Poc1 stabilizes microtubules in the proximal region of the basal body. (A) Wild-type and poc1Δ BBs from whole cells expanded and imaged by U-ExM-SIM, stained with anti-acetylated tubulin antibody. Individual TMTs are observable in both wild-type and poc1Δ BBs. Wild-type BBs at 30 and 38°C generally maintain all nine TMTs and an overall circular shape (100 ±0% and 88.8 ±13.2%, respectively). poc1Δ BBs are mostly comprised of nine TMTs at 30°C (93.3% ±6.0% of BBs) but occasionally show loss of one or two TMTs. At 38°C, poc1Δ BBs lose uniform shape and tubulin signal is uneven, indicative of TMT loss (14.3 ±22.5% of BBs are normal). Scale bar is 500 nm. (B) Overall subtomogram average of poc1Δ TMTs in the proximal region shows loss of protofilaments. Left, poc1Δ TMT only (green). Right, poc1Δ overlaid with wild-type (gray). (C) Classification of poc1Δ TMTs focused on the B-tubule (demarcated by a dashed red circle) into six classes displaying different states of B-tubule disintegration. The most severe protofilament loss is at the luminal side of the triplet, consistent with Poc1’s binding location on the TMT. The numbers of subtomogram in the classes are indicated. (D) Classification of poc1Δ TMTs focused on the C-tubule (demarcated by a dashed red circle) into five classes displaying different states of C-tubule disintegration. Classes 1–3 are complete TMTs. The combined average of these classes was used for density loss comparison in Fig. 2 E and Fig. 4 B. The number of subtomogram in the classes are indicated.