Schmid M F, Matsudaira P, Jeng T W, Jakana J, Towns-Andrews E, Bordas J, Chiu W
Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, TX 77030.
J Mol Biol. 1991 Sep 20;221(2):711-25. doi: 10.1016/0022-2836(91)80082-6.
The acrosomal process of Limulus sperm contains a bundle of filaments composed of actin and a 102 kDa protein in a 1:1 molar ratio. The structure of the bundle in true discharge was investigated by electron cryomicroscopy, X-ray scattering and crystallographic image analysis. A bundle can be characterized as a quasi-crystal with continuously varying views along the bundle axis. Each segment of the bundle is found to obey the symmetry of space group P1, with a = b = 147 A, c = 762 A, alpha = 90 degrees, beta = 90.6 degrees, gamma = 120 degrees. A unit cell contains a helical repeat of the filament with a selection rule following that of an actin filament. A 24 A projection map based on the h0l view was reconstructed after averaging 5300 unit cells from six electron images. Filaments in this projection are well separated and clearly display a 21 screw symmetry. This screw symmetry results from the helical parameters of the bundle filament and is found to be a non-crystallographic symmetry element present in the unit cell. Our structural analysis has led to the proposal that the assembly of a stable bundle with a defined maximum diameter can be controlled by the crystallographic packing of the twisted filaments.
鲎精子的顶体突起包含一束由肌动蛋白和一种102 kDa蛋白质以1:1摩尔比组成的细丝。通过电子冷冻显微镜、X射线散射和晶体学图像分析研究了真实释放状态下束状结构。一束细丝可被表征为沿束轴具有连续变化视图的准晶体。发现束的每个片段遵循空间群P1的对称性，a = b = 147 Å，c = 762 Å，α = 90°，β = 90.6°，γ = 120°。一个晶胞包含细丝的螺旋重复，其选择规则遵循肌动蛋白细丝的规则。在对六个电子图像中的5300个晶胞进行平均后，重建了基于h0l视图的24 Å投影图。该投影图中的细丝分隔良好，清晰显示出21螺旋对称性。这种螺旋对称性源于束状细丝的螺旋参数，并且是晶胞中存在的非晶体学对称元素。我们的结构分析提出，具有确定最大直径的稳定束的组装可由扭曲细丝的晶体学堆积控制。
