Myosin is a contractile protein that interacts with actin to bring about contraction of muscle or cell movement. The type of myosin molecule found in muscle fibres consists of a tail, by which it aggregates with other myosin molecules to form socalled thick filaments and a globular head, which has sites for the attachment of actin and ATP molecule. Troponin, tropomyosin and $\alpha$-actinin are the actin in the thin filament.