Actomyosin

Actomyosin is a complex formed by actin filaments and myosin motor proteins that converts ATP energy into mechanical force. The interaction between actin and myosin underlies muscle contraction as well as numerous non-muscle processes that shape and move cells.

In muscle cells, myosin II forms thick filaments that interact with actin thin filaments organized into sarcomeres. According to the sliding filament theory, myosin heads bind to actin, perform a power stroke as they hydrolyze ATP, detach when a new ATP binds, and rebind to generate repetitive filament sliding, shortening the muscle. Calcium regulates this interaction through troponin and tropomyosin, which control access to myosin-binding sites on actin.

In non-muscle cells, actomyosin networks, especially myosin II–driven cortical networks and contractile rings, generate traction, tension, and contractility essential for cell shape changes, cytokinesis, and migration. Regulation occurs via phosphorylation of the myosin regulatory light chains by kinases such as MLCK in response to calcium, and through Rho-family GTPases that organize actin networks.

Actomyosin is thus a central driver of cellular mechanics, development, and tissue remodeling, with its activity tightly controlled in space and time.