Making sense of cilia and flagella

Abstract

Data reported at an international meeting on the sensory and motile functions of cilia, including the primary cilium found on most cells in the human body, have thrust this organelle to the forefront of studies on the cell biology of human disease.

Figure 1.

Transmission electron micrographs of (left) a Chlamydomonas flagellum and (right) the connecting cilum from a vertebrate rod cell. IFT particles in the flagellum and connecting cilium are shown by arrows. Flagellum micrograph courtesy of Karl A. Johnson and vertebrate rod reprinted with permission from Elsevier (Sandborn, E.B. 1970. Cells and Tissues by Light and Electron Microscopy. Vol. 1. Academic Press, New York. 366 pp.).

Figure 2.

Diagram showing the main features of intraflagellar transport (IFT). IFT particles and the associated cargo required for flagellar assembly and maintenance gather around the basal body region at the base of the flagellum where they associate with each other for their trip past the transition zone (flagellar pore) and into the flagellum. They are transported by anterograde IFT to the flagellar tip, the axonemal assembly site, powered by the motor protein kinesin-2. Flagellar membrane proteins, components of vesicles that have budded from the Golgi and subsequently fused with the plasma membrane adjacent to the basal body, are picked up by IFT particles for their transport past the transition zone and onto the flagellar membrane. The membrane proteins are then moved within the plane of the ciliary membrane bilayer by IFT (Qin et al., 2005). At the flagellar tip, anterograde cargo is unloaded, turnover cargo is picked up, the kinesin-2 motor is inactivated for transport back to the cytoplasm, and cytoplasmic dynein 2, itself an anterograde cargo protein, is activated to power the retrograde trip back to the cytoplasm. Adapted from Rosenbaum and Witman [2002].