(i) Contrast transfer function (CTF) estimation. We estimated the microscope defocus using two different programs: Gctf (Zhang, 2016 ▸) and CTFFIND4 (Rohou & Grigorieff, 2015 ▸). We only selected those micrographs for which both estimates agreed up to 2.1 Å resolution (Marabini et al., 2015 ▸).