ATP binding and crossbridge structure in muscle

Thick filaments extracted from insect flight muscle were used in examining whether the dependence of actin-myosin crossbridge structure on nucleotide, generally presumed to underlie the power-stroke, is exhibited by myosin alone. The strongly periodic crossbridge arrangement seen in the presence of ATP (corresponding to relaxed muscle) is reversibly lost in conditions that induce rigor in intact muscle fibres. These observations suggest that the power-stroke may involve changes in the steric relation of the myosin head to the thick as well as to the thin filament.