Intraflagellar transport

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Intraflagellar transport in the cilia of the nematode C. elegans

Intraflagellar transport or IFT is a bidirectional motility along axonemal microtubules that is essential for the formation (ciliogenesis) and maintenance of most eukaryotic cilia and flagella.[1] It is thought to be required to build all cilia that assemble within a membrane projection from the cell surface. Plasmodium falciparum cilia and the sperm flagella of Drosophila are examples of cilia that assemble in the cytoplasm and do not require IFT. The process of IFT involves movement of large protein complexes called IFT particles or trains from the cell body to the ciliary tip and followed by their return to the cell body. The outward or anterograde movement is powered by kinesin-2 while the inward or retrograde movement is powered by cytoplasmic dynein 2/1b. The IFT particles are composed of about 20 proteins organized in two subcomplexes called complex A and B.[2]

IFT was first reported in 1993 by graduate student Keith Kozminski while working in the lab of Dr. Joel Rosenbaum at Yale University.[3][4] The process of IFT has been best characterized in the biflagellate alga Chlamydomonas reinhardtii as well as the sensory cilia of the nematode Caenorhabditis elegans.[5]

It has been suggested based on localization studies that IFT proteins also function outside of cilia.[6]

Biochemistry[edit]

A simplified model of intraflagellar transport.

IFT describes the bi-directional movement of non-membrane-bound particles along the doublet microtubules of the flagellar axoneme, between the axoneme and the plasma membrane. Studies have shown that the movement of IFT particles along the microtubule is carried out by two different microtubule-based motors; the anterograde (towards the flagellar tip) motor is heterotrimeric kinesin-2, and the retrograde (towards the cell body) motor is cytoplasmic dynein 1b. IFT particles carry axonemal subunits to the site of assembly at the tip of the axoneme; thus, IFT is necessary for axonemal growth. Therefore, since the axoneme needs a continually fresh supply of proteins, an axoneme with defective IFT machinery will slowly shrink in the absence of replacement protein subunits. In healthy flagella, IFT particles reverse direction at the tip of the axoneme, and are thought to carry used proteins, or "turnover products," back to the base of the flagellum.[7][8]

The IFT particles themselves consist of two sub-complexes,[9] each made up of several individual IFT proteins. The two complexes, known as 'A' and 'B,' are separable via sucrose centrifugation (both complexes 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:

The biochemical properties and biological functions of these IFT subunits are just beginning to be elucidated, for example they interact with components of the basal body like CEP170 or proteins which are required for cilium formation like tubulin chaperone and membrane proteins.[10]

Physiological importance[edit]

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 cause an autosomal-recessive polycystic kidney disease model phenotype in mice. Other human diseases such as retinal degeneration, situs inversus (a reversal of the body's left-right axis), Senior-Loken syndrome, liver disease, primary ciliary dyskinesia, nephronophthisis, Alstrom syndrome, Meckel-Gruber syndrome, Sensenbrenner syndrome, Jeune syndrome, and Bardet-Biedl syndrome, which causes both cystic kidneys and retinal degeneration, have been linked to the IFT machinery. This diverse group of genetic syndromes and genetic diseases are now understood to arise due to malfunctioning cilia, and the term "ciliopathy" is now used to indicate their common origin.[11] These and possibly many more disorders may be better understood via study of IFT.[12]

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 signaling pathways. Specifically, IFT has been implicated as a mediator of Sonic Hedgehog signaling, one of the most important pathways in embryogenesis.[13]

References[edit]

  1. ^ [1]
  2. ^ [2], Cole DG, Diener DR, Himelblau AL, Beech PL, Fuster JC, Rosenbaum JL. Chlamydomonas kinesin-II-dependent intraflagellar transport (IFT): IFT particles contain proteins required for ciliary assembly in Caenorhabditis elegans sensory neurons. J Cell Biol. 1998 May 18;141(4):993-1008.
  3. ^ Bhogaraju, S.; Taschner, M.; Morawetz, M.; Basquin, C.; Lorentzen, E. (2011). "Crystal structure of the intraflagellar transport complex 25/27". The EMBO Journal 30 (10): 1907–1918. doi:10.1038/emboj.2011.110. PMC 3098482. PMID 21505417.  edit
  4. ^ Kozminski, KG; Johnson KA; Forscher P; Rosenbaum JL. (1993). "A motility in the eukaryotic flagellum unrelated to flagellar beating". Proc Natl Acad Sci U S A. 90: 5519–23. doi:10.1073/pnas.90.12.5519. PMC 46752. PMID 8516294. 
  5. ^ Orozco, JT; Wedaman KP; Signor D; Brown H; Rose L; Scholey JM (1999). "Movement of motor and cargo along cilia". Nature 398 (6729): 674. doi:10.1038/19448. PMID 10227290. 
  6. ^ Sedmak T, Wolfrum U (April 2010). "Intraflagellar transport molecules in ciliary and nonciliary cells of the retina". J. Cell Biol. 189 (1): 171–86. doi:10.1083/jcb.200911095. PMC 2854383. PMID 20368623. 
  7. ^ Rosenbaum, JL; Witman GB (2002). "Intraflagellar Transport". Nat Rev Mol Cell Biol 3: 813–25. doi:10.1038/nrm952. PMID 12415299. 
  8. ^ Scholey, JM (2008). "Intraflagellar transport motors in cilia: moving along the cell's antenna". Journal of Cell Biology 180 (1): 23–29. doi:10.1083/jcb.200709133. PMC 2213603. PMID 18180368. 
  9. ^ Lucker BF, Behal RH, Qin H, et al. (July 2005). "Characterization of the intraflagellar transport complex B core: direct interaction of the IFT81 and IFT74/72 subunits". J. Biol. Chem. 280 (30): 27688–96. doi:10.1074/jbc.M505062200. PMID 15955805. 
  10. ^ Lamla S (2009). "Functional characterisation of the centrosomal protein Cep170". Dissertation. LMU Muenchen: Fakultät für Biologie. 
  11. ^ Badano, Jose L.; Norimasa Mitsuma; Phil L. Beales; Nicholas Katsanis (September 2006). "The Ciliopathies : An Emerging Class of Human Genetic Disorders". Annual Review of Genomics and Human Genetics 7: 125–148. doi:10.1146/annurev.genom.7.080505.115610. PMID 16722803. 
  12. ^ Rosenbaum, JL; Witman GB (2002). "Intraflagellar Transport". Nat Rev Mol Cell Biol 3: 813–25. doi:10.1038/nrm952. PMID 12415299. 
  13. ^ Haycraft CJ, Zhang Q, Song B, et al. (January 2007). "Intraflagellar transport is essential for endochondral bone formation". Development 134 (2): 307–16. doi:10.1242/dev.02732. PMID 17166921. 

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