Tilt-series were filtered according to the cumulative radiation dose (Grant and Grigorieff, 2015) and aligned on the basis of the patch tracking algorithm using the IMOD ETOMO package (Kremer et al., 1996). Tomograms were reconstructed from aligned stacks using weighted back-projection in IMOD. Tomograms were further 4x binned, resulting in pixel sizes of 21.8 Å. Tomograms were denoised by edge enhancing diffusion (bnad command in Bsoft (Heymann et al., 2008)).