CRYO-EM MAP FILTERING

这个子项目是许多研究子项目中的一个 由NIH/NCRR资助的中心赠款提供的资源。子项目和 研究者（PI）可能从另一个NIH来源获得了主要资金， 因此可以在其他CRISP条目中表示。所列机构为 中心，不一定是研究者的机构。 众所周知，低温EM数据的傅立叶振幅不如相应的相位准确。通常，低分辨率振幅倾向于被过度加权，而高分辨率振幅倾向于被低估。Crowther的早期工作表明，使用高斯函数锐化图谱使B型肝炎衣壳核心的7.4图谱中的螺旋更加明显（B ttcher et al.，1997年）。在许多重建图中已经采用了类似的方法（例如（Zhang等人，2008年b））。然而，用于锐化这些图的实际B因子参数有些特殊。在EMAN过程中，当我们合并2-D中的数据以进行类平均时，我们应用维纳滤波器（Ludtke等人，2004年）。当对这些类平均值进行3-D重建时，由于在重建期间发生的额外平均，该维纳滤波器不再完全准确。这导致在后处理之前EMAN重建的适度过度过滤，即使所有的CTF参数已经被准确地确定。类似地，MPSA算法还包括当通过修改的EMAN 3D重建程序合并每个粒子图像时每个粒子图像的实验B因子的加权因子（Liu等人，2007年b）。为了补偿，我们通常在最终解释和发布之前对地图进行重新过滤（锐化）。我们正在评估不同的方法，如如何锐化地图，以带出隐藏的功能。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. It is well recognized that the Fourier amplitudes of cryo-EM data are not as accurate as the corresponding phases. In general, low resolution amplitudes tend to be overweighted and high resolution amplitudes tend to be underweighted. The early work of Crowther showed that sharpening a map using a Gaussian function made the helices more pronounced in their 7.4 ¿ map of the hepatitis B capsid core (B¿ttcher et al., 1997). A similar approach has been adopted in many of the reconstruction maps (e.g. (Zhang et al., 2008b)). However, the actual B factor parameter used for sharpening those maps was somewhat ad hoc. In the EMAN procedure, when we merge the data in 2-D to make class-averages, we apply a Wiener filter (Ludtke et al., 2004). When 3-D reconstruction of these class-averages is performed, this Wiener filter is no longer fully accurate due to the additional averaging that occurs during reconstruction. This results in a moderate overfiltration of EMAN reconstructions before post-processing, even if all of the CTF parameters have been determined accurately. Similarly, the MPSA algorithm also includes a weighting factor of the experimental B factor for each particle image when they are merged through a modified EMAN 3D reconstruction program (Liu et al., 2007b). To compensate, we generally perform a refiltering (sharpening) of the map prior to final interpretation and publication. We are assessing different approaches as how to sharpen the map to bring out the hidden features.