A Natively Unfolded Toxin Domain Uses Its Receptor as a Folding Template*

Natively unfolded proteins range from molten globules to disordered coils. They are abundant in eukaryotic genomes and commonly involved in molecular interactions. The essential N-terminal translocation domains of colicin toxins from Escherichia coli are disordered bacterial proteins that bind at least one protein of the Tol or Ton family. The colicin N translocation domain (ColN-(1–90)), which binds to the C-terminal domain of TolA (TolA-(296–421)), shows a disordered far-UV CD spectrum, no near-UV CD signal, and non-cooperative thermal unfolding. As expected, TolA-(296–421) displays both secondary structure in far-UV CD and tertiary structure in near-UV CD. Furthermore it shows a cooperative unfolding transition at 65 °C. CD spectra of the 1:1 complex show both increased secondary structure and colicin N-specific near-UV CD signals. A new cooperative thermal transition at 35 °C is followed by the unchanged unfolding behavior of TolA-(296–421). Fluorescence and surface plasmon resonance confirm that the new unfolding transition accompanies dissociation of ColN-(1–90). Hence upon binding the disordered structure of ColN-(1–90) converts to a cooperatively folded domain without altering the TolA-(296–421) structure.