Figure 6. Structural features of the S.rosetta central pair complex.

(A–B) Tomographic slices at two different positions of the averaged S. rosetta CPC. The slice in (A) highlights CPC projections C1a, C2b, and the central bridge, whereas (B) highlights C1b-c, C2a, and C2c-e. Averages were generated using 1323 particles from 28 different tomograms (Resolution information in Figure 3—figure supplement 1 and Table 1). (C) Isosurface rendering of the averaged central pair complex; projection colors follow (Carbajal-González et al., 2013). Black lines and rotation arrows indicate the viewing directions of (D) and (E). (D–E) Isosurface renderings showing longitudinal side-views of the averaged S. rosetta CPC. Note: panel (D) is oriented with the distal side of the flagellum to the left, and proximal to the right (D and P, respectively). The orientation of the CPC in relation to the 5–6 bridge, vane, and barb structures is shown in Figure 6—figure supplement 1. Additional species comparisons are provided in Figure 6—figure supplement 2. Scale bar: 10 nm (B, applies also to A).

Figure 6—figure supplement 1. Relative orientation of CPC to DMTs and extra flagellar features.

(A–I) Tomographic slices through individual S. rosetta flagella shown in cross-section. The white dotted lines indicate the CPC plane (i.e. the plane that contains both CPC microtubules). DMTs are labeled (1-9), with the 5–6 bridge highlighted with a green ellipse. The cyan brackets indicate the position on the membrane where vane filaments protrude, and the red arrowheads indicate the position of barbs (within ~315 nm model depth). (J–K) The angle of the CPC (yellow) and vane (cyan) were measured relative to the ice layer (J) and plotted (K). Black horizontal lines indicate the median values. (L) Schematic representing the likely orientation of the CPC, 5–6 bridge (green ellipse), vane (cyan filaments), and barbs (red) in relation to the direction of flagellar motion (black arrows). Scale bar: 100 nm (C, applies to A-I; J).

Figure 6—figure supplement 2. Evolutionary comparison of the central pair complex between Chlamydomonas reinhardtii, choanoflagellate (S.rosetta), and sea urchin (S. purpuratus).

Isosurface renderings are viewed in cross-section from proximal (top row) and rotated to generate the longitudinal views as indicated. Black and white arrowheads represent the presence and absence of indicated structures, respectively. The Chlamydomonas and sea urchin (S. purpuratus) data were previously published with EM Data Bank accession codes EMD-20160 and EMD-9385, respectively (Carbajal-González et al., 2013; Fu et al., 2019).