Actin is a protein found inside the cells of all living things whose cells contain a membrane-bound nucleus. The protein is a component of two kinds of cell filaments: microfilaments and thin filaments. Microfilaments contribute to the cytoskeleton — a structure inside of cell membranes that helps the cell maintain its shape. Thin filaments, which are found in muscle cells, are involved in muscle contraction.
The way in which actin filaments facilitate muscle contraction can be explained by using the sliding filament theory. Within each muscle cell, these protein chains form passive thin filaments that work in conjunction with thick filaments of myosin — a motor or movement protein that produces the force of muscle contraction. To do so, the myosin filaments slide back and forth along the actin filaments within a unit inside the muscle cell, called the sarcomere. Each muscle cell can contain hundreds of thousands of sarcomeres — a band-like structure that expands and contracts as a unit as the actin and myosin filaments slide past each other. It is the bands of sarcomeres that give muscles their striated appearance.
Under the sliding filament model, myosin filaments are alternated with actin filaments in horizontal lines, much like the red and white stripes on the American flag. The myosin proteins slide along the actin, releasing calcium ions that allow the head of each myosin protein to bind to a site on the actin filament. Once the myosin binds to these sites — much like a crew of rowers in a scull pulling their oars simultaneously — the myosin pulls the two filaments past each other, resulting in an overall shortening of the sarcomere. This collective shortening is made possible by the hydrolysis of adenosine triphosphate (ATP) — the body's main energy source for many cellular functions — and results in the contraction of the muscle cell.
Once the actin and myosin filaments bind and the stroke occurs — pulling the actin filaments toward the center of the sarcomere — the myosin heads detach and the ATP is recharged in these filaments, then causing the next stroke of the filaments. The protein tropomyosin could cover the actin filaments and block the binding sites; this process would prevent myosin from binding and result in the relaxation of the muscles. This mechanism of myosin and actin filaments binding and sliding is also how cytokinesis, or cell division, takes place, with the sliding filament action resulting in the pinching off of one cell into two during mitosis.