Myosin is not a microfilament; they are distinct components that work together to generate force and movement within a cell. Microfilaments are structural elements of the cell’s internal skeleton, while myosin is a motor protein that moves along them. The confusion arises because this pair forms the fundamental machinery for processes like muscle contraction and cell division. Understanding their individual structures and roles is necessary to grasp how they cooperate in cellular movement.
The Structure of Microfilaments
Microfilaments, also known as actin filaments, are the thinnest components of the cytoskeleton, the internal scaffolding that gives a cell its shape and stability. They are long, flexible polymers composed primarily of the globular protein G-actin. These G-actin molecules link end-to-end to form long strands called F-actin (filamentous actin). Two F-actin strands then twist around each other in a helical orientation to form the complete microfilament structure, which has a diameter of about 7 nanometers.
This polymerization process is dynamic, allowing microfilaments to rapidly assemble and disassemble. This enables the cell to quickly change its shape or move. Their primary function is to provide tensile strength, resisting tension and crushing forces. They are also involved in creating cellular extensions and play a significant role in cell division by forming the contractile ring.
Myosin: The Molecular Motor
Myosin is a family of molecular motor proteins, fundamentally different from structural microfilaments. Its primary role is to convert chemical energy, stored in adenosine triphosphate (ATP), into mechanical force and movement. This energy conversion classifies myosin as an ATPase enzyme, which hydrolyzes ATP to power its function.
A typical myosin molecule, such as Myosin II, has a distinct structure consisting of an elongated tail, a flexible neck, and a globular head. The tail domain is often involved in forming thick filaments or attaching to other cellular structures. The neck acts as a lever arm, while the globular head is the active site. The head contains the pocket that binds to the microfilament (actin) and the site for ATP hydrolysis, allowing myosin to “walk” along the microfilament and generate movement.
Working Together: The Sliding Filament Mechanism
The mechanism that joins the microfilament and myosin into a functional unit is the sliding filament theory, best known for explaining muscle contraction. This process relies on myosin heads repeatedly binding to, pulling on, and detaching from the actin microfilaments. The microfilaments themselves do not shorten; instead, the myosin motor pulls the actin filaments past the myosin filaments, causing the entire structure, such as a muscle sarcomere, to shorten.
The process begins when a myosin head, having hydrolyzed ATP into adenosine diphosphate (ADP) and inorganic phosphate, attaches to an exposed binding site on the actin microfilament, forming a cross-bridge. The release of the phosphate group initiates the “power stroke,” a change in the myosin head’s shape that pulls the actin filament toward the center. A new ATP molecule must then bind to the myosin head to cause it to detach from the microfilament, ending the cross-bridge.
This cycle—attach, pull, detach—repeats continuously as long as ATP is available and a contraction signal is present. While highly organized in muscle cells, this interaction also powers non-muscle cell activities. For instance, the actin-myosin interaction is responsible for the pinching action that divides a cell during cytokinesis and allows white blood cells to crawl across surfaces.