Data processing and analyses of the CP. (A) Diagram of the subtomogram averaging workflow used in this study. (B) FSC curves of the tip CP and main CP structures. (C) Front and side views of Tetrahymena main CP (this study, dark salmon), sea urchin CP (EMD 9385, navy blue), and both overlaid (Tetrahymena CP is transparent). (D) Measured twist angles for WT main (blue bars) and tip (gray bars, transparent) CP. The dotted line is drawn at the highest point of the curve fitted to the histogram (green line for main CP, dark salmon line for tip CP). (E) Example of measurement of the distance between both CP microtubules along the length of a tomogram. The blue dots represent the coordinates of the center of a microtubule subtomogram. The red lines represent the shortest distance between two blue points. (F) Masks were used to refine the structure of the tip CP in this study. The resolution obtained after masking is indicated on the figure for each mask. (G) Fitting of a tubulin dimer in the electron density map of the subtomogram average of the tip CP. α-Tubulin is colored green and the β-tubulin is colored blue. For comparison, the same tubulin dimer was fitted in a map at 4 Å resolution, which was then lowpass filtered to 9 Å. (H) Local resolution map for C1 and C2 colored according to the scale bar on the left-hand side.