Actin is a protein found in all living things that contributes to the cytoskeleton and muscle contraction. The sliding filament theory explains how actin and myosin filaments slide past each other, shortening the sarcomere and causing muscle contraction. ATP is necessary for this process, and tropomyosin can prevent muscle contraction by blocking binding sites. The same mechanism is involved in cytokinesis during cell division.
Actin is a protein found within the cells of all living things whose cells contain a membrane-bound nucleus. Protein is a component of two types of cell filaments: microfilaments and fine filaments. Microfilaments contribute to the cytoskeleton, a structure within cell membranes that helps the cell maintain its shape. The thin filaments, found in muscle cells, are involved in muscle contraction.
The way actin filaments facilitate muscle contraction can be explained using the sliding filament theory. Within each muscle cell, these protein chains form thin passive filaments that work in conjunction with thick filaments of myosin, a motor or movement protein that produces the force of muscle contraction. To do this, myosin filaments slide back and forth along actin filaments within a unit within the muscle cell, called a sarcomere. Each muscle cell may contain hundreds of thousands of sarcomeres, a band-like structure that expands and contracts as a unit as actin and myosin filaments slide past each other. It is the bands of sarcomeres that give muscles their striated appearance.
Under the sliding filament pattern, myosin filaments alternate with actin filaments in horizontal lines, much like the red and white stripes on the American flag. Myosin proteins glide along actin, releasing calcium ions that allow the head of each myosin protein to bind to a site on the actin filament. Once myosin binds to these sites, much like a crew of rowers in a crew pulling on the oars at the same time, the myosin pulls the two strands past each other, resulting in 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 causes the muscle cell to contract.
Once the actin and myosin filaments bind and stroke occurs – pulling the actin filaments towards the center of the sarcomere – the myosin heads detach and ATP is reloaded into these filaments, thus causing the subsequent stroke of the filaments. The tropomyosin protein could cover the actin filaments and block the binding sites; this process would prevent myosin from binding and would result in relaxation of the muscles. This binding and sliding mechanism of myosin and actin filaments is also how cytokinesis, or cell division, occurs, with the action of the sliding filament causing a cell to be pinched in two during mitosis.
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