Cold denaturation of CheY.

The thermal stability of the bacterial chemotaxis protein CheY from Salmonella typhimurium has been examined by thermal denaturation at pH 7.0 in the presence of guanidine-HCl and urea. For both denaturants, thermal denaturation monitored by circular dichroism spectropolarimetry consists of transitions both above and below 25 degrees C, which is strong evidence for a heat capacity change that is > or = 1500 cal/(mol K) upon unfolding. While many data for chemical and thermal denaturation are consistent with data for CheY from Escherichia coli, the observation of cold denaturation for S. typhimurium CheY is inconsistent with the small heat capacity change, 600 to 850 cal/(mol K), reported for denaturation of the E. coli protein.