Algal Flagella

Abstract

Flagella are highly conserved organelles comprised of several hundred proteins that are assembled using the equally conserved mechanism called intraflagellar transport (IFT). The molecular motors dynein and kinesin drive IFT‐mediated flagellar assembly. In addition, flagellar movement is powered by the flagellar dynein motors. The outer and inner dynein arms function in generating flagellar beat frequency and flagellar waveform that are the two main features of flagellar bending. The biflagellate algae Chlamydomonas reinhardtii is a powerful model organism for the study of flagellar assembly and function. Recent advances have revealed that cilia and flagella play vital motile and sensory roles in the developing human embryo and in adult tissue function. Defects in assembly or function of mammalian cilia/flagella underlie an expanding set of human diseases and syndromes, collectively called ‘the ciliopathies’. Much of our current understanding of flagellar assembly, motility and the ciliopathies has come from studies in Chlamydomonas.

Key Concepts:

  • Cilia and flagella are highly conserved organelles.

  • Dynein motors drive flagellar movement.

  • Chlamydomonas reinhardtii is a powerful model experimental system for studying flagella.

  • Defects in assembly or function of cilia/flagella result in multiple pathologies called ‘the ciliopathies’.

Keywords: kinesin; dynein; microtubules; basal bodies; intraflagellar transport; ciliopathies

Figure 1.

Chlamydomonas and its organelles. Chlamydomonas is a unicellular green alga that is about 10 μm in length with a large cup‐shaped choloroplast (green) shown on electron micrograph of a cell. Inside the chloroplast is the pyrenoid (Brown and tan), which stores starch (tan) and the eyespot filled with cartenoid pigment granules (orange). There is a centrally placed nucleus (light blue) with a nucleolus (dark blue). Contractile vacuoles (gold) are found at the anterior end of the cell and mitochondria (yellow) are distributed throughout. At the anterior end is the basal body complex (pink) and the two motile, sensory flagella (red). The flagella emerge from a cell wall that encircles the cell (purple).

Figure 2.

Tracing of flagellar waveforms. (a) Wild‐type cells; (b) oda mutant cells that are lacking outer dynein arms show no obvious difference in flagellar waveforms; (c) ida mutant cells that are lacking the I1 or f dynein arms. The amplitude of the initial waveform is reduced resulting in a less effective waveform; and (d) sinusoidal waveform from backward swimming wild‐type cells.

Figure 3.

Flagellar assembly is mediated by intraflagellar transport (IFT). IFT is the mechanism by which flagella are generated, maintained and resorbed. Flagellar proteins are synthesised in the cell body and assembled into precursor complexes (yellow). These flagellar complexes then attach to the IFT machinery, which includes the IFT motors (kinesin in orange and dynein in red) and IFT particles (blue), for transport into the flagellum and subsequent assembly into the flagellar structure. IFT particles are then returned to the cell body by the dynein IFT motor along with flagellar turnover products (purple).

Figure 4.

Cross section of a Chlamydomonas axoneme. The axoneme shows the 9+2 arrangement of microtubules that is characteristic of motile flagella. The outer doublet microtubules consist of an A‐ and a B‐tubule. Attached to the A‐tubules are the outer dynein arms (ODA), the inner dynein arms (IDA), the radial spokes (RS) and the nexin‐dynein regulatory complex (N‐DRC), which extends to the neighbouring outer doublet. The central pair apparatus (CP) is shown with its two microtubules, C1 and C2, along with their associated projections. In addition, the Chlamydomonas axoneme contains a structure called the 5–6 bridge and B‐tubule projections found in the proximal part of the flagellum on outer doublets 1, 5 and 6.

Figure 5.

Longitudinal view of the 96 nm repeat. The diagram depicts the axonemal structures and densities that form the 96 nm repeat seen in longitudinal sections of the Chlamydomonas axoneme. At the top are the outer dynein arms (ODA) in blue. The heterogeneous nature of the inner dynein arm system (IDA) is most evident in longitudinal views. The I1 dynein (dynein f) is shown in red, whereas the other inner arm structures are shown in light brown. There are no known mutants for some of the inner arm dynein motors so their molecular composition and exact location is unknown. The two radial spokes, S1 and S2, are positioned just below I1 dynein and the N‐DRC (green), respectively. The N‐DRC forms a large complex that projects from the A‐tubule, where it is anchored, out towards the adjacent outer doublet microtubule.

Figure 6.

A diagram of an outer dynein arm that shows the arrangement of the different polypeptides within the outer dynein arm. The outer dynein arm consists of three DHCs (α, β and γ in yellow), two ICs (69 and 78 in blue) and at least 10 LCs (8, 11, 14, 16, 18, 19, 20 and 22 in pink). The small green spheres at the bottom of the figure represent the microtubule lattice. Reprinted with permission of The Rockefeller University Press. The Journal of Cell Biology, 1997, 137: 1069–1080.

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Further Reading

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How to Cite close
Wirschell, Maureen, and Dutcher, Susan K(Sep 2011) Algal Flagella. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000319.pub3]