Harford J, Luther P, Squire J
Biophysics Section, Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
J Mol Biol. 1994 Jun 17;239(4):500-12. doi: 10.1006/jmbi.1994.1391.
It has been known for many years that the vertebrate striated muscle A-bands and I-bands both contribute to the observed equatorial X-ray diffraction patterns. Despite this, the observed equatorial patterns, with the exception of the clearly distinct Z-reflection, have often been analysed as coming solely from the A-band, since it has not been possible to separate the observed intensity distribution into individual A-band and I-band contributions. Here we show, for the case of diffraction from the highly ordered muscles in bony fish, that it is possible to separate these contributions to the diffraction patterns from intact muscles and to compute separate electron density maps for the A and I-bands. Difference A-band density maps between resting and active muscles are distinctly altered when the I-band contribution is removed from the observed equatorial intensity. Results from resting and fully active fish muscle A-bands are compared and interpreted in terms of myosin crossbridge movement; the observations are consistent with specific crossbridge labelling of actin filaments, with a strong azimuthal component of crossbridge movement towards actin. From electron microscopy of freeze-substituted fish muscle, it is shown that the I-band X-ray diffraction pattern probably arises mainly from the thin filament arrangement immediately adjacent to the Z-band.
多年来人们已经知道，脊椎动物横纹肌的A带和I带都对观察到的赤道X射线衍射图案有贡献。尽管如此，除了清晰可辨的Z反射外，观察到的赤道图案通常被分析为仅来自A带，因为无法将观察到的强度分布分离为单独的A带和I带贡献。在这里，我们针对硬骨鱼高度有序肌肉的衍射情况表明，可以将这些对完整肌肉衍射图案的贡献分离出来，并计算A带和I带各自的电子密度图。当从观察到的赤道强度中去除I带贡献时，静息肌肉和活动肌肉之间的差异A带密度图会明显改变。比较并根据肌球蛋白横桥运动解释了静息和完全活动的鱼肌肉A带的结果；这些观察结果与肌动蛋白丝的特定横桥标记一致，横桥运动朝着肌动蛋白有很强的方位角分量。从冷冻替代鱼肌肉的电子显微镜观察可知，I带X射线衍射图案可能主要源于紧邻Z带的细肌丝排列。
