Multiple kinesins are found in eukaryotic cilia and flagella. Two of these are tightly associated with the microtubular central pair apparatus of motile 9+2 flagella. The function of the central pair kinesins is unknown, though it is hypothesized that they may contribute to the known rotation of the central pair apparatus within the axoneme. A third flagellar kinesin, Kinesin-2 (formerly kinesin-II or KRP85/95), is found in both motile 9+2 cilia and nonmotile 9+0 sensory cilia where it is associated with a transport process known as intraflagellar transport (IFT).
First identified in Chlamydomonas using video-enhanced DIC microscopy, IFT is characterized by the bidirectional movement of proteinaceous IFT particles along the length of the outer doublet microtubules. In Chlamydomonas, the anterograde movement of the IFT particles to the flagellar tip is powered by the plus end-directed FLA10 Kinesin-2 and proceeds at 2 um/sec while retrograde movement back to the cell bodies is powered by the minus end-directed cytoplasmic dynein 1b and proceeds at 3.5 um/sec. This is the first kinesin- and cytoplasmic dynein-powered transport to be described in which the cargo has been positively identified and in which mutants are available for both the anterograde and retrograde transport motors as well as in the cargo particle polypeptides. Anterograde ciliary IFT has also been directly confirmed in the nematode, C. elegans, using the green fluorescent protein (GFP) fused to Kinesin-2 and OSM6, one of the IFT particle subunits.
Click on the image to view a movie of the IFT process. (Quicktime format, 4256K)
A common function of Kinesin-2 mediated IFT has been identified in diverse organisms. The elimination of Kinesin-2 activity severely inhibits the assembly of motile and nonmotile (sensory neuronal) cilia in green algae, ciliated protozoa, nematodes, echinoderms and vertebrates. These results support the hypothesis that IFT functions to deliver axonemal precursors to the distal end of the organelle, which is the site of axonemal assembly. A second function of IFT may be to deliver or interact with membrane proteins distributed along the ciliary/flagellar membranes and the membranes of cilia-derived sensory neurons such as the rod outer segment of the vertebrate retina.
The Chlamydomonas IFT particles are composed of two large (~16 S) protein complexes, complex A (550 kD) and complex B (750 kD), consisting of 4 and 11 subunits, respectively. A role for IFT in Chlamydomonas flagellar assembly was recently confirmed when a null mutant in one of the complex B subunits was recently found to have either short or no flagella. A universal role of IFT in axonemal assembly is strongly supported by the fact that four IFT particle proteins found in C. elegans have been identified as either OSM or CHE mutants, all of which display assembly defects in their sensory-cilia neurons.
Contributed by D. Cole, W. Marshall, and J. Rosenbaum
Acknowledgements: M. Bernstein and K. Kozminski performed the experiments that led to the discovery of flagellar kinesins.
Referencees
Rosenbaum, J.L., D.G. Cole, and D.R. Diener. 1999. Intraflagellar transport: the eyes have it. J. Cell Biol. 144:385-388.
Cole, D.G., D.R. Diener, A. Himelblau, P.L. Beech, J.C. Fuster, and J.L. Rosenbaum. 1998. Chlamydomonas kinesin-II-dependent intraflagellar transport (IFT): IFT particles contain proteins required for ciliary assembly in Caenorhabditis elegans sensory neurons. J. Cell Biol. 141:993-1008.
Pazour, G.J., Wilkerson, C.G., and Witman, G.B. (1998). A dynein light chain is essential for the retrograde particle movement of intraflagellar transport (IFT). J. Cell Biol. 141:979-992.
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