Unlike most organelles, which are surrounded by cytoplasm, the flagellum protrudes from the cell surface extending tens or even hundreds of microns into the external medium. This elongated organelle must import all the macromolecules required for its assembly, maintenance, and function including >200 polypeptides that make up the microtubular axoneme (Dutcher, 1995), all the constituents of the flagellar membrane, as well as a prodigious amount of ATP to supply the thousands of dynein motors that drive flagellar motility.
A dramatic example of the delivery of molecules into the flagellum is seen during flagellar regeneration in the biflagellate alga Chlamydomonas: flagella 10 μm long are assembled in ∼1 h. As the organelle elongates, flagellar precursors must reach the site of assembly at the distal tip (Rosenbaum and Child, 1967; Johnson and Rosenbaum, 1992), which grows farther and farther away from the site of protein...
