The protein actin is abundant in all eukaryotic cells. It was first discovered in skeletal muscle, where actin filaments slide along filaments of another protein called myosin to make the cells contract. (In nonmuscle cells, actin filaments are less organized and myosin is much less prominent.) Single actin monomers are called G-actin, these monomers polymerize into long filaments called F-actin which are about 7nm in diameter. Actin filaments are made up of identical actin proteins arranged in a long spiral chain. Like microtubules, actin filaments have plus and minus ends, with more ATP-powered growth occurring at a filament's plus end. Once the filament has been synthesized it folds into a U-shape containing a binding site for ATP or ADP in the middle.
Actin-binding proteins interact with the actin filaments or monomers and regulate their assembly and organisation, they can convert actin from one form to another. These proteins include: severing proteins, filament capping proteins, crosslinking proteins, anchoring proteins, filament bundling proteins and monomer binding proteins.
In many types of cells, networks of actin filaments are found beneath the cell cortex, which is the meshwork of membrane-associated proteins that supports and strengthens the plasma membrane. Such networks allow cells to hold - and move - specialized shapes, such as the brush border of microvilli. Actin filaments are also involved in cytokinesis and cell movement due to their flexible and dynamic nature. For example. crawling cells have structures called lamellipodia and filopodia; polymerisation of the actin at the plus end extends the lamellipodium, this puts stress under the unpolymerized actin in the cortex causing retraction at the tail of the cell. This enables the cell to move along a surface.