Myosin Motors: How These Cellular Engines Work

Myosin motors are microscopic engines inside our cells that generate the force and movement necessary for life. These proteins convert chemical energy into mechanical work, powering a range of activities from muscle contraction to the transport of materials within a single cell. They are the driving force behind physical motion in cells, making their activity foundational to the structure and dynamics of all eukaryotic organisms.

The Fundamental Structure of Myosin

A myosin molecule is composed of three distinct domains: a head, a neck, and a tail. The head is the motor domain, containing the machinery for binding to cellular tracks called actin filaments. This globular region has the enzymatic ability to break down adenosine triphosphate (ATP), the cell’s primary energy currency, which fuels its movement. The head is the most conserved part of the myosin protein across its many classes.

Adjacent to the head is the neck domain, which acts as a rigid lever arm. This section, stabilized by smaller proteins called light chains, amplifies the small conformational changes that occur within the head. The length of the neck can vary between myosin types, which influences how far the motor moves in a single step.

The tail domain is the most variable part of the molecule and dictates the myosin’s specific function. In some myosins, the tails are long and fibrous, enabling them to assemble into large filaments. In others, the tail is specialized to bind to specific cellular cargo, such as vesicles or organelles, allowing it to act as a transport vehicle.

The Power Stroke Mechanism

The force-generating action of myosin is a cyclical process known as the power stroke mechanism, which is coupled to the consumption of ATP. The cycle begins with the myosin head detached from the actin filament with an ATP molecule bound to it. The head’s enzyme then hydrolyzes the ATP into adenosine diphosphate (ADP) and an inorganic phosphate (Pi).

This reaction releases energy, causing the myosin head to “cock” into a high-energy position. Now in its energized state, the myosin head attaches to an adjacent actin filament, forming a cross-bridge. This binding triggers the release of the inorganic phosphate, which strengthens the bond between myosin and actin and initiates the power stroke.

During the power stroke, the head pivots and pulls the actin filament approximately 10 nanometers. Following this action, the ADP molecule is released, but the head remains tightly bound to the actin in a rigor conformation. The cycle can only restart when a new molecule of ATP binds to the myosin head, causing it to detach from the actin.

Myosin’s Role in Muscle Contraction

The most recognized function of myosin is powering muscle contraction, a process carried out by Myosin II. Within muscle cells, these Myosin II molecules assemble into structures called thick filaments. These filaments are arranged in a repeating pattern alongside thinner actin filaments, forming the contractile unit of striated muscle called the sarcomere.

Muscle contraction occurs according to the sliding filament theory. This model explains that the filaments themselves do not change in length; rather, the thin actin filaments slide past the thick myosin filaments. This sliding action is driven by the coordinated power strokes of the numerous myosin heads extending from the thick filaments.

As countless myosin heads perform their power strokes, they collectively pull the actin filaments toward the center of the sarcomere, the M line. This causes the Z discs, which mark the sarcomere’s boundaries, to be drawn closer together, shortening the entire unit. When millions of sarcomeres shorten simultaneously, the result is the macroscopic contraction of the muscle.

Beyond Muscle: Diverse Cellular Functions

The myosin family is versatile, performing many other jobs within nonmuscle cells. One major function is the transport of intracellular materials. Myosins from the Myosin V class act like miniature trucks, “walking” along the actin filament network to deliver cargo such as vesicles and organelles to specific destinations. The tail of Myosin V binds to the cargo, while its two head domains move step-by-step along the actin track.

Myosin is also central to cell division, specifically during the final step called cytokinesis. After the genetic material has been separated, a contractile ring of actin and Myosin II forms at the cell’s equator. This ring functions like a purse string, with Myosin II motors pulling the actin filaments to constrict the ring, ultimately dividing the cell into two.

Myosins also contribute to maintaining cell shape and tension. Myosin I, for instance, has a tail that can bind to the cell membrane, linking the inner actin cytoskeleton to the outer membrane. These single-headed motors can regulate membrane tension and influence cell shape and motility.

Myosin Dysfunction and Human Health

Proper functioning of myosin is necessary for health, and errors in their genetic code can lead to a variety of human diseases. Mutations can alter the protein’s structure and impair its ability to generate force or interact with actin and cargo. The consequences are often specific to the tissues where a particular myosin isoform is most active.

A prominent example is in heart muscle, where mutations in the gene for β-cardiac myosin (MYH7) cause familial hypertrophic cardiomyopathy. This condition is characterized by an abnormal thickening of the heart muscle, which can interfere with its ability to pump blood effectively and lead to serious complications.

Defects are not limited to muscle myosins. Mutations in the MYO7A gene, which codes for Myosin VIIA, are responsible for Usher syndrome type 1B, a condition characterized by congenital deafness and progressive vision loss. Myosin VIIA is active in the sensory cells of the inner ear and the retina, and its dysfunction compromises these tissues.