通过图像平移的优化来对准单个颗粒的冷冻电子显微镜电影
Alignment of cryo-EM movies of individual particles by optimization of image translations
摘要：
直接检测器装置（DDD）相机彻底改变了单粒子电子低温显微镜（冷冻电子显微镜）。除了改进的相机检测量子效率，DDD电影的采集允许校正的试样的运动，由于在显微镜试样阶段和电子束引起的运动的不稳定性。与样品台漂移，光束引起的运动并不总是均匀的图像内。附近粒子轨迹的局部相关性表明，束流诱导的运动是由于冰层的变形。已经描述了可以校正帧的大区域和> IMDa蛋白质颗粒的运动的算法。另一种算法允许估计单个< 1 MDa的蛋白质颗粒轨迹，但需要从帧计算滚动平均值并拟合颗粒的线性轨迹。在这里，我们描述了一种算法，该算法允许单个< 1 MDa粒子图像在没有帧平均或线性轨迹的情况下进行对齐。该算法最大化了移位帧与移位帧之和的总体相关性。在这个单一的目标函数的最佳有效地发现，利用分析计算的衍生物的功能。为了平滑粒子轨迹的估计，在目标函数中惩罚帧之间的粒子位置的快速变化，并且附近轨迹的加权平均确保轨迹中的局部相关性。这种单独的粒子运动校正，结合加权傅立叶分量以说明在后面的帧中增加的辐射损伤，可以用于改进来自单粒子冷冻电子显微镜的3-D图。
英文摘要：
Direct detector device (DDD) cameras have revolutionized single particle electron cryomicroscopy (cryo-EM). In addition to an improved camera detective quantum efficiency, acquisition of DDD movies allows for correction of movement of the specimen, due both to instabilities in the microscope specimen stage and electron beam-induced movement. Unlike specimen stage drift, beam-induced movement is not always homogeneous within an image. Local correlation in the trajectories of nearby particles suggests that beam-induced motion is due to deformation of the ice layer. Algorithms have already been described that can correct movement for large regions of frames and for > 1 MDa protein particles. Another algorithm allows individual < 1 MDa protein particle trajectories to be estimated, but requires rolling averages to be calculated from frames and fits linear trajectories for particles. Here we describe an algorithm that allows for individual < 1 MDa particle images to be aligned without frame averaging or linear trajectories. The algorithm maximizes the overall correlation of the shifted frames with the sum of the shifted frames. The optimum in this single objective function is found efficiently by making use of analytically calculated derivatives of the function. To smooth estimates of particle trajectories, rapid changes in particle positions between frames are penalized in the objective function and weighted averaging of nearby trajectories ensures local correlation in trajectories. This individual particle motion correction, in combination with weighting of Fourier components to account for increasing radiation damage in later frames, can be used to improve 3-D maps from single particle cryo-EM.
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