Recognition of the intrinsically flexible addiction antidote MazE by a dromedary single domain antibody fragment. Structure, thermodynamics of binding, stability, and influence on interactions with DNA.

The Escherichia coli mazEF operon defines a chromosomal addiction module that programs cell death under various stress conditions. It encodes the toxic and long-lived MazF and the labile antidote MazE. The denaturation of MazE is a two-state reversible dimer-monomer transition. At lower concentrations the denatured state is significantly populated. This leads to a new aspect of the regulation of MazE concentration, which may decide about the life and death of the cell. Interactions of MazE with a dromedary antibody domain, cAbMaz1 (previously used as a crystallization aid), as well as with promoter DNA were studied using microcalorimetric and spectroscopic techniques. Unique features of cAbMaz1 enable a specific enthalpy-driven recognition of MazE and, thus, a significant stabilization of its dimeric native conformation. The MazE dimer and the MazE dimer-cAbMaz1 complex show very similar binding characteristics with promoter DNA, i.e. three binding sites with apparent affinities in micromolar range and highly exothermic binding accompanied by large negative entropy contributions. A working model for the MazE-DNA assembly is proposed on the basis of the structural and binding data. Both binding and stability studies lead to a picture of MazE solution structure that is significantly more unfolded than the structure observed in a crystal of the MazE-cAbMaz1 complex.