Actin

Microfilament

Actin is a contractile protein filament important for cell movements. It is expressed in all body cells, but especially in muscle cells. Actin can polymerize into microfilament, which are essential for the cytoskeleton, for cell motility, and for contraction of the cell during cell division.

Together with myosin filaments it forms Actomyosin, which provides the mechanism for muscle contraction. Actin uses ATP for energy. The ATP allows the myosin head to extend up and bind with the actin filament. This process is called hydrolisis. The myosin head then releases after moving the actin filament in a relaxing or contracting movement by usage of ADP.

Non-muscle actin is found in virtually every cell in every eukaryotic organism. It is important for cell shape, motility, and intercellular transport. In order to carry out such diverse functions, actin binds with hundreds of different actin binding proteins. Myosin is an example of an actin binding protein. Another example is villin, which can either cross-link actin into bundles or cut the actin filaments, depending on the calcium concentration in its environment.

Actin is also one of the most highly conserved proteins, differing by no more than 5% in species as diverse as algae and humans.