A procedure for building protein chains into maps produced by single-particle electron cryo-microscopy (cryo-EM) is described. The procedure is similar to the way an experienced structural biologist might analyze a map, focusing first on secondary structure elements such as helices and sheets, then varying the contour level to identify connections between these elements. Since the high density in a map typically follows the main-chain of the protein, the main-chain connection between secondary structure elements can often be identified as the unbranched path between them with the highest minimum value along the path. This chain-tracing procedure is then combined with finding side-chain positions based on the presence of density extending away from the main path of the chain, allowing generation of a Cα model. The Cα model is converted to an all-atom model and is refined against the map. We show that this procedure is as effective as other existing methods for interpretation of cryo-EM maps and that it is considerably faster and produces models with fewer chain breaks than our previous methods that were based on approaches developed for crystallographic maps.

中文翻译：

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低温电磁图解释和使用迭代图分割的蛋白质模型构建。

描述了将蛋白质链构建到由单粒子电子冷冻显微镜（cryo-EM）生成的图谱中的过程。该过程类似于经验丰富的结构生物学家分析地图的方式，首先关注二级结构元素（例如螺旋和薄片），然后更改轮廓级别以识别这些元素之间的连接。由于图谱中的高密度通常遵循蛋白质的主链，因此二级结构元素之间的主链连接通常可以被识别为它们之间的无支链路径，且沿着该路径具有最高的最小值。然后将此链跟踪过程与基于远离链主路径延伸的密度的存在找到侧链位置相结合，从而生成Cα模型。Cα模型将转换为全原子模型，并根据图进行精化。我们表明，该程序与其他现有的解释低温电磁图方法一样有效，并且比基于结晶图方法开发的以前的方法要快得多，并且生成的链断裂更少。