Myosin

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.