Myosin is a motor protein that converts chemical energy from adenosine triphosphate (ATP) into mechanical force, facilitating movement on a cellular level. This large protein family is responsible for a wide range of movements in eukaryotic cells, from muscle contraction to cell division. Functioning like microscopic engines, myosins generate the force needed to move along protein filaments.
The Molecular Structure of Myosin
Most myosin molecules are composed of a head, neck, and tail domain. Each region has a distinct structure and purpose. While there are many variations of myosin, this basic three-part structure is a common feature for the types involved in muscle contraction and cellular transport.
The head region is also known as the motor domain. This globular structure contains a binding site for actin, a protein that forms filaments that serve as tracks for myosin to move along. The head also has a pocket that binds and hydrolyzes ATP, releasing the energy needed to power its movement.
Connecting the head to the tail is the neck region, which acts as a flexible linker and a lever arm. This part of the molecule is where smaller proteins called light chains bind, which can help regulate the myosin’s activity. The neck amplifies the small conformational changes that occur in the head, translating them into a larger mechanical movement. The length of this lever arm can influence the step size of the myosin.
The tail domain’s primary role is to interact with other molecules or structures. In some myosins, the tails of many molecules intertwine to form thick filaments. For other myosins, the tail domain serves as an anchor, attaching to cellular cargo like vesicles or organelles to transport them along the actin filament network. This diversity in tail structure allows different myosins to perform a wide array of functions.
Myosin’s Function in Muscle Contraction
Muscle contraction is driven by the interaction between myosin and actin filaments, a process known as the sliding filament model. Within muscle cells, myosin molecules are bundled together to form “thick filaments” with their heads pointing outwards. These are positioned alongside “thin filaments” made of actin. When a muscle contracts, the myosin heads pull the actin filaments, causing them to slide past each other and shorten the muscle’s functional unit, the sarcomere.
This movement is powered by a series of molecular events called the cross-bridge cycle. The cycle begins when a signal, such as an influx of calcium ions, exposes binding sites on the actin filaments. Once these sites are open, an energized myosin head can attach to the actin, forming a cross-bridge.
Following attachment, the myosin head undergoes a conformational change known as the power stroke. During this step, the head pivots and pulls the actin filament toward the center of the sarcomere, releasing adenosine diphosphate (ADP) and a phosphate molecule. Billions of these power strokes occurring simultaneously result in the shortening of the muscle.
For the muscle to relax or continue contracting, the myosin head must detach from the actin filament. This is accomplished when a new molecule of ATP binds to the myosin head. After detaching, the myosin hydrolyzes the ATP, which releases energy to “re-cock” the head, returning it to its starting position, ready to begin the cycle anew.
Cellular Roles Beyond Muscle Tissue
While known for its role in muscle, myosin’s functions extend to nearly every eukaryotic cell. One such process is cytokinesis, the final stage of cell division. During cytokinesis, a contractile ring of actin and non-muscle myosin II forms at the cell’s equator. This ring constricts, progressively pinching the cell membrane inward until the parent cell divides into two daughter cells.
Myosin also acts as a transport system, moving materials from one location to another. Certain classes of myosin, such as myosin V, are specialized for this task. These motor proteins attach to organelles or vesicles and “walk” along the network of actin filaments. This directed movement ensures that cellular components reach their intended destinations.
The ability of cells to migrate and change their shape also depends on myosin. Cell migration involves the coordinated extension of the cell in one direction and retraction at the rear. Myosin II generates contractile forces within the cell’s actin cytoskeleton, creating the tension needed to pull the trailing edge of the cell forward. These forces also help the cell adhere to surfaces and navigate through tissues, a process important in wound healing and immune responses.
The Different Classes of Myosin
“Myosin” refers to a large superfamily of motor proteins, not a single molecule. Scientists have identified numerous classes, categorized based on the amino acid sequence in their head domains, which reflects their evolutionary relationships.
Myosin II is often called conventional myosin. It is the class responsible for muscle contraction, where its molecules assemble into thick filaments. This class also drives the contractile ring during cytokinesis in non-muscle cells.
Other classes, termed unconventional myosins, perform a wider range of tasks. Myosin I is a smaller, single-headed myosin involved in connecting the actin cytoskeleton to cell membranes. It plays a part in membrane trafficking and maintaining the structure of cell surface projections like microvilli.
Myosin V is recognized as a processive cargo transporter. Unlike Myosin II, Myosin V can take many “steps” along an actin filament without detaching, making it an efficient motor for carrying vesicles and organelles over long distances. This specialization highlights how the myosin superfamily has evolved to meet diverse cellular needs.
Myosin’s Importance in Health and Disease
Because myosins are widespread, malfunctions in these motor proteins can lead to various human diseases. These conditions often arise from genetic mutations that alter the structure and function of a specific myosin protein, affecting the tissues where that myosin is most active.
A well-documented example involves mutations in the gene for cardiac myosin, a form of myosin II found in the heart muscle. These mutations are a leading cause of hypertrophic cardiomyopathy (HCM), a condition characterized by the abnormal thickening of the heart muscle. The altered myosin can lead to hypercontractility, where the heart muscle contracts too forcefully, causing the muscle cells to enlarge and leading to inefficient blood pumping.
Defects in unconventional myosins are linked to other genetic disorders. For instance, mutations in the MYO7A gene, which codes for Myosin VIIA, are responsible for a form of hereditary deafness and blindness known as Usher syndrome type 1B. Myosin VIIA is active in the sensory hair cells of the inner ear, where it helps maintain their intricate structure. When this myosin is defective, the hair cells cannot function properly, leading to hearing impairment.