myosin 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 so-called `thick filaments'; and a globular head, which has sites for the attachment of actin and ATP molecules. See sarcomere; sliding filament theory.

myosin The predominant protein of the myofibrils of muscle cells. It has an unusual shape for a protein, having a globular head and a rod-like tail.

myosin (my-oh-sin) n. the most abundant protein in muscle fibrils, having the important properties of elasticity and contractility. See striated muscle.