Actin

Definition

Actin filaments (microfilament) are a major structural component of the cellular cytoskeleton. The monomeric globular form (G-actin) polymerizes to form long helical filaments (F-actin), 7–9 nm in diameter. All subunits are oriented in the same direction resulting in a structural polarity where the ends of the filament are different. The structural polarity has important functional implications where the barbed end (+ end) of the filament has a faster rate of growth than the pointed (-) end. Actin is also the name of one of the two contractile proteins implicated in muscle contraction. Actin (sometimes also referred to as the thin filament) consists of two chains of serially linked actin globules that are wrapped around each other in a helical fashion. Actin also contains tropomyosin, a long fibrous protein that lies in the groove formed by the actin chains and three sub-units of troponin, troponin T, I and C. Tropomyosin and troponin are regulatory proteins associated with...