Actin

Actin is a globular protein that assembles into long filaments called microfilaments, one of the three main parts of the cytoskeleton of eukaryotic cells. It is among the most abundant proteins in many cells, and it is involved in shape, movement and division.

Single actin molecules (G-actin) bind ATP and join to form two-stranded helical filaments (F-actin) about 7 nanometers across. The filaments have polarity, with a fast-growing barbed end and a slow-growing pointed end. Hydrolysis of ATP after assembly makes the older parts of the filament less stable, and filaments can treadmill, adding units at one end while losing them at the other.

A mesh of actin under the plasma membrane, the cortex, gives the cell its shape and supports the membrane. Growth of actin networks pushes out sheet-like extensions called lamellipodia and spikes called filopodia when cells crawl, and actin is also used for endocytosis. During cell division a contractile ring of actin and myosin pinches the cell in two.

In muscle, thin actin filaments slide past thick filaments of myosin to shorten the sarcomere. Toxins show its importance: phalloidin from the death cap mushroom locks filaments in place, while cytochalasins and latrunculin prevent assembly. Fluorescent versions of these compounds are used to image the cytoskeleton, and actin is found in all eukaryotes.

• Cytoskeleton — the network it belongs to
• Myosin — the motor that moves along it
• Cell Division — a process that depends on actin