DRC subunit localization and model of the DRC interactions. (A–J) Averaged repeats from WT (A, F, and H–J) and drc mutants (B–E) are visualized by isosurface rendering. (A–E) Bottom views of the DRC structure in WT (A), sup-pf-4 (B), sup-pf-3 (C), pf2 (D), and pf3 (E; coloring as in Fig. 4). The dashed white line represents the shape of the WT DRC for better comparison (C–E and G). (F–J) Proposed locations of the DRC subunits, as suggested by our WT mutant comparisons. We correlated previously published data on missing subunits (Huang et al., 1982; Piperno et al. 1994) with structural data on the presence or absence of specific DRC features in the different drc mutants (see Discussion section “Location of DRC subunits within the NDRC”). The suggested locations of DRC subunits are summarized and colored (see color legend in H) within the WT DRC structure (F and H–J), which is shown from the bottom (F), in cross section (H), and as and overview (I) and close-up (J) in a longitudinal front-bottom view. The WT mutant comparison also revealed DRC structures that could not be correlated to the published biochemical DRC components, suggesting that these densities are unknown DRC subunits (colored red in F–J and highlighted in G). Note that subunit DRC7 was not included in the model because of uncertainty about its presence or absence in different mutants. (K) Simplified model that summarizes the structural connections between the WT DRC with other axonemal structures, indicating potential regulatory interactions. The IDA includes IA2–6 and I1 dynein with IC–LC complex and 1-α- and 1-β-dyneins. A_t, A-tubule; BP, base plate protrusion; B_t, B-tubule; dL, distal lobe; L1, linker protrusion 1; pL, proximal lobe. Bar, 10 nm.