Figure 1.

Central pair structure. (Left) Diagrammatic representation of the multiple projections associated with the central pair apparatus; redrawn from Mitchell and Sale (1999). The C1 microtubule (left) is associated with two 18-nm-long projections (1a and 1b) and two smaller projections (1c and 1d). Projections 1b and 1d appear to be linked by a thin sheath. The C2 microtubule (right) is associated with two 8-nm-long projections (2a and 2b) and one smaller projection (2c). The C1 and C2 microtubules appear to be linked by a two-membered bridge and a diagonal link that extends from the C2 microtubule to the C1 microtubule near the base of the C1–1b projection. (Middle) Image average of central pair structure from wild-type (wt) axonemes (n = 32). (Right) Image average of central pair structure from pf6-2 axonemes (n = 27). Note the absence of the 1a projection on the C1 microtubule.