The Thermodynamic Basis of the Fuzzy Interaction of an Intrinsically Disordered Protein.

Many intrinsically disordered proteins (IDP) that fold upon binding retain conformational heterogeneity in IDP-target complexes. The thermodynamics of such fuzzy interactions is poorly understood. Herein we introduce a thermodynamic framework, based on analysis of ITC and CD spectroscopy data, that provides experimental descriptions of IDP association in terms of folding and binding contributions which can be predicted using sequence folding propensities and molecular modeling. We show how IDP can modulate the entropy and enthalpy by adapting their bound-state structural ensemble to achieve optimal binding. This is explained in terms of a free-energy landscape that provides the relationship between free-energy, sequence folding propensity, and disorder. The observed "fuzzy" behavior is possible because of IDP flexibility and also because backbone and side-chain interactions are, to some extent, energetically decoupled allowing IDP to minimize energetically unfavorable folding.

Substrate Recognition and Activity Regulation of the Escherichia coli mRNA Endonuclease MazF.

Escherichia coli MazF (EcMazF) is the archetype of a large family of ribonucleases involved in bacterial stress response. The crystal structure of EcMazF in complex with a 7-nucleotide substrate mimic explains the relaxed substrate specificity of the E. coli enzyme relative to its Bacillus subtilis counterpart and provides a framework for rationalizing specificity in this enzyme family. In contrast to a conserved mode of substrate recognition and a conserved active site, regulation of enzymatic activity by the antitoxin EcMazE diverges from its B. subtilis homolog. Central in this regulation is an EcMazE-induced double conformational change as follows: a rearrangement of a crucial active site loop and a relative rotation of the two monomers in the EcMazF dimer. Both are induced by the C-terminal residues Asp-78-Trp-82 of EcMazE, which are also responsible for strong negative cooperativity in EcMazE-EcMazF binding. This situation shows unexpected parallels to the regulation of the F-plasmid CcdB activity by CcdA and further supports a common ancestor despite the different activities of the MazF and CcdB toxins. In addition, we pinpoint the origin of the lack of activity of the E24A point mutant of EcMazF in its inability to support the substrate binding-competent conformation of EcMazF.

Differences in Unfolding Energetics of CcdB Toxins From V. fischeri and E. coli.

Ccd system is a toxin-antitoxin module (operon) located on plasmids and chromosomes of bacteria. CcdBF encoded by ccd operon located on Escherichia coli plasmid F and CcdBVfi encoded by ccd operon located on Vibrio fischeri chromosome are members of the CcdB family of toxins. Native CcdBs are dimers that bind to gyrase-DNA complexes and inhibit DNA transcription and replication. While thermodynamic stability and unfolding characteristics of the plasmidic CcdBF in denaturant solutions are reported in detail, the corresponding information on the chromosomal CcdBVfi is rather scarce. Therefore, we studied urea-induced unfolding of CcdBVfi at various temperatures and protein concentrations by circular dichroism spectroscopy. Global model analysis of spectroscopic data suggests that CcdBVfi dimer unfolds to the corresponding monomeric components in a reversible two-state manner. Results reveal that at physiological temperatures CcdBVfi exhibits lower thermodynamic stability compared to CcdBF. At high urea concentrations CcdBVfi, similarly to CcdBF, retains a significant amount of secondary structure. Differences in thermodynamic parameters of CcdBVfi and CcdBF unfolding can reasonably be explained by the differences in their structural features.