Intraflagellar transport

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. 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.

IFT was first reported in 1993 by graduate student Keith Kozminski while working in the lab of Dr. Joel Rosenbaum at Yale University. 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.

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

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.

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