The structure of the Tetrahymena basal body. (A) Cartoon diagram of a Tetrahymena cell with BBs in black on the left. On the right, a negative stain image of a single BB isolated for cryoET showing that BB appendage structures are maintained. Scale bar is 400 nm. (B) Cartoon of the BB showing the three regions for which structures were determined: Proximal, Core, and Distal. The proximal region lumen harbors the cartwheel and the core contains the inner scaffold, indicated in the cartoon. (C–E) CryoET structure of the Tetrahymena BB in the proximal (C), core (D), and distal (E) regions. Left: cross-sectional view of a BB with the TMT subtomogram average mapped onto the TMTs in ice. Note that in the proximal and core regions, the BB is circular but the distal end is flattened. This is likely due to the force from freezing the BB on the EM grid in ice and a lack of an interconnecting structure at the distal end. Middle: Cross-sectional view of the TMT subtomogram average from each region. For the core region, this is the subclass of complete TMTs. For a view of the structure without classification, see Fig. S2 C and Fig. S4 A. For the distal region, no complete TMTs were observed. Right: luminal side view of the TMT subtomogram average structure. Arrowheads point to features of the TMTs: green, A/C linkers; cyan, pinhead; magenta, A–B inner junction; yellow, B–C inner junction; lavender, inner scaffold.