Actin

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Actin filaments (polymerized from actin monomers) form the major protein constituent of the cytoskeleton of eukaryotic cells (for which they are known as microfilaments) and play a pivotal role in muscle contraction and cell movement. They form thin, flexible fibers and are approximately 7 nm in diameter. 

Actin exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin). G-actin is responsible for the formation of the actin filament, while F-actin forms the cytoskeleton and contractile apparatus of muscle cells.

Actin filaments organize into bundles or dynamic networks which play fundamental roles throughout different cellular processes. Actin bundles are cross-linked and closely packed into parallel arrays, while in actin networks, actin filaments are loosely cross-linked and form three-dimensional mesh works.

The main functions of actin filaments include:

• Forming a dynamic cytoskeleton to provide structural support to cells.
• Supporting and allowing cell motility.
• Supporting muscle contractions as actin filaments slide alongside myosin filaments. In muscle, actin molecules twist together to form a 'thin filament' which interdigitate with thick filament bundles of myosin (muscle protein). Together actin and myosin filaments are known as myofilaments.