NaCl. This effect is larger than generally observed for proteins. The midpoint of the thermal denaturation curve (Tm) is shifted from about 37 degrees C in the absence of NaCl to about 68 degrees C in 1 M NaCl. After a first non-linear increase of the Tm with the NaCl concentration (up to about 0.2 M NaCl) the Tm varies linearly with the salt concentration. Assuming a two-state mechanism for the thermal denaturation, enthalpies of 30-36 kcal/mol have been determined. The decrease of the circular dichroism signal due to the tryptic cleavage follows pseudo first-order kinetics for all salt concentrations studied. The half-life time of hydrolysis increased by about 40-times from 2 mM to the highest NaCl concentration we have used (655 mM). Assuming that only the unfolded state of the headpiece is a good substrate for trypsin, the observed stabilization against proteolytic degradation may be explained by a shift of the unfolding equilibrium of the headpiece due to the salt, and a subsequent decrease of the concentration of the unfolded state. The unusual stabilization of the headpiece is discussed with respect to its positive charge and to its function to bind to DNA.