Intraflagellar transport and the flagellar tip complex.

Intraflagellar transport (IFT) is the term that refers to the microtubule dependent particle motility that is common to almost all flagella and cilia and is distinct from the mechanism of flagellar beating. IFT involves the rapid, bi-directional transport of molecular motors and their cargo proteins from the base to the tip of the flagellum and back again. While the basic mechanism of IFT is well established, the varied functions of this process are continually being elucidated. For example, although IFT plays a clear role in flagellar assembly, disassembly and stability, the exact sequence of events that take place when tubulin subunit addition and loss occur during flagellar assembly and disassembly, respectively, are unknown. Key to furthering our understanding of IFT is greater knowledge of the flagellar tip complex (FTC) because it is at the FTC that flagellar assembly and disassembly, cargo loading and unloading, and motor protein regulation occur. Yet these related processes may only represent one aspect of the importance of IFT in flagellar dynamics. IFT may also provide the basic elements of a signal transduction mechanism that functions to provide the nucleus with information about the outside environment and even about the state of the flagellum itself. Thus, IFT may function as the central component of a signal transduction system that controls flagellar gene transcription.