Crowther R A, Padrón R, Craig R
J Mol Biol. 1985 Aug 5;184(3):429-39. doi: 10.1016/0022-2836(85)90292-x.
Thick filaments from leg muscle of tarantula, maintained under relaxing conditions (Mg-ATP and EGTA), were negatively stained and photographed with minimal electron dose. Particles were selected for three-dimensional image reconstruction by general visual appearance and by the strength and symmetry of their optical diffraction patterns, the best of which extend to spacings of 1/5 nm-1. The helical symmetry is such that, on a given layer-line, Bessel function contributions of different orders start to overlap at fairly low resolution and must therefore be separated computationally by combining data from different views. Independent reconstructions agree well and show more detail than previous reconstructions of thick filaments from Limulus and scallop. The strongest feature is a set of four long-pitch right-handed helical ridges (pitch 4 X 43.5 nm) formed by the elongated myosin heads. The long-pitch helices are modulated to give ridges with an axial spacing of 14.5 nm, lying in planes roughly normal to the filament axis and running circumferentially. We suggest that the latter may be formed by the stacking of a subfragment 1 (S1) head from one myosin molecule on an S1 from an axially neighbouring molecule. Internal features in the map indicate an approximate local twofold axis relating the putative heads within a molecule. The heads appear to point in opposite directions along the filament axis and are located very close to the filament backbone. Thus, for the first time, the two heads of the myosin molecule appear to have been visualized in a native thick filament under relaxing conditions.
在松弛条件（Mg-ATP和EGTA）下保存的狼蛛腿部肌肉粗肌丝，经负染并以最小电子剂量进行拍照。通过总体视觉外观以及光学衍射图案的强度和对称性来选择用于三维图像重建的颗粒，其中最佳的颗粒延伸至1/5 nm - 1的间距。螺旋对称性使得在给定的层线上，不同阶的贝塞尔函数贡献在相当低的分辨率下开始重叠，因此必须通过组合来自不同视图的数据进行计算分离。独立重建结果吻合良好，并且比之前对鲎和扇贝粗肌丝的重建显示出更多细节。最显著的特征是由伸长的肌球蛋白头部形成的一组四个长螺距右手螺旋脊（螺距4×43.5 nm）。长螺距螺旋被调制以形成轴向间距为14.5 nm的脊，这些脊位于大致垂直于细丝轴的平面内并沿圆周延伸。我们认为，后者可能是由一个肌球蛋白分子的亚片段1（S1）头部堆叠在轴向相邻分子的S1头部上形成的。图谱中的内部特征表明分子内假定头部之间存在近似的局部二重轴。头部似乎沿细丝轴指向相反方向，并且非常靠近细丝主干。因此，首次在松弛条件下的天然粗肌丝中可视化了肌球蛋白分子的两个头部。
