Myosin

Tiny parts in your body move.

These parts help your muscles work. They pull on things to make you move. This helps you run and play.

Inside your cells, there are tiny motors.

These motors use energy to walk. They walk along thin lines in your cells.

One motor is called myosin. It has a head, a neck, and a tail. The head grabs onto a line. Then, the neck pulls.

This pull is like a tiny step.

Many different kinds of these motors exist. Some help your muscles move. Others carry things around the cell. They are very busy!

These tiny motors are everywhere in living things. They help life go on. It is amazing how they work!

Inside your cells, tiny motors help you move. These motors are called myosin.

Most myosin molecules have three main parts. They have a head, a neck, and a tail.

The head grabs onto thin lines called actin. The neck acts like a lever arm. This neck helps turn power into movement.

The tail part connects to other things. It can carry cargo like tiny packages through the cell.

To move, myosin uses a special fuel called ATP. This fuel provides the power for a "power stroke."

First, the myosin head binds to the actin line. Then, it lets out a part called phosphate. This change makes the neck pull forward. This pull is the power stroke.

There are many types of myosin. Myosin II is famous for making muscles contract. Other types, like Myosin V, walk along lines to move cargo.

Some myosins move one way, while others move the opposite way. Even different species use these same motor parts to stay alive.

Myosins are a large family of tiny motor proteins. They are famous for helping muscles contract and move. Most of these proteins are found in eukaryotic cells, which are the cells that make up plants and animals. These motors work by using a special fuel called ATP. They use this fuel to move along thin lines called actin. This movement helps living things do many important jobs.

Most myosin molecules have three main parts: a head, a neck, and a tail. The head binds to the actin filament to create movement. The neck acts like a lever arm to turn that energy into a pull. This part is also called a linker. The tail domain usually connects to cargo or other myosin subunits. Some tails even help regulate how the motor works.

To move, myosin goes through a cycle called a power stroke. First, the myosin head binds tightly to the actin. Then, the molecule releases a part called phosphate. This release causes a shape change that pulls against the actin. This is the actual power stroke that creates force. After this, the myosin releases a molecule called ADP. Finally, a new ATP molecule binds to let the myosin release the actin so it can start again.

Scientists have been studying these proteins for a long time. In 1864, Wilhelm Kühne discovered the first myosin, known as M2. He found a thick protein in skeletal muscle that kept the muscle under tension. Later, in 1973, researchers found similar proteins in a tiny organism called Acanthamoeba castellanii. This showed that myosin is not just for muscles. Today, we know there is a huge superfamily of many different myosin genes.

There are many different types of myosin with unique jobs. Myosin II is the most well-known because it makes muscles contract. Myosin V is an unconventional type that walks along actin to carry cargo like vesicles. This motor has a step size of 36 nm. Some myosins, like Myosin VI, move in the opposite direction of others. Even different species use these same parts. For example, a rabbit's muscle myosin can still bind to the actin of an amoeba.

Myosins are a diverse superfamily of motor proteins found in almost all eukaryotic cells. While they are most famous for causing muscle contraction, they perform many other essential tasks. These proteins act as tiny molecular machines that convert chemical energy into physical movement. They do this by using a molecule called ATP, which serves as their fuel. Specifically, myosins are responsible for actin-based motility, meaning they move along thin protein filaments called actin.

The physical structure of a myosin molecule typically consists of three distinct domains: a head, a neck, and a tail. The head domain is the catalytic motor. It binds to the actin filament and uses the energy from ATP hydrolysis—the process of breaking down ATP—to generate force. The neck domain acts as a linker and a lever arm. It translates the energy from the head into a physical pull. This neck can also bind to myosin light chains, which are separate proteins that help regulate how the motor works. Finally, the tail domain usually connects the motor to cargo molecules or other myosin subunits.

To create movement, myosin undergoes a repeating mechanical cycle known as the power stroke. This process begins when the myosin head binds to actin. After ATP hydrolysis occurs, the myosin molecule releases a phosphate group. This release triggers a conformational change, which is a change in the shape of the protein. This shape change pulls the lever arm against the actin filament, creating force. Following this, the myosin releases a molecule called ADP. The cycle only resets when a new molecule of ATP binds to the head, causing the myosin to release its grip on the actin so it can start the process again.

Scientific understanding of myosin has evolved significantly since its discovery. In 1864, Wilhelm Kühne identified the first myosin, which he called M2. He extracted a viscous protein from skeletal muscle and believed it was responsible for maintaining muscle tension. For a long time, scientists thought myosin was only found in muscle cells. However, in 1973, researchers discovered enzymes with myosin-like functions in a single-celled organism called *Acanthamoeba castellanii*. This discovery proved that myosin is a global feature of eukaryotic life. Today, we know that myosin is not just one protein, but a massive superfamily of many different genes.

There are many different classes of myosin, often categorized by their evolutionary relationships. Myosin II is the most abundant and is responsible for contraction in skeletal, cardiac, and smooth muscle. It forms thick filaments within the sarcomere, which is the functional unit of a muscle. In contrast, "unconventional" myosins, such as Class I, often act as single units called monomers. These are frequently used for transporting vesicles within a cell. Myosin V is another important unconventional motor. It acts as a dimer and has a step size of 36 nm as it walks along actin to carry cargo like mitochondria or organelles.

Different myosin types are specialized for specific speeds and directions. Most myosins walk toward the "barbed" or positive (+) end of an actin filament. However, Myosin VI is a notable exception because it moves toward the "pointed" or negative (-) end. The speed of a myosin motor depends on how quickly it completes its kinetic cycle of ATP binding and ADP release. Additionally, the length of the neck domain affects how far a motor moves. A longer lever arm allows the motor to move its cargo a greater distance with every step, much like a person with longer legs covers more ground per stride.

The molecular machinery of myosin is remarkably well-preserved across the tree of life. This is known as being globally conserved. Because the basic mechanism of moving along actin is so effective, the head domains of different species remain very similar. In fact, the myosin II from a rabbit can successfully bind to the actin filaments of an amoeba. This deep connection shows how fundamental these tiny motors are to the survival and movement of almost all complex living organisms.