Monitoring the rotary mechanism of a single FoF1-ATP synthase in the ABELtrap

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FoF1-ATP synthases produce and maintain the [ATP] level in cells by a mechanochemical mechanism using internal rotation of subunits. We have investigated the stepwise rotation of the gamma and epsilon subunits in the F1 part and of the c-ring in the Fo part of single enzymes. Two subunits were labeled specifically with different fluorophores to monitor the internal distance changes during rotation using Förster resonance energy transfer (FRET). Our confocal single-molecule FRET approach used freely diffusing enzymes reconstituted in liposomes with a diameter of about 120 nm. Limited by a mean transit time of 30 ms for the proteoliposomes through the detection volume, we could study subunit rotation during ATP hydrolysis at 1 mM ATP and ATP synthesis at high Delta_ph and Delta_psi only.The group of W. E. Moerner in Stanford has developed an Anti-Brownian electrokinetic trap (ABELtrap) which holds a small fluorescent particle (i.e. a protein) in place for up to 10 seconds in a microfluidic device. Single-molecule FRET measurements in the ABELtrap allow to answer the open mechanistic questions of possible substeps of 120° rotation of epsilon during ATP hydrolysis at lower [ATP], to clarify the identical or different dwell times of the sequential 36° steps of c and to measure the internal elastic deformation of the two coupled rotor subunits with appropriate single-molecule statistics. The applicant participates in these ABELtrap experiments as a Stanford Visting Scholar.