Myosin is a motor protein found across all forms of life. It converts chemical energy into mechanical force, enabling a wide array of cellular and tissue-level movements. Myosin’s actions are widespread, underpinning processes from precise movements within single cells to coordinated contractions of large muscle groups.
The Fundamental Components of Myosin
A typical myosin molecule, such as Myosin II prominent in muscle cells, is structured as a hexamer composed of two identical heavy chains and two pairs of light chains: essential and regulatory. Each heavy chain features three distinct regions: the globular head, the neck, and the elongated tail.
The globular head, or motor domain, is located at the N-terminal end of the heavy chain. This domain contains sites for binding both ATP and actin filaments. The binding and hydrolysis of ATP within this head region provide the energy for myosin’s mechanical work.
Connected to the head is the neck domain, which acts as a lever arm. The essential light chains stabilize this neck region, while the regulatory light chains help in the movement of the globular heads. This domain amplifies the small movements occurring within the head during its chemical cycle.
The C-terminal tail, or rod domain, is an alpha-helical structure that extends from the neck. In Myosin II, the two heavy chain tails form a coiled-coil structure, allowing the molecules to dimerize. This tail region is also responsible for forming larger filament structures important in muscle contraction.
How Myosin’s Structure Powers Cellular Movement
Myosin’s structure enables movement through a cyclical process known as the “power stroke.” This mechanism involves a series of interactions between the myosin head and actin filaments, driven by ATP hydrolysis. The process begins with the myosin head strongly bound to an actin filament.
When an ATP molecule binds to the myosin head, it causes the head to detach from the actin filament. The ATP is then hydrolyzed into ADP and an inorganic phosphate (Pi), leading to a conformational change in the myosin head, which “cocks” it into a high-energy state. In this state, the myosin head reattaches to the actin filament at a new position further along the filament.
The release of the inorganic phosphate from the myosin head triggers the power stroke. This causes the neck region, acting as a lever arm, to swing, pulling the actin filament along. ADP is then released, and the myosin head remains strongly bound to the actin filament, ready for another cycle upon ATP binding. This cycle of attachment, pivoting, and detachment generates the force and movement seen in processes like muscle contraction, where many myosin molecules work in concert to slide actin filaments.
Diverse Myosin Forms and Roles
Myosin is not a singular protein but belongs to a large superfamily with diverse classes, categorized by variations in their tail regions. While all myosins share a conserved motor head domain that interacts with actin and ATP, differences in their tail domains dictate their varied cellular functions beyond muscle contraction. There are at least 24 recognized classes of myosins, with several, including Myosin I, Myosin II, Myosin V, Myosin VI, Myosin VII, and Myosin X, being well-characterized.
For instance, Myosin I is a monomeric protein with a shorter tail that can bind to lipid membranes, playing a role in intracellular transport of molecules and vesicles. It is also found in the microvilli of intestinal cells, contributing to their structure. Myosin V, another class, is known for its role in transporting organelles and vesicles within the cell by moving along actin filaments.
Myosin II, besides its role in muscle contraction, is also involved in forming contractile rings during cell division (cytokinesis). Other myosins contribute to cell polarization, signal transduction, and maintaining cell shape. This structural diversity allows the myosin superfamily to perform a wide array of tasks within eukaryotic cells.