Actin filaments are the thinnest of the three components of the cytoskeleton in respect to its diameter (measuring 7-8nm in diameter in stained sections of muscle cells) and thus, makes it the most flexible. It is required for the movement on the surface of the cells and also to maintain the shape of the cell, through a hign concentration of actin filaments beneath the cell membrane known as the cell cortex. They are found in cells such as microvilli, filopodia as well as contractile bundles, the plasma membrane and also muscle cells. In some preparations they are arranged in rosettes around myosin filaments; however they are most often gathered into bundles or cables,which are many microns long and often merge or branch and terminate by penetrating into a dense body/dense band.
Actin filament is a polymer made by polymerisation of actin monomers by hydrolysis of ATP. The filament is polarised and the monomers are more readily added to the positive side of the filament. And polymerisation is also regulated by different set of binding proteins such as bundle protein, cross linked protein, motor proteins etc.
Actin is an abundant protein in typical animal cells and accounts for arround 5% of all proteins in the cell. Half of this can be found in filaments and the other half remains as monomers in the cytosol of the cell.
The actin filament's motor proteins are the myosin. Myosin have a head and a tail, the head binds to the actin filament and the tail is bound to plasma membrane, vesicles and other yosin proteins. The hydrolysis of ATP drives the myosin resulting in movement.
Also see actin