Conway J F, Steven A C
Laboratory of Structural Biology, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, Maryland 20892, USA.
J Struct Biol. 1999 Dec 1;128(1):106-18. doi: 10.1006/jsbi.1999.4168.
Recently the resolution attainable in density maps calculated from cryo-electron micrographs of free-standing virus capsids has advanced to resolutions below 1 nm. This represents a significant milestone in that resolutions of this order potentially allow direct visualization of individual elements of protein secondary structure (i.e., alpha-helices), in addition to the shapes and connectivity of subdomains. We describe here a computational strategy for structural analyses at this level of detail: its principal innovation is a procedure for correcting the contrast transfer function of the electron microscope. Also important is the practice of combining data from pairs of differently defocused micrographs to improve the signal-to-noise ratio of the images, thereby allowing more precise determinations of the particles' orientations and origins and contributing to higher resolution reconstructions. These procedures proved instrumental in our analysis of the capsid of hepatitis B virus at 9-A resolution (Conway et al., 1997, Nature 386, 91-94). Finally, we discuss the prospects for achieving comparable resolutions for isolated macromolecular complexes with lower symmetry or no symmetry and for further extension of the resolution.
最近，从独立病毒衣壳的冷冻电子显微照片计算得到的密度图中可达到的分辨率已提高到低于1纳米。这是一个重要的里程碑，因为这种分辨率有可能直接观察到蛋白质二级结构的单个元素（即α螺旋），以及亚结构域的形状和连接性。我们在此描述一种用于这种详细程度结构分析的计算策略：其主要创新是一种校正电子显微镜对比度传递函数的程序。同样重要的是将来自不同散焦显微照片对的数据进行组合以提高图像信噪比的做法，从而能够更精确地确定颗粒的取向和起源，并有助于进行更高分辨率的重建。这些程序在我们以9埃分辨率分析乙型肝炎病毒衣壳时发挥了重要作用（Conway等人，1997年，《自然》386卷，91 - 94页）。最后，我们讨论了对于对称性较低或无对称性的分离大分子复合物实现可比分辨率以及进一步提高分辨率的前景。
