X-ray evidence for two structural states of the actomyosin cross-bridge in muscle fibers.

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Abstract
Biochemical data, stiffness measurements and equatorial X-ray diffraction patterns provide evidence that actomyosin cross-bridges form in relaxed skinned rabbit fibers at low ionic strength (20 mM). The structure of these cross-bridges was examined by using 2-dimensional X-ray diffraction. In contrast to rigor cross-bridges, which significantly weaken the myosin-based reflections characteristic of relaxed fibers at 120 mM ionic strength (notably the 86-.ANG. and 108-.ANG. layer lines and the 72-.ANG. and 143-.ANG. meridionals), the formation of low ionic strength cross-bridges produced only small changes in these reflections. These cross-bridges did not produce the additional intensity on the 59-.ANG. actin-based layer line near the meridian that is associated with rigor cross-bridges. The formation of low ionic strength cross-bridges caused the 215-.ANG. meridional reflection to decrease in intensity, as is also the case when rigor cross-bridges are formed. The structure of the low ionic strength cross-bridge is significantly different from that of the rigor cross-bridge, and they raise the possibility that contractile force may be generated by a transition between these 2 actomyosin configurations.