For Biophysics Week, members of the Early Careers Committee have written short summaries of classical biophysics studies, accessible to scientists and non-scientists alike.

This lay summary regarding Motor Proteins and Insights from Optical Tweezers was written by Early Careers Committee Chair Bert Tanner, Washington State University.

Motor Proteins and Insights from Optical Tweezers

Motor proteins are a class of biomolecules that use chemical energy to generate forces and motion inside a cell.  Much like we put gas into a car to power the engine for a car to drive, motor proteins harness chemical energy by splitting up adenosine triphosphate (or ATP) molecules to produce piconewton-forces over nanometer-distances. Some of the most well-known motor proteins include myosin, kinesin, and dynein.  In our hearts and skeletal muscles millions of myosins work together to bind actin filaments, thereby powering muscle contraction, blood flow, and human locomotion.  Kinesin and dynein play important roles in transporting cargo molecules around a cell, and these motors help properly separate chromosomal DNA as cells divide.

While there is still a lot to learn about the complexities of the motor proteins, how they power life in the cell, and how things can go wrong with disease, we have learned a lot about these motors.  For example, we know that the activity of these motors can be modulated by loads that they experience and the forces that they experience as they operate in a cell (Finer et al. 1994; Svoboda and Block 1994).  Similar to driving up a hill, resistive loads can slow down these motors and even call them to stall out altogether.  The opposite can also occur, where smaller loads or assistive forces among a team of motors can make them move faster or more consistently.

Biophysicists who study these motors owe a lot to Arthur Ashkin, who won the Nobel Prize in Physics in 2018 for “the optical tweezers and their application to biological systems."  It was Ashkin who worked to focus sharp beams of laser light, thus inventing optical tweezers that can be used to grab particles, atoms, molecules, and living cells with these “laser beam fingers” (Ashkin et al. 1986). Optical tweezers are now widely used to investigate biological systems, and the ability to use light to make force measurements in the realm of piconewtons with nanometer precision has been a truly remarkable development.  As a point of reference, the forces created by a laser pointer shining on a wall are in the ballpark of 2-10 piconewtons (depending upon intensity of pointer).  These force levels are equivalent to optical tweezers, making it a very useful tool to measure, monitor, and/or influence individual motor proteins.

Ashkin A et al. (1986) Observation of a single-beam gradient force optical trap for dielectric particles.  Optics Letters 11:288-290.  •https://doi.org/10.1364/OL.11.000288

Finer JT et al. (1994) Single myosin molecule mechanics: piconewton forces and nanometre steps. Nature 368:113-119. doi:    10.1038/368113a0

Svoboda K and Block SM (1994) Force and velocity measured for single kinesin molecules. Cell 77:773-784.  DOI:    10.1016/0092-8674(94)90060-4