By Mara Casebeer
Just like a city has highways to transport goods and people from one area to another, your cells have their own set of highways to transport important components like proteins and genetic material. These cellular highways are called microtubules, and they are traversed not by trucks and cars, but by molecular motors – proteins that can carry cargo and step along microtubules (Figure 1). In eukaryotic cells, such as those found in plants, animals, and fungi, this transport requires energy, unlike passive processes like diffusion. As with any energy-dependent process, our body needs to optimize the efficiency of molecular motor transport in our cells.
Molecular motors are orders of magnitude smaller than your typical car engine – at around 30 nanometers long! – and produce much less energy. A car engine can produce around 100,000 Joules each minute, whereas a molecular motor produces closer to a few Joules per minute. However, if we consider the size difference between the two motors, molecular motors are remarkably efficient machines.
As a result of their much smaller size, molecular motors need to optimize for a different sort of physics compared to a car engine. In particular, molecular motors are subject to the whims of Brownian motion.
What is Brownian Motion?
When Scottish botanist Robert Brown looked at pollen grains underneath one of the first microscopes in 1827, he noticed that, despite being suspended in a stationary liquid, each grain was wiggling and jiggling. While he originally attributed this motion to the “force of life”, later inspection of rock and inorganic materials showed that non-living materials also moved back and forth.
This observed motion, now called “Brownian motion,” was in fact a result of thermodynamic fluctuations. In fluids like water, the molecules that make up that fluid are constantly moving, colliding with each other and changing directions. The higher the temperature, the faster the molecules move. When bigger particles like pollen grains are suspended in a fluid, they will be constantly bombarded by the moving molecules of the fluid, which will push them a tiny bit randomly in all directions, resulting in the jiggling motion. As a result, the fluctuations of the larger, suspended particle is also directly related to the temperature of the system.
Brownian motion is only observed in microscopic particles. When you go swimming, for example, you are far too big to feel the effect of individual water molecules bumping into you. A molecular motor, on the other hand, may be able to feel those effects. But, can a molecular motor take advantage of Brownian motion to make it go faster? If we imagine a microscopic paddle that is free to rotate when experiencing collisions with water molecules, it doesn’t seem like the paddle would move forward or backward substantially. Instead, it would just shift back and forth when a molecule hits it one way or another, like in Figure 2, producing no net energy transfer, known as work. One model to extract work from Brownian motion is the Brownian ratchet.
What is a Brownian Ratchet?
The Brownian ratchet is a theoretical framework by which we can try to understand how biological systems harness Brownian motion. In Figure 2, we see particles exhibiting Brownian motion and hitting a paddle, causing it to move. In Figure 3, on the other hand, we have another object in the system: a ratchet. While a paddle is symmetrical and can rotate in either direction, a ratchet is asymmetric, meaning that the device can only rotate in a single direction. This asymmetry is created by the pawl, which blocks the movement of the ratchet in the other direction.
What happens if we pair the two devices with the goal of doing work – in this case, lifting the weighted load? When Brownian motion causes the microscopic particles to hit the paddle, moving it in the right direction, the connected ratchet will move to the next tooth and the load will lift, without any danger of moving back thanks to the pawl, which is locked into place. If the paddle is at a higher temperature than the ratchet, meaning the paddle is experiencing more Brownian motion than the ratchet, this mechanism will extract work from the temperature difference by lifting the load. In Figure 3, for example, the temperature difference would lift the load as the paddle and ratchet rotates. How would a motor protein use a similar mechanism?
Could Motor Proteins Behave as Brownian Ratchets?
For a motor protein to behave as a Brownian ratchet, the ratchet itself would look slightly different. The motor protein contains binding sites that lock onto microtubules when they are bombarded from one direction by water molecules, but disengage when they are bombarded in the other direction. It takes energy for the motor protein to release its grip from the microtubule and propel itself forward. Once it does, water molecules randomly hit the motor protein, eventually causing the locking mechanism to re-engage further ahead on the microtubule track. Like the ratchet, this locking mechanism prevents the motor from being pushed off track and allows the motor to take a step forward. Overall, this process of energy release and thermal ratcheting drives the molecular motor’s motion.
While the initial conception of a Brownian ratchet is old, it is still actively being studied today. A 2020 Nature paper showcased how dynein, a particular kind of motor protein, could harness Brownian motion to power its movement. To test whether dynein acts as a Brownian ratchet, the scientists used optical traps. Optical traps are like microscopic tweezers – a tightly focused laser beam that can be used to trap a protein or small object and hold or push it around. They showed that dynein can act as a ratchet on microtubules, only moving in one direction, even when pulled in the other direction by these optical traps. The scientists also applied small oscillating forces to the dynein to mimic the effects of Brownian motion. By increasing or decreasing this small oscillating force, the scientists intended to find out if random fluctuations help or hurt the transport process. The result of this experiment was that dynein moves faster in its natural direction along microtubules and outputs more power when it is experiencing more fluctuations. Taken together, these results provide evidence that dynein does indeed behave as a mechanical ratchet and harnesses random thermal fluctuations to propel itself along microtubules.
Overall, Brownian ratchets are a clever way for molecular motors to harness the constantly changing thermal energy of their surroundings and drive directed transport, which is critical for keeping our cells alive. However, the experiments by Exber et al. used isolated proteins outside of an actual cell. With more advances in microscopy, we may begin to be able to see individual motor proteins inside of living cells and learn more about how our cells use Brownian motion.
Mara Casebeer is a PhD student in the Biophysics Program at Harvard University.