Energetics of structural transitions of the addiction antitoxin MazE: is a programmed bacterial cell death dependent on the intrinsically flexible nature of the antitoxins?

The Escherichia coli mazEF addiction module plays a crucial role in the cell death program that is triggered under various stress conditions. It codes for the toxin MazF and the antitoxin MazE, which interferes with the lethal action of the toxin. To better understand the role of various conformations of MazE in bacterial life, its order-disorder transitions were monitored by differential scanning calorimetry, spectropolarimetry, and fluorimetry. The changes in spectral and thermodynamic properties accompanying MazE dimer denaturation can be described in terms of a compensating reversible process of the partial folding of the unstructured C-terminal half (high mean net charge, low mean hydrophobicity) and monomerization coupled with the partial unfolding of the structured N-terminal half (low mean net charge, high mean hydrophobicity). At pH<or=4.5 and T<50 degrees C, the unstructured polypeptide chains of the MazE dimer fold into (pre)molten globule-like conformations that thermally stabilize the dimeric form of the protein. The simulation based on the thermodynamic and structural information on various addiction modules suggests that both the conformational adaptability of the dimeric antitoxin form (binding to the toxins and DNA) and the reversible transformation to the more flexible monomeric form are essential for the regulation of bacterial cell life and death.