Helical reconstruction in RELION

We describe a new implementation for the reconstruction of helical assemblies in the empirical Bayesian framework of RELION. Our approach calculates optimal linear filters for the 3D reconstruction by embedding helical symmetry operators in Fourier-space, and deals with deviations from perfect helical symmetry through Gaussian-shaped priors on the orientations of individual segments. By incorporating our approach into the standard pipeline for single-particle analysis in RELION, our implementation aims to be easily accessible for non-experienced users. Although our implementation does not solve the problem that grossly incorrect structures can be obtained when the wrong helical symmetry is imposed, we show for four different test cases that it is capable of reconstructing structures to near-atomic resolution. Abbreviations CARD Caspase Activation and Recruitment Domain EM Electron Microscopy FSC Fourier Shell Correlation IHRSR Iterative Helical Real-Space Reconstruction MAVS Mitochondrial Antiviral Signaling Protein TMV Tobacco Mosaic Virus