Intraflagellar transport at a glance.

Intraflagellar transport (IFT) is the bidirectional transport of multisubunit protein complexes, called IFT particles, along axonemal microtubules (MTs) beneath the ciliary membrane. IFT plays essential roles in the assembly and function of cilia and flagella by contributing to cell motility, sensory perception and ciliumbased signaling (Rosenbaum and Witman, 2002; Scholey, 2003). IFT was first observed as the bidirectional movement of ‘particles’ along Chlamydomonas reinhardtii flagella (Kozminski et al., 1993). Subsequently, time-lapse fluorescence-microscopy assays in transgenic Caenorhabditis elegans allowed observations of the transport of specifically tagged IFT proteins along sensory cilia (Orozco et al., 1999). IFT is now understood to be highly conserved among a broad range of eukaryotes; mutations in IFT proteins in these organisms disrupt cilium biogenesis (Absalon et al., 2008; Avidor-Reiss et al., 2004; Blacque et al., 2008; Follit et al., 2006; Jekely and Arendt, 2006; Nachury et al., 2007; Rosenbaum and Witman, 2002; Scholey, 2003), although, in rare cases, flagella are assembled in the cytoplasm using IFTindependent mechanisms (Briggs et al., 2004; Han et al., 2003; Sarpal et al., 2003). Importantly, defects in IFT and ciliogenesis have been linked to various human diseases such as retinal degeneration, polycystic kidney disease, Bardet-Biedl syndrome (BBS), Jeune asphyxiating thoracic dystrophy, respiratory disease and defective determination of the left-right axis (Blacque and Leroux, 2006).