Myosin is found in all cells in the human body but they are particularly abundant in muscle cells, where they are located in myofibrils, a major component of the contractile unit of a muscle cell. Myosin is a superfamily of proteins that play an important role in binding actin, hydrolyzingATP and transducing force. In muscle cells, myosin molecules bind to actin filaments, using energy from ATP to pull on actin, causing a muscle contraction.