Modulation of allosteric behavior through adjustment of the differential stability of the two interacting domains in E. coli cAMP receptor protein.

The communication mechanism(s) responsible for the allosteric behavior of E.coli cAMP binding receptor protein, CRP, is still a subject of intense investigation. As a tool to explore the communication mechanism, the mutations at various positions in the cAMP-binding (K52N, D53H, S62F and T127L) or the DNA- binding (H159L) domain or both (K52N/H159L) were generated. The sites and specific nature of side chain substitutions were defined by earlier genetic studies, the results of which show that these mutants have a similar phenotype i.e. they are activated without exogenous cAMP. Presently, no significant changes in the structures of WT and mutant CRPs have been observed. Hence, the pressing issue is to identify a physical parameter that reflects the effects of mutations. In this study, the stability of these various CRP species in the presence of GuHCl was monitored by three spectroscopic techniques, namely, CD, tryptophan fluorescence and FT-IR which could provide data on the stability of α-helices and β-strands separately. Results of this study led to the following conclusions: 1. The α-helices can be grouped into two families with different stabilities. Mutations exert a differential effect on the stability of helices as demonstrated by a biphasic unfolding curve for the helices. 2. Regardless of the locations of mutations, the effects can be communicated to the other domain resulting in a perturbation of the stability of both domains, although the effects are more significantly expressed in the stability of the helices. 3. Although in an earlier study [Gekko, et al. Biochemistry 43 (2004) 3844] we showed that cooperativity of cAMP binding is generally correlated to the global dynamics of the protein and DNA binding affinity, in this study we found that generally there is no clear correlation between functional energetics and stability of secondary structures. Thus, results of this study imply that modulation of allostery in CRP is entropic in nature.