Figure 2.
Figure 2. Views of the DRC in WT and a rescued drc mutant with WT phenotype (pWT). (A–F) Tomographic slices through averages of the 96-nm repeat obtained from WT (A–C) and a rescued pf2 mutant, pWT (D–F), showing the DRC in cross (A and D) and longitudinal sections (B, C, E, and F). The dashed red lines in A and D indicate the positions of the slices shown in the longitudinal views (B, C, E, and F); i.e., slices B and E are located close to the neighboring B-tubule (Bt; left), whereas slices C and F were taken closer to the A-tubule (At) of the doublet to which the DRC is attached. (G–K) 3D visualizations using isosurface rendering of averaged 96-nm repeats from WT (G and H) and pWT (I–K) in longitudinal front view (H and I), longitudinal bottom view (K), and cross-sectional view (G and J; for additional explanation of bottom and front views, see Fig. 1). (G–I) Overviews showing the general shape of the DRC in WT (G and H) and pWT (I) and its location within the 96-nm repeat in relation to other axonemal structures, including the two OID linkers (red arrowheads). Note that the IDAs just below the DRC are IA4 and -5. The light green–colored protrusion is an uncharacterized feature, which was previously proposed as a candidate for the nexin link (Bui et al., 2008). (J and K) Simplified surface-rendering visualization of the DRC (gold) represented by the pWT average, which has the highest resolution among our data. All structures except the DRC are either shown in gray transparency (J) or were removed (K). The DRC structures of WT and pWT are very similar (within the resolution limits) and share the same morphology, which consists of two parts, a base plate reaching from the B-tubule structure B11 to the A-tubule protofilament 4 and a linker that reaches from the A-tubule into the gap between neighboring MTDs. Please note that several different systems for numbering the MT protofilaments in the A- and B-tubules have been used (for review see Linck and Stephens, 2007). For consistency and clarity, we have chosen the system recently proposed by Linck and Stephens (2007). The linker ends in two lobes, the larger proximal lobe and the smaller distal lobe (dL). Four major protrusions can be distinguished: the base plate protrusion (BP), the OID linker (red arrowhead), and the two protrusions (L1 and L2) attached to the linker region. The only difference between WT and pWT is a small additional protrusion at the beginning of the base plate in pWT, which was not visible in WT or the other mutants; thus, it is not part of the common model of the DRC and was excluded from J and K. The inset explains the orientation of the DRC shown in K. Descriptions of the B11 structure have been controversial (Nicastro et al., 2006; Sui and Downing, 2006). Although we clearly observe density at this location, the structure appears thinner than a regular protofilament. The A-tubule is labeled as protofilaments 1–5; the B-tubule is labeled as protofilaments 8–10 and filament 11. Bar, 20 nm.

Views of the DRC in WT and a rescued drc mutant with WT phenotype (pWT). (A–F) Tomographic slices through averages of the 96-nm repeat obtained from WT (A–C) and a rescued pf2 mutant, pWT (D–F), showing the DRC in cross (A and D) and longitudinal sections (B, C, E, and F). The dashed red lines in A and D indicate the positions of the slices shown in the longitudinal views (B, C, E, and F); i.e., slices B and E are located close to the neighboring B-tubule (Bt; left), whereas slices C and F were taken closer to the A-tubule (At) of the doublet to which the DRC is attached. (G–K) 3D visualizations using isosurface rendering of averaged 96-nm repeats from WT (G and H) and pWT (I–K) in longitudinal front view (H and I), longitudinal bottom view (K), and cross-sectional view (G and J; for additional explanation of bottom and front views, see Fig. 1). (G–I) Overviews showing the general shape of the DRC in WT (G and H) and pWT (I) and its location within the 96-nm repeat in relation to other axonemal structures, including the two OID linkers (red arrowheads). Note that the IDAs just below the DRC are IA4 and -5. The light green–colored protrusion is an uncharacterized feature, which was previously proposed as a candidate for the nexin link (Bui et al., 2008). (J and K) Simplified surface-rendering visualization of the DRC (gold) represented by the pWT average, which has the highest resolution among our data. All structures except the DRC are either shown in gray transparency (J) or were removed (K). The DRC structures of WT and pWT are very similar (within the resolution limits) and share the same morphology, which consists of two parts, a base plate reaching from the B-tubule structure B11 to the A-tubule protofilament 4 and a linker that reaches from the A-tubule into the gap between neighboring MTDs. Please note that several different systems for numbering the MT protofilaments in the A- and B-tubules have been used (for review see Linck and Stephens, 2007). For consistency and clarity, we have chosen the system recently proposed by Linck and Stephens (2007). The linker ends in two lobes, the larger proximal lobe and the smaller distal lobe (dL). Four major protrusions can be distinguished: the base plate protrusion (BP), the OID linker (red arrowhead), and the two protrusions (L1 and L2) attached to the linker region. The only difference between WT and pWT is a small additional protrusion at the beginning of the base plate in pWT, which was not visible in WT or the other mutants; thus, it is not part of the common model of the DRC and was excluded from J and K. The inset explains the orientation of the DRC shown in K. Descriptions of the B11 structure have been controversial (Nicastro et al., 2006; Sui and Downing, 2006). Although we clearly observe density at this location, the structure appears thinner than a regular protofilament. The A-tubule is labeled as protofilaments 1–5; the B-tubule is labeled as protofilaments 8–10 and filament 11. Bar, 20 nm.

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