FIGURE 2:

FAP44, but not FAP43, is required for the assembly of the T/TH in Chlamydomonas. (A–L) Tomographic slices of averaged 96-nm-long axonemal repeats of wild-type Chlamydomonas reinhardtii (A–D) and its fap43 (E–H) and fap44 (I–L) mutants viewed in cross-sectional (A, B, E, F, I, and J) and longitudinal (C, D, G, H, K, and L) orientations. Blue lines indicate the locations of the slices in the respective panels. Note that fap43_C1 is a class average containing the majority (86%) of the axonemal repeats from the fap43 mutant (see also related Supplemental Figure S2 for classification analysis of the T/TH complex in fap43). Electron densities corresponding to tether (T, red arrowheads in A, E, C, and G) and tether head (TH, red arrowheads in B, D, F, and H) were observed in wild type and fap43_C1 but were missing in fap44 (white arrowheads in I–L). (M–R) Isosurface renderings show the three-dimensional structures of the averaged axonemal repeat of the wild type and fap43 and fap44 mutants in front (M, N, and O) and bottom (P, Q, and R) views. Black arrows in P, Q, and R indicate the connection between inner dynein arm d (IDA d, rose) and radial spoke 3 stand-in (RS3S, orange). Other labels: A_t and B_t, A- and B-tubule; a–e and g, inner dynein arm isoforms; ICLC (purple), intermediate and light chain complex; N-DRC, nexin dynein regulatory complex; ODA, outer dynein arm. Scale bar: 20 nm (valid for A–L).