Coupling between the enzymatic site of myosin and the mechanical output of muscle

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When Mg-imido-ATP binds to the active site of myosin in glycerol-extracted insect muscle fibres it causes a rapid, fully reversible and stress-independent increase in rest length of 2 nm per half sarcomere, whilst the isotonic stiffness remains within 2% of the rigor value. Mg-ADP and H-imido-ATP bind but have less mechanical effect and also less effect on the equatorial X-ray diffraction pattern. Coupling of binding to length change is quantitatively reversible, since stress of 200 nm per fibre doubles the amount of either Mg-imido-ATP or Mg-ATP bound at low concentrations but has no effect on Mg-ADP or H-imido-ATP binding. Linkage between the nucleotide and actin binding sites is shown by studies with subfragment-1 in solution. By fluorometric titration the nucleotide affinity for subfragment-1 was shown to increase in the order of H-imido-ATP, Mg-ADP and Mg-imido-ATP, while by a sedimentation technique the affinity of actin for subfragment-1-nucleotide was shown to decrease in the same order. The data are interpreted in terms of a simple cross-bridge model and their relevance to the mechanism of muscle contraction is discussed.