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© The Rockefeller University Press,
0021-9525/1999//385 $5.00
The Journal of Cell Biology, Volume 144, Number 3,
, 1999 385-388
Mini-Reviews |
Intraflagellar Transport: The Eyes Have It
Abbreviations used in this paper: DIC, differential interference-contrast; IFT, intraflagellar transport; RIS, rod inner segment; ROS, rod outer segment.
Address correspondence to Joel L. Rosenbaum, Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520-8103. Tel.: (203) 432-3472. Fax: (203) 432-5059. E-mail: joel.rosenbaum{at}yale.edu
| The Problem: An Organelle whose Assembly Site Is Distant from the Site of Protein Synthesis |
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A dramatic example of the delivery of molecules into the flagellum is seen during flagellar regeneration in the biflagellate alga Chlamydomonas: flagella 10 µm long are assembled in
1 h. As the organelle elongates, flagellar precursors must reach the site of assembly at the distal tip (Rosenbaum and Child, 1967; Johnson and Rosenbaum, 1992), which grows farther and farther away from the site of protein synthesis. The site of tubulin addition during flagellar assembly was identified by fusing cells with half-length flagella to cells containing epitope-tagged tubulin: all the tagged tubulin incorporated into the growing flagella at their distal tips. When cells with full-length flagella lacking radial spokes were fused to wild-type cells, radial spokes from the wild-type cytoplasm entered the spokeless flagella, assembled at the distal tips of the flagella, and gradually continued assembly toward the base (Johnson and Rosenbaum, 1992). Similar results were obtained with inner dynein arms (Piperno et al., 1996). Thus, there appears to be a mechanism for transporting axonemal precursors to the distal tip of the flagellum, whether or not it is elongating.
Extensive transport also occurs into nonmotile sensory cilia of certain neuronal cells. The outer segments of retinal rods (ROS),1 for example, are modified cilia replete with the membrane-associated photoreceptor machinery for receiving and transducing light signals. These stacks of membranes are constantly being replenished: in mouse, the membranes of the ROS are completely replaced every 2 wk. The only connection between the ROS and the rod inner segment (RIS), in which synthesis of all the components occurs, is the connecting cilium composed of a membrane bound 9+0 axoneme. All the materials required for continual turnover of the ROS must pass through the connecting cilium.
The following briefly reviews recent literature that addresses how the cell rapidly mobilizes over 200 polypeptides required for flagellar assembly, and how it transports these polypeptides to the flagellar tip assembly site.
| The Solution: Intraflagellar Transport of Preassembled Flagellar Complexes |
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Intraflagellar transport (IFT) appears to be the mechanism that moves flagellar precursors to the flagellar tip. IFT, visualized with high-resolution video-enhanced differential interference-contrast (DIC) microscopy, is a motility located between the flagellar membrane and axoneme (Kozminski et al., 1993, 1995, 1998). Particles of variable size travel to the flagellar tip (anterograde transport) at 2.0 µm/s and smaller particles return from the tip to the base (retrograde transport) at 3.5 µm/s (Kozminski et al., 1993). The movement is continuous and linear along the entire flagella. IFT is neither affected by mutations that cause immotile flagella, e.g., mutations affecting dynein arms, radial spokes, or central pair microtubules, nor does it appear to be related to other motilities associated with the flagellar membrane, e.g., gliding of whole cells on surfaces by means of their outstretched flagella or the movement of polystyrene beads on the flagellar surface (Bloodgood, 1992; Kozminski et al., 1993). IFT continues unabated in cells in which gliding or bead movement has been blocked chemically (Kozminski et al., 1993) or genetically (Kozminski, 1995). All three motitities, however, are reversibly inhibited by increasing the osmolarity with NaCl (100 mM) or sucrose (6%) (Kozminski et al., 1993).
Thin sectioning and electron microscopy indicate that the material moving beneath the flagellar membrane by IFT consists of "lollipop-shaped" particles occurring in groups of varying particle numbers, called "rafts" (Fig. 1; Kozminski et al., 1993). The rafts, initially observed by Ringo (1967) during ultrastructural analysis of the Chlamydomonas flagellum, are attached by a thin connection to the B-subfibers of the outer-doublet microtubules and to the overlying flagellar membrane (Kozminski et al., 1993). Electron microscopy of IFT particles observed by DIC in the flagellum of a single embedded cell confirmed that the particles observed by DIC are, indeed, the rafts observed by electron microscopy (Kozminski et al., 1995).
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70% of the rafts observed by electron microscopy also disappear (Kozminski et al., 1995; Vashishtha et al., 1996). Together, these results indicate that (a) the IFT particles observed by DIC microscopy are the rafts observed in thin sections by electron microscopy; (b) the movement of the rafts is dependent on FLA10; and (c) the movement of the rafts, IFT, is essential for assembly and maintenance of the flagella. Sequence analysis reveals that FLA10 is most closely related to the kinesin-II subfamily of kinesins; some of these kinesins have been purified as heterotrimeric complexes consisting of two distinct, though related, motor subunits and one nonmotor subunit (Scholey, 1996). FLA10 is part of a similar heterotrimeric complex, FLA10 kinesin-II (Cole et al., 1998). As is typical for kinesins and their cargoes, FLA10 kinesin-II does not copurify with IFT particles; however, a small amount of FLA10 coprecipitates with immunoprecipitates of IFT particle polypeptides, suggesting a weak interaction between FLA10 kinesin-II and IFT particles (Cole et al., 1998).
The IFT particles were purified from flagellar extracts taking advantage of the fact that they are greatly decreased in flagella of fla10 cells maintained at the restrictive temperature. 15 polypeptides sedimenting at
16 S are reduced in flagella of fla10 cells incubated at 32°C (Piperno and Mead, 1997; Cole et al., 1998). These 15 polypeptides form two complexes: complex A, composed of 4 polypeptides; and complex B, composed of 11 polypeptides. Analysis of new mutants with defects in IFT have identified a fifth polypeptide in complex A (Piperno et al., 1998). Considering the size of the IFT particles seen in the electron microscope, there are probably multiple copies of the 16 S complexes in each single IFT particle, several of which compose the rafts.
Perhaps the strongest evidence that the 16 S particles are, indeed, the IFT particles observed in DIC, came from the work of Pazour et al. (1998). They isolated a Chlamydomonas mutant, fla14, that lacks LC8, a component of both flagellar (Piperno and Luck, 1979) and cytoplasmic dyneins (King et al., 1996). In fla14, kinesin-powered anterograde IFT is normal, but retrograde particle movement is missing (Pazour et al., 1998). The flagella of fla14 are immotile, approximately half length, and deficient in dynein arms and radial spokes. Most importantly, these flagella contain massive accumulations of the rafts, and, biochemically, contain 10–20-fold the amount of IFT particle polypeptides and FLA10 found in wild-type flagella (Pazour et al., 1998). Apparently, the rafts, composed of the 16 S particles, are brought into the flagella by the FLA10 kinesin-II, and accumulate because they cannot be moved out in the absence of retrograde IFT.
Although analysis of fla14 suggested that cytoplasmic dynein was the retrograde IFT motor, this interpretation was clouded by the fact that LC8 is also a component of myosin-V (Espindola, F.S., R.E. Cheney, S.M. King, D.M. Suter, and M.S. Mooseker. 1996. ASCB Meeting, San Francisco. Abstract 2160) and flagellar inner and outer arm dyneins (Piperno and Luck, 1979; Harrison et al., 1998). The importance of cytoplasmic dynein in IFT has now been more convincingly shown by mutations in DHC1b, a cytoplasmic dynein heavy chain. The effect of these mutations is similar to, though even more severe than, fla14: the flagella of these mutants, dhc1b, stfl-1, and stfl-2 are only 1–2 µm long and the space between the axoneme and the flagellar membrane is filled with rafts (Pazour et al., 1999; Porter et al., 1999). Therefore, IFT represents one of the only microtubule-based motility systems in which there are mutants in the motors responsible for both the anterograde and retrograde transport of defined, isolable particles.
| The Cellular Localization of IFT Particles and Motors |
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| IFT in Other Motile Cilia |
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| IFT in Nonmotile Sensory Cilia |
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Interestingly, similar phenotypes are found in two additional C. elegans mutants, osm3 and che3, which have defects in a subunit of heterotrimeric kinesin-II (Shakir et al., 1993; Tabish et al., 1995) and in the cytoplasmic dynein heavy chain DHC1b (Grant, W., personal communication), respectively. Thus, IFT particle proteins and the motors that move them, kinesin-II and cytoplasmic dynein DHC1b, are essential for the assembly of neuronal nonmotile sensory cilia of C. elegans.
IFT particles and their motors also appear to be present in vertebrate cilia-containing sensory neurons. Using antibodies to KIF3A, kinesin-II has been localized by fluorescence and immunogold electron microscopies in the connecting cilium between the RIS and ROS of fish retinal rod cells (Beech et al., 1996). Rafts similar to those observed in flagella of Chlamydomonas can also be seen in the connecting cilia in the vertebrate eye (Fig. 1; Sandborn, 1970). Mouse and human expressed sequence tags with homologies to IFT particle polypeptides have also been identified (Cole, D., unpublished observations), so IFT may play a similar role in mammals. Recently, targeted knockout of a kinesin-II (KIF3A) in the mouse retina resulted in degeneration of ROS, starting at the proximal portion where new material is added (Marszalek, J.R., X. Liu, E. Roberts, D. Chui, J. Marth, D.S. Williams, and L.S.B. Goldstein. 1998. ASCB Meeting, San Francisco. Abstract 756). Thus, kinesin-powered IFT is probably present in the connecting cilium between the RIS and the ROS of vertebrates and ROS assembly and maintenance is apparently dependent on IFT. The implications of this for studies of retinal pathologies leading to blindness are clear.
| Hypothesis: The Role of IFT in the Assembly and Maintenance of Flagella |
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Although the above discussion has emphasized the importance of IFT in transporting flagellar axonemal precursors into the flagella, it is important to note that (a) the IFT rafts are clearly associated with the flagellar membrane as well as the B tubule of the outer doublets; (b) axonemal components, e.g., radial spokes, dynein arms, and central pair microtubules, are absent from immotile sensory cilia of C. elegans and vertebrate photoreceptors; and (c) the first phenotype that appears (in
30 min) in fla10 cells at the restrictive temperature is a membrane defect: the cells lose the ability to mate (Piperno et al., 1996) by use of their flagella. It is known that the flagellar mating molecules must be moved onto the flagellar surface and activated before mating (Hunnicutt et al., 1990). Therefore, IFT is almost certainly involved in flagellar membrane maintenance and function.
IFT is required for the assembly and maintenance of Chlamydomonas flagella and probably functions in a similar manner in the formation of the ciliated sensory neurons in C. elegans and other higher organisms including vertebrates (Kozminski et al., 1995; Cole et al., 1998; J.R. Marszalek, X. Liu, E. Roberts, D. Chui, J. Marth, D.S. Williams, and L.S.B. Goldstein. 1998. ASCB Meeting, San Francisco. Abstract 756). Exactly how IFT functions in the assembly and maintenance of motile cilia and nonmotile sensory cilia and what roles the individual IFT particle polypeptides play in this process are not yet known.
| Acknowledgments |
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Submitted: 9 December 1998
Revised: 21 January 1999
Douglas G. Cole's current address is Department of Microbiology, Molecular Biology and Biochemistry, University of Idaho, Moscow, ID 83844-3052.
| References |
|---|
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|
|---|
Adams GMW, Huang B & Luck DJL. Temperature-sensitive, assembly-defective flagella mutants of Chlamydomonas reinhardtii. , Genetics, 1982, 100, 579–586.
Afzelius BA. A human syndrome caused by immotile cilia, Science, 1976, 193, 317–319.
Beech PL, Pagh-Roehl K, Noda Y, Hirokawa N, Burnside B & Rosenbaum JL. Localization of kinesin super family proteins to the connecting cilium of fish photoreceptors, J Cell Sci, 1996, 109, 889–897.[Abstract]
Bernstein M, Beech PL, Katz SG & Rosenbaum JL. A new kinesin-like protein (Klp1) localized to a single microtubule of the Chlamydomonasflagellum, J Cell Biol, 1994, 125, 1313–1326.
Bloodgood RA. Directed movements of ciliary and flagellar membrane components: a review, Biol Cell, 1992, 76, 291–301.[Medline]
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 eleganssensory neurons, J Cell Biol, 1998, 141, 993–1008.
Criswell PS, Ostrowski LE & Asai DJ. A novel cytoplasmic dynein heavy chain: expression of DHC1b in mammalian ciliated epithelial cells, J Cell Sci, 1996, 109, 1891–1898.[Abstract]
Dutcher SK. Flagellar assembly in two hundred and fifty easy-to-follow steps, Trends Genet, 1995, 11, 398–404.[Medline]
Fok AK, Wang H, Katayama A, Aihara MS & Allen RD. 22S axonemal dynein is preassembled and functional prior to being transported to and attached on the axonemes, Cell Motil Cytoskel, 1994, 29, 215–224.[Medline]
Fowkes ME & Mitchell DR. The role of preassembled cytoplasmic complexes in assembly of flagellar dynein subunits, Mol Biol Cell, 1998, 9, 2337–2347.
Fox LA, Sawin KE & Sale WS. Kinesin-related proteins in eukaryotic flagella, J Cell Sci, 1994, 107, 1545–1550.[Abstract]
Gibbons BH, Asai DJ, Tang WJ, Hays TS & Gibbons IR. Phylogeny and expression of axonemal and cytoplasmic dynein genes in sea urchins, Mol Biol Cell, 1994, 5, 57–70.[Abstract]
Goodenough UW & St. Clair HS. Bald-2: a mutant affecting the formation of doublet and triplet sets of microtubules in Chlamydomonas reinhardtii. , J Cell Biol, 1975, 66, 480–491.
Han JW, Park JH, Kim M & Lee J. mRNAs for microtubule proteins are specifically colocalized during the sequential formation of basal body, flagella, and cytoskeletal microtubules in the differentiation of Naegleria gruberi. , J Cell Biol, 1997, 137, 871–879.
Harrison A, Olds-Clarke P & King SM. Identification of the t complex–encoded cytoplasmic dynein light chain Tctex1in inner arm I1 supports the involvement of flagellar dyneins in meiotic drive, J Cell Biol, 1998, 140, 1137–1147.
Huang B, Rifkin MR & Luck DJL. Temperature-sensitive mutations affecting flagellar assembly and function in Chlamydomonas reinhardtii. , J Cell Biol, 1977, 72, 67–85.
Hunnicutt GR, Kosfiszer MG & Snell WJ. Cell body and flagellar agglutinins in Chlamydomonas reinhardtii: the cell body plasma membrane is a reservoir for agglutinins whose migration to the flagella is regulated by a functional barrier, J Cell Biol, 1990, 111, 1605–1616.
Johnson KA & Rosenbaum JL. Polarity of flagellar assembly in Chlamydomonas. , J Cell Biol, 1992, 119, 1605–1611.
Johnson KA, Haas MA & Rosenbaum JL. Localization of a kinesin-related protein to the central pair apparatus of the Chlamydomonas reinhardtiiflagellum, J Cell Sci, 1994, 107, 1551–1556.[Abstract]
King SM, Barbarese E, Dillman JF III, Patel-King RS, Carson JH & Pfister KK. Brain cytoplasmic and flagellar outer arm dyneins share a highly conserved Mr 8,000 light chain, J Biol Chem, 1996, 271, 19358–19366.
Kozminski, K.G. 1995. Beat-independent flagellar motilities in Chlamydomonas and an analysis of the function of alpha-tubulin acetylation. Ph.D. thesis. Department of Biology, Yale University, New Haven, CT. 151 pp.
Kozminski KG, Johnson KA, Forscher P & Rosenbaum JL. A motility in the eukaryotic flagellum unrelated to flagellar beating, Proc Natl Acad Sci USA, 1993, 90, 5519–5523.
Kozminski KG, Beech PL & Rosenbaum JL. The Chlamydomonaskinesin-like protein FLA10 is involved in motility associated with the flagellar membrane, J Cell Biol, 1995, 131, 1517–1527.
Kozminski, K.G., P. Forscher, and J.L. Rosenbaum. 1998. Three flagellar motilities in Chlamydomonas unrelated to flagellar beating. Cell Motil. Cytoskel. (Video Suppl. 5). 39:347–348.
Lux FG III & Dutcher SK. Genetic interactions at the FLA10 locus: suppressors and synthetic phenotypes that affect the cell cycle and flagellar function in Chlamydomonas reinhardtii. , Genetics, 1991, 128, 549–561.[Abstract]
Morris RL & Scholey JM. Heterotrimeric kinesin-II is required for the assembly of motile 9+2 ciliary axonemes on sea urchin embryos, J Cell Biol, 1997, 138, 1009–1022.
Nonaka S, Tanaka Y, Okada Y, Takeda S, Harada A, Kanai Y, Kido M & Hirokawa N. Randomization of right-left asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein, Cell, 1998, 95, 829–837.[Medline]
Pazour GJ, Wilkerson CG & Witman GB. A dynein light chain is essential for retrograde particle movement in intraflagellar transport (IFT), J Cell Biol, 1998, 141, 979–992.
Pazour GJ, Dickert BL & Witman GB. The DHC1b (DHC2) isoform of cytoplasmic dynein is required for flagellar assembly, J Cell Biol, 1999, 144, 473–481.
Perkins LA, Hedgecock EM, Thomson JN & Culotti JG. Mutant sensory cilia in the nematode Caenorhabditis elegans. , Dev Biol, 1986, 117, 456–487.[Medline]
Piperno G & Luck DJL. Axonemal adenosine triphosphatases from flagella of Chlamydomonas reinhardtii. Purification of two dyneins, J Biol Chem, 1979, 254, 3084–3090.
Piperno G & Mead K. Transport of a novel complex in the cytoplasmic matrix of Chlamydomonasflagella, Proc Natl Acad Sci USA, 1997, 94, 4457–4462.
Piperno G, Mead K & Henderson S. Inner dynein arms but not outer dynein arms require the activity of kinesin homologue protein KHP1FLA10 to reach the distal part of flagella in Chlamydomonas. , J Cell Biol, 1996, 133, 371–379.
Piperno G, Siuda E, Henderson S, Segil M, Vaananen H & Sassaroli M. Distinct mutants of retrograde intraflagellar transport (IFT) share similar morphological and molecular defects, J Cell Biol, 1998, 143, 1591–1601.
Porter, M.E., R. Bower, J.A. Knott, P. Byrd, and W. Dentler. 1999. Cytoplasmic dynein heavy chain 1b is required for flagellar assembly in Chlamydomonas. Mol. Biol. Cell. In press.
Ringo DL. Flagellar motion and fine structure of the flagellar apparatus in Chlamydomonas. , J Cell Biol, 1967, 33, 543–571.
Rosenbaum JL & Child FM. Flagellar regeneration in protozoan flagellates, J Cell Biol, 1967, 34, 345–364.
Sandborn, E.B. 1970. Cells and Tissues by Light and Electron Microscopy. Vol. 1. Academic Press, New York. 366 pp.
Scholey JM. Kinesin-II, a membrane traffic motor in axons, axonemes, and spindles, J Cell Biol, 1996, 133, 1–4.
Shakir MA, Fukushige T, Yasuda H, Miwa J & Siddiqui SS. C. elegans osm-3gene mediating osmotic avoidance behaviour encodes a kinesin-like protein, Neuroreport, 1993, 4, 891–894.[Medline]
Tabish M, Siddiqui ZK, Nishikawa K & Siddiqui SS. Exclusive expression of C. elegans osm-3kinesin gene in chemosensory neurons open to the external environment, J Mol Biol, 1995, 247, 377–389.[Medline]
Vashishtha M, Walther Z & Hall JL. The kinesin-homologous protein encoded by the Chlamydomonas FLA10gene is associated with basal bodies and centrioles, J Cell Sci, 1996, 109, 541–549.
Walther Z, Vashishtha M & Hall JL. The Chlamydomonas FLA10gene encodes a novel kinesin-homologous protein, J Cell Biol, 1994, 126, 175–188.
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