Three-dimensional bead position histograms reveal single-molecule nanomechanics

Area of Science:

• Biophysics
• Molecular Biology
• Structural Biology

Background:

• Understanding macromolecular structure and elasticity is crucial in molecular biology.
• Myosin-II is a motor protein essential for muscle contraction and cellular motility.
• Characterizing the mechanical properties of myosin-II provides insights into its function.

Purpose of the Study:

• To develop and apply a novel method combining single molecule experiments and kinematic modeling.
• To investigate the structure and elasticity of full-length myosin-II.
• To determine the location and flexibility of intramolecular hinges within myosin-II.

Main Methods:

• Utilized a photonic force microscope to record spatial position histograms of a microsphere tethered to myosin-II.
• Developed a kinematic model based on robot kinematics, assuming myosin-II consists of concatenated rigid segments.
• Related experimental histograms to molecular segment lengths and bending stiffnesses.

Main Results:

• The model successfully related experimental data to myosin-II's structural parameters.
• Calculated position distributions exhibited asymmetry, characteristic of a mixed entropic-enthalpic spring.
• The best-fit model identified two intramolecular hinges and a summed bending stiffness of approximately 3 k(B)T/rad.

Conclusions:

• The combined experimental and modeling approach effectively probes macromolecular structure and elasticity.
• Myosin-II exhibits a mixed entropic-enthalpic spring behavior due to its intramolecular flexibility.
• The identified hinges and stiffness provide a quantitative mechanical description of myosin-II.