Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibres and regulate muscle contraction by regulating the binding of myosin. In resting muscle, tropomyosin overlays the myosin binding sites on actin, with a single tropomyosin molecule spanning seven actin subunits and is "locked" down in this position by troponin-T (tropomyosin binding troponin) and troponin-I (inhibitory troponin). Upon release of calcium from the sarcoplasmic reticulum, calcium binds to troponin-C (calcium binding troponin). This "unlocks" tropomyosin from actin, allowing it to move away from the binding groove. Myosin heads can now access the binding sites on actin. Once one myosin head binds, this fully displaces tropomyosin and allows additional myosin heads to bind, initiating muscle shortening and contraction. Once calcium is pumped out of the cytoplasm and calcium levels return to normal, tropomyosin again binds to actin, preventing myosin from binding.