FIGURE 1.

Summary of previously proposed locations of Ribbons of PFs. PFs (black) and tektin fibrils/filament (green) are shown. A, A-tubule; B, B-tubule; IJ, inner junction; PFs numbered according to Ref. 58. The approximate location and shape of the partition-associated material (stippled) is redrawn from Ref. 59 and likely contains the MIP4 density (11). a, the 3-PF Ribbon was originally suggested to arise from the PFs forming the partition between the A- and B-tubules (12). b–e, given the unknown structure of the A-tubule/B-tubule junctions prior to tannic acid fixation (60) and the lack of three-dimensional information about the origin of these PFs, it was uncertain whether the stable Ribbon corresponded to PFs A10-11-12 (data not shown) or to PFs A11-12-13 (b), or PFs A12-13-1 (c), or whether there were two kinds of Ribbons. Several observations pointed to a different or second location of Ribbon PFs closer to the inner dynein arms (see “Discussion”), e.g. PFs A13-1-2 (d) or PFs A1–3 (e). f–h, different locations have also been proposed for tektins, i.e. f, the IJ structure or thin fibrils either between PFs or in the grooves between PFs (15, 27, 32, 61), or g, the partition material on the A-lumen side of tubulin PFs A11-12-13-1 (redrawn from Ref. 62), or h, one of the PFs of the Ribbon (15, 44, 63).