Comparison of the DRC structure between WT and drc mutants. (A–T) The WT DRC is displayed in yellow (A, F, and P; and pWT in K), and the mutant DRCs are shown in different colors and ordered from left to right by increasing size of structural changes: sup-pf-4 (B, G, L, and Q), sup-pf-3 (C, H, M, and R), pf2 (D, I, N, and S), and pf3 (E, J, O, and T). The averaged 96-nm repeats are visualized by isosurface rendering and are shown from four different viewing directions: longitudinal close-up view from the front (slightly rotated toward the bottom view; A–E), cross-sectional overview from distal end (F–J), and longitudinal close-up from the top (K–O) and bottom (P–T). Different parts of the DRC and associated structures are missing in the mutants. In sup-pf-4, the structural changes are small, with only the L2 protrusion (G and L) and most of the distal lobe (dL) missing (B and L). In all of the other mutants, both the proximal and distal lobes (pL and dL) and the L2 protrusion are missing; i.e., these strains have no DRC connection to the B-tubule (B_t) of the neighboring doublet (M–O). In addition, the DRC linker, including the OID linker (F and K, red arrowheads) to the ODAs, is reduced in length to varying degrees (compare H–J with M–O). In pf3, the entire DRC base plate (E, J, and T), including its protrusion (BP; J and T), connections 2 and 8 (see Fig. S2 J), and the hole (red arrow/dashed red circle) through the B11 structure (T), is missing. The base plate protrusion is also lacking in pf2 (I). The defects in the assembly of IDAs also increase from left to right; i.e., IA4 (A–E, left arrows) is reduced in sup-pf-3 (C) and pf2 (D) and missing in pf3 (E), whereas IA5 (right arrows) is reduced in pf2 (D) and missing in pf3 (E). A-tubule (A_t) includes protofilament A2; B-tubule includes filaments B10 and -11. The light green protrusion in A is an uncharacterized structure previously proposed as a candidate for the nexin link (Bui et al., 2008). L1, linker protrusion 1.