X-ray evidence for two structural states of the actomyosin cross-bridge in muscle fibers.
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). In the present study we examined the structure of these cross-bridges by using two-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-A and 108-A layer lines and the 72-A and 143-A meridionals), the formation of low ionic strength cross-bridges produced only small changes in these reflections. In addition, these cross-bridges did not produce the additional intensity on the 59-A actin-based layer line near the meridian that is associated with rigor cross-bridges. However, the formation of low ionic strength cross-bridges caused the 215-A meridional reflection to decrease in intensity, as is also the case when rigor cross-bridges are formed. These observations show that 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 two actomyosin configurations.
D000205:Actomyosin / Q000378:metabolism*; D000818:Animals; D009132:Muscles / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D009994:Osmolar Concentration; D011487:Protein Conformation; D011817:Rabbits; D014961:X-Ray Diffraction
D000205:Actomyosin; D009218:Myosins