What does being alive mean to you? Perhaps the ability to think, breathe, eat and sleep? From a scientist’s perspective, being alive means that your cells can use energy, respond to your environment, and reproduce.
Cells are tiny. And yet, they’re able to do the extremely complex tasks it takes to stay alive. That’s because cells have evolved special types of proteins. They’re called molecular motors, and they’re constantly at work to keep your cells alive.
Molecular motors are often called the workhorses of the cell. They are, quite literally, tiny machines made out of protein.
Some motors walk in straight lines on protein filaments, which are like highways within the cell. They actually walk in a foot-over-foot fashion, just like humans do! These motors are called cytoplasmic motors. There are three families of them: kinesins, myosins and dyneins.
Did you know? Scientists believe that humans have 40 myosin, 13 dynein, and 45 kinesin genes.
Cytoplasmic molecular motors have many important jobs. Here are some examples:
Kinesin-1: The Resource Distributor
One of the important jobs cytoplasmic motors are tasked with is resource distribution - that is, giving different parts of the cell what it needs to be healthy.
For example, kinesin-1 is a two-legged molecular motor that actually walks on filaments in the cell called microtubules. Often, kinesin-1 can be found walking away from the cell nucleus towards the edge of the cell. While doing so, it carries things called vesicles, which are huge cargos full of things the cell needs to survive (such as proteins).
Did you know? Malfunctioning kinesin motors are involved in many cancers. Because of this, they’re the target of many chemotherapy treatments.
Myosin-2: Muscle movers
Meanwhile, another cytoplasmic molecular motor called myosin-2 is responsible for muscle contraction. Large groups of myosin-2 motors work together to tug on filaments within muscle cells. When all of these filaments are tugged on at the same time, your muscles contract.
Why the large groups? Imagine you needed to pull a bike out of a ditch with a rope. If you were the only person pulling, the bike might not move. If a neighbour came by and tried to pull, and then your sibling came out and tried to pull, the bike still might not move. But if all three of you (and maybe some more neighbours) pulled at the same time, the force on the bike would be much larger, and the bike would move.
Similarly, many myosin-2 motors work together to pull on filaments, making your muscles contract. When your heart beats, when you walk, when you smile, that’s many myosin-2 motors at work.
Dynein: Airway cleaners
You might not know it, but you rely on dynein to clear your airways of tiny debris. Your body has cilia, tiny hairlike organelles that extend outward from your cell surfaces. To move mucus around, or to clear debris from your lungs into your throat, all the cilia in your airways move in unison - thanks to dynein.
Dynein motors work behind the scenes. They’re attached to every cilia, and they work in groups and synchronize much the way that myosin-2 motors do. When these dyneins stop doing their job, you can develop a respiratory disease, which can cause problems breathing. See what I mean when I say that molecular motors have some pretty important jobs?
Also, like kinesin, dynein motors move resources around. But while kinesin motors almost always move away from the cell nucleus, dynein motors move in the opposite direction towards the cell nucleus. This allows the two motors to move resources back and forth, just like on a two-lane highway with cars driving in opposite directions.
Did you know? Not all molecular motors walk. Rotary motors sit stationary in the cellular membrane and - you guessed it! - rotate.
Now you know that as you go about your day, kinesin-1, myosin-2 and dynein are at work. But the tasks you’ve read about are only three examples. There are actually hundreds of important jobs molecular motors do.
And it’s not just humans that rely on molecular motors. They’re also working in the cells of all other animals, plants, fungi, bacteria, and protists (one-celled organisms). No matter what living organism you look at, molecular motors are in there, working hard to keep its cells alive!