protein - any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes; "a diet high in protein"

actin - one of the proteins into which actomyosin can be split; can exist in either a globular or a fibrous form

myosin - the commonest protein in muscle; a globulin that combines with actin to form actomyosin