THE WORKING STROKE OF MYOSIN CROSSBRIDGES

Recent x-ray diffraction measurements of the axial periodicities of th e actin and myosin filaments in contracting muscles show that they are stretched by small but significant amounts by the developed tension, so that at least one half, possibly more, of the observed compliance o f a sarcomere must reside in the filaments themselves. This implies th at the movement steps of a crossbridge deduced from quick-release expe riments may be shorter than some previous estimates, necessitating at least two steps to account for current in vitro single-molecule measur ements. Intensity measurements of the wider angle x-ray reflections al so show some unexpected features.