Actin

Actin is a protein that is important in two types of filaments in biological cells:

1. Microfilaments - which are one of the three main components of the cytoskeleton, and
2. Thin Filaments - which are important parts of the contractile apparatus in muscle cells.

Actin is one of the protein molecules that (together with myosin, tropomyosin, and troponin) forms the cylindrical structures called myofibrils that, together, form muscle fibers.

Myofibrils consist of two types of protein filaments, called thin filaments, and thick filaments. Actin molecules, together with tropomyosin and actin molecules, form the thin filaments which have a helix structure - as shown in the diagram below.

Above: Diagram of Actin molecules within a Thin Filament.

Above: Diagram of Actin molecules within a Thin Filament.

Actin molecules are the points of attachment (on the thin filaments) for the myosin heads (which are part of the thick filaments).

Actin has an important combination of strength and sensitivity. Actin filaments are and continually constructed and disassembled as needed and directed for the correct function of the tissue of which actin is a part. This dynamic character of actin is controlled by a molecule of Adenosine Triphosphate (ATP) bound to each actin monomer. The state of the ATP determines the stability of the actin filament.