The thin filament protein that slides against myosin during muscle contraction is called actin. In skeletal and cardiac muscle, actin is arranged into thin filaments inside repeating contractile units called sarcomeres. Myosin forms the thick filaments and pulls on actin, causing the sarcomere to shorten.
Actin exists as individual globular subunits called G-actin and as the assembled filamentous form called F-actin. In a resting muscle, the regulatory proteins troponin and tropomyosin cover myosin-binding sites on actin. A nerve signal causes calcium ions to be released inside the muscle cell; calcium binds to troponin, shifting tropomyosin and exposing those sites.
Myosin heads then attach to actin and use energy from ATP to pull the thin filament toward the center of the sarcomere. The filaments themselves do not significantly shorten; they slide past one another. Actin is also widely used outside muscle, where it forms microfilaments involved in cell shape, movement, transport, and cell division. Elastin, collagen, and keratin are different structural proteins and do not serve as the muscle’s thin filament.