Reconstruction of heterogeneous and small macromolecules by cyro-EM

项目总结 单粒子电子冷冻显微镜(Cryo-EM)最近加入了X射线结晶学和 核磁共振波谱作为生物大分子的高分辨率结构方法。在……里面 此外，低温电子显微镜可以产生单个分子的图像，因此有可能 解决构象变化。该提案旨在开发新的算法和软件，以 将冷冻-EM的应用扩展到太小或太灵活的分子 由现有的低温电磁计算工具绘制。此扩展需要解决两个 低温EM提出的最具挑战性的计算问题。 首先，绘制大分子的结构可变性被广泛认为是主要的 低温电磁中的计算挑战。结构变化对生物学家来说意义重大， 因为它们提供了对分子机器功能的洞察。现有的计算工具 仅限于少量不同的构象，因此无法处理 具有多个连续构象变化光谱的高流动性生物分子。第一 这个项目的研究领域是开发一个计算框架来分析 连续不断的变化。所提出的方法基于一种新的数学表示法 基于马尔可夫链蒙特卡罗的连续变化结构及其有效估计 (MCMC)算法。MCMC算法在许多其他科学领域都取得了巨大的成功 然而，在低温EM单粒子分析中，它们大多被忽视。 其次，目前低温电磁研究的一个主要限制因素是分子大小。小的图像 分子(~50 kDa以下)的信号太小，现有方法无法提供有效的3-D 重建。人们普遍认为，冷冻-EM不能用于太 体积小，可从显微照片中可靠地检测和挑选。挑战这种广为流传的信念， 第二个调查领域的重点是开发一种开创性的方法 直接从显微图像重建小分子，而不需要拾取粒子。新的 该方法基于自相关分析，完全绕过了粒子拾取和 方向分配，只需要对数据进行一次传递。单次通过法 为数据采集过程中的实时处理提供了新的可能性。

PROJECT SUMMARY Single-particle electron cryomicroscopy (cryo-EM) has recently joined X-ray crystallography and NMR spectroscopy as a high-resolution structural method for biological macromolecules. In addition, cryo-EM produces images of individual molecules, and therefore has the potential to resolve conformational changes. The proposal aims to develop new algorithms and software for extending the application of cryo-EM to molecules that are either too small or too flexible to be mapped by existing computational tools for cryo-EM. This extension requires solving two of the most challenging computational problems posed by cryo-EM. First, mapping the structural variability of macromolecules is widely recognized as the main computational challenge in cryo-EM. Structural variations are of great significance to biologists, as they provide insight into the functioning of molecular machines. Existing computational tools are limited to a small number of distinct conformations, and therefore are incapable of tackling highly mobile biomolecules with multiple, continuous spectra of conformational changes. The first area of investigation in this project is the development of a computational framework to analyze continuous variability. The proposed approach is based on a new mathematical representation of continuously changing structures and its efficient estimation using Markov chain Monte Carlo (MCMC) algorithms. MCMC algorithms have found great success in many other scientific disciplines, yet they have been mostly overlooked for cryo-EM single particle analysis. Second, a major limiting factor for present cryo-EM studies is the molecule size. Images of small molecules (below ~50kDa) have too little signal to allow existing methods to provide valid 3-D reconstructions. It is commonly believed that cryo-EM cannot be used for molecules that are too small to be reliably detected and picked from micrographs. Challenging that widespread belief, the second area of investigation focuses on developing a groundbreaking approach for reconstructing small molecules directly from micrographs without particle picking. The new approach is based on autocorrelation analysis and completely bypasses particle picking and orientation assignment and requires just one pass over the data. The single-pass approach opens new possibilities for real-time processing during data acquisition.