Stepped versus continuous rotatory motors at the molecular scale

Area of Science:

• Biophysics
• Molecular Biology
• Biochemistry

Background:

• Nature utilizes molecular rotary devices, such as the flagellar motor and ATP synthase.
• Photoselection techniques are common for detecting protein rotational random walks, but less so for subunit rotational drift.
• Existing theories predict polarization anisotropy (r) oscillations for unidirectional drift and monotonic relaxation for bidirectional random walks, assuming continuous angles.

Purpose of the Study:

• To develop a theory for stepped rotatory devices, addressing the limitations of continuous angular assumptions.
• To investigate the rotational behavior of subunits in molecular machines.
• To experimentally validate theoretical predictions using the F-ATPase motor.

Main Methods:

• Developed a new theory for stepped rotatory devices, predicting damped oscillations in polarization anisotropy (r).
• Investigated the relationship between damping, number of steps, and relaxation behavior.
• Conducted photoselection experiments using active F-ATPase with a spectroscopic probe on the central beta-subunit.

Main Results:

• The developed theory predicts damped oscillations of polarization anisotropy (r) for unidirectional drift in stepped rotary devices.
• Damping of oscillations increases as the number of steps decreases; a three-step device predicts quasi-monotonic relaxation for both random walk and drift.
• Experimental photoselection on F-ATPase showed relaxation of r, consistent with a three-stepped rotary mechanism.

Conclusions:

• The rotational motion of molecular machines like F-ATPase may occur in discrete steps rather than a continuous rotation.
• A three-step rotation model accurately explains the observed relaxation of polarization anisotropy in F-ATPase experiments.
• This work provides a theoretical framework and experimental evidence for stepped rotation in biological rotary motors.