Cryo-electron microscopy (cryo-EM) enables macromolecular structure determination in vitro and in situ. In addition to aligning individual particles, accurate registration of sample motion and 3D deformation during exposures is crucial for achieving high resolution. Here we describe M, a software tool that establishes a reference-based, multi-particle refinement framework for cryo-EM data and improves the results of structure determination. M provides a unified optimization framework for both in vitro frame series and in situ tomographic tilt series data. We show that tilt series data can provide the same resolution as frame series, indicating that the alignment step no longer limits the resolution obtainable from tomographic data. In combination with Warp and RELION, M improves upon previous methods, and resolves a 70S ribosome bound to an antibiotic inside bacterial cells at a nominal resolution of 3.7 Å. Thus, computational tools are now available to resolve structures from tomographic in situ cryo-EM data at residue level.Correspondence should be addressed to D.T. (dteguno@mpibpc.mpg.de), P.C. (patrick.cramer@mpibpc.mpg.de), and J.M. (julia.mahamid@embl.de).
Figure 1| Th e Warp-RELION-M pipeline for frame and tilt series cryo-EM data refi nement Electron microscopy data are pre-processed on-the-fl y in Warp, which then exports particles as images or sub-tomograms. Particles are imported in RELION, where they can be subjected to a multitude of processing strategies, resulting in 3D reference maps, global particle pose alignments, and class assignments. Th e particle population encompassing all classes is then imported in M, where reference-based frame or tilt image alignments are performed simultaneously with further refi nement of particle poses and CTF parameters to improve resolution. Finally, M produces high-resolution reconstructions that can be used for model building. Alternatively, the improved alignments can be used in Warp to re-export particles for further, more accurate classifi cation in RELION.