Regulatory conformational changes of the ε subunit in single FRET-labeled FoF1-ATP synthase.

Subunit e is an intrinsic regulator of the bacterial FoF1-ATP synthase, the ubiquitous membrane-embedded enzyme that utilizes a proton motive force in most organisms to synthesize adenosine triphosphate (ATP). The C-terminal domain of e can extend into the central cavity formed by the α and β subunits, as revealed by the recent X-ray structure of the F1 portion of the Escherichia coli enzyme. This insertion blocks the rotation of the central γ subunit and, thereby, prevents wasteful ATP hydrolysis. Here we aim to develop an experimental system that can reveal conditions under which e inhibits the holoenzyme FoF1-ATP synthase in vitro. Labeling the C-terminal domain of e and the γ subunit specifically with two different fluorophores for single-molecule Forster resonance energy transfer (smFRET) allowed monitoring of the conformation of e in the reconstituted enzyme in real time. New mutants were made for future three-color smFRET experiments to unravel the details of regulatory conformational changes in e.