Intraflagellar Transport or IFT refers to the cellular process essential for the formation and maintenance of eukaryotic cilia and flagella. IFT, first discovered in 1993 by graduate student Keith Kozminski while working in the lab of Dr. Joel Rosenbaum ^* at Yale University, is phylogenically well-conserved and seems to be present in the cilia and flagella of all species.

Biochemistry

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IFT describes the bi-directional movement of non-membrane-bound particles along the doublet microtubles of the flagellar axoneme, between the axoneme and the plasma membrane. Studies have shown that the movement of IFT particles (polypeptides consisting of 16-18 subunits) along the microtubule is carried out by two different microtubule-based motors; the anterograde (towards the flagellar tip) motor is kinesin-2, and the retrograde (towards the cell body) motor is cytoplasmic dynein 1b. IFT particles carry axonemal subunits to the site of assebly at the tip of the axoneme. Thus, IFT is necessary for axonemal growth; furthermore, due to the constant turnover of proteins within the axoneme, an axoneme with defective IFT machinery will slowly shrink in the absence of replacement subunits. In healthy flagella, IFT particle reverse direction at the tip of the axoneme, and are thought to carry these protein turnover products back to the base of the organelle. For a time-lapse microscopic Quicktime movie and schematic cartoon of IFT, see "Rosenbaum Lab IFT webpage" ^*

The IFT particles themselves consist of two sub-complexes, A and B, separable via sucrose centrifugation (complex A sediments at approximately 16S, but under increased ionic strength complex B sediments more slowly, thus segregating the two complexes). The many subunits of the IFT complexes have been named according to their molecular weights; complex A contains IFT144, 140, 139, and 122, while complex B contains IFT 172, 88, 81, 80, 74/72, 57/55, 52, 46, 27, and 20. The biochemical properties and biological functions of these IFT subunits are just beginning to be elucidated.

Physiological Importance

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Due to the importance of IFT in maintaining functional cilia, defective IFT machinery has now been implicated in many disease phenotypes generally associated with non-functional (or absent) cilia. IFT88, for example, encodes a protein known as Tg737 in mouse and human, and the loss of this protein has been found to be the cause of autosomal-recessive polycystic kidney disease. Other conditions such as retinal degeneration and loss of left-right body axis determination are also strongly tied to improper ciliary function. Thus, human diseases such as situs inversus (a reversal of the body's left-right axis), Senior-Loken syndrome, Jeune syndrome, and Bardet-Biedl syndrome, which cause both cystic kidneys and retinal degeneration, may be much better understood on the cellular level as a malfunction of IFT.

One of the most recent discoveries regarding IFT is its potential role in signal transduction. IFT has been shown to be necessary for the movement of other signaling proteins within the cilia, and therefore may play a role in many different signalling pathways. Specifically, IFT has been implicated as a mediator of Sonic Hedgehog signaling, one of the most important pathways in embryonic development.

References

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"Intraflagellar Transport"; Joel L. Rosenbaum and George B. Witman, Nature Reviews, 2002

"Chlamydomonas Kinesin-II-dependent Intraflagellar Transport (IFT): IFT Particles Contain Proteins Required for Ciliary Assembly in Caenorhabditis elegans Sensory Neurons"; Douglas G. Cole et al, The Journal of Cell Biology, 1998

"Intraflagellar Transport is Required for the Vectorial Movement of TRPV Channels in the Ciliary Membrane"; Hongmin Qin et al, Current Biology, 2005

"Gli2 and Gli3 Localize to Cilia and Require the Intraflagellar Transport Protein Polaris for Processing and Function"; Courtney J. Haycraft et al, 2005

Cell biology