Actin

Actin is a globular protein (G-actin) which is divided by a central cleft that binds ATP. An actin filament appears as two strands of subunits. One repeating unit consists of 28 subunits (14 in each strand) covering a distance of 72nm in length. The filament has a clockwise helical twist such that the filament has symmetry every 36nm. The ATP binding cleft always binds to the opposite side of the adjoining actin molecule. This gives the filament polarity with the actin binding cleft exposed at the minus end. Filamentous actin is called F-actin.