a, Representative cryo-micrographs for the TnsC•ATP heptamer at various defoci; the corresponding CTF is shown (top right), b, Representative reference-free 2D class averages with four types of views identified: top and bottom views, a lateral side view and intermediate views. The size of circular masks used during the classification run is indicated in the first class average for each type. c, Image processing workflow employed for the identification and high-resolution refinement of a homogenous class of particles for the TnsC•ATP heptamer. One of the seven TnsC•ATP monomers exhibited weaker density (orange asterisk/arrow).