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Abstract

WE have attempted to establish a correlation between the binding of an ATP analogue, β, γ-imido-ATP (hereafter termed imido-ATP), and a structural change induced in muscle in the presence of this analogue. Imido-ATP binds tightly at the enzymatic site of myosin, but is not hydrolysed there1. When applied to glycerol-extracted muscle, it binds to the myosin in situ without causing the muscle to relax—that is, a ternary actin–myosin–nucleotide complex is formed2. Muscle fibres in this condition have the same mechanical stiffness as in rigor, but their zero tension point is displaced to a slightly greater muscle length, and both X-ray diffraction and electron microscopy indicate that the conformation of the attached myosin has altered, as if the head was rotated relative to the actin2–5.