Microtubules flagellar

Cytoplasmic dyneins may have multiple roles in the neuron. The identification of kinesin as a plus-end directed microtubule motor suggested that it is involved in anterograde transport but left the identity of the retrograde motor an open question. Since flagellar dynein was known to be a minus-end-directed motor, interest in cytoplasmic dyneins was renewed. Identification of the cytoplasmic form of dynein in nervous tissue came as an indirect result of the discovery of kinesin. 

Summary of the Kinetic and Equilibrium Constants Characterizing Microtubule Elongation from Chlamydomonas Flagellar Axonemes"... 

The movement is processive, kinesin motors typically taking 100 steps before dissociating from the microtubule.201 2123 Kinesin is bound to the microtubule continuously. Its duty ratio is nearly 1.0 (the same is true for the bacterial flagellar motor ... 

When microtubules were visualized by electron microscopy (EM), after the improvement of methods of fixation, it was realized that they formed the structural basis of flagellar axonemes and of so-called spindle fibers, as well as occurring as individual filaments in the cytoplasm. Their designation as part of the cytoskeleton suggested that they acted mainly as fixed structural supports. Subsequent research has focused more and more on their dynamic behavior and on their role as tracks for motor proteins, which may, for example, transport chromosomes during cell division. Microtubules are found in all eukaryotic cells and are essential for many cellular functions, such as motility, morphogenesis, intracellular transport, and cell division. It is that dynamic behavior that allows microtubules to fulfill all of these functions in specific places and at appropriate times in the cell cycle. 

Figure 34.22. Microtubule Arrangement. Electron micrograph of a cross section of a flagellar axoneme shows nine microtubule doublets surrounding two singlets. [Courtesy of Dr. Joel Rosenbaum.]...

Ciliary and Flagellar Beating Are Produced by Controlled Sliding of Outer Doublet Microtubules... 

Figure 2. A three-dimensional reconstitution of the flagellar structure. Red, peripheral microtubule doublets orange, central microtubule singlets purple, nexin bridges blue, outer dynein arms yellow, inner dynein arms green, radial spokes. (Kindly provided by Dr Jacky Cosson, CNRS, France.]...

Fig. 4. Microtubules reconstituted from a Sarkosyl solution of subunits of flagellar outer fibers (from sperm tails of the sea urchin Strongyfocentrotus droebachiens/s). The arrow indicates an example of the microtubule fragments added to the subunit solution which apparently are necessary nucleation centers for ordered polymerization. Approximately X110,000. (From Stephens. 1969. Quant. Rev. Biophys., 377-390.)...

I. Neo-centric activity in chromosome arms at mitosis. Hereditas (Lund), 47 563-598. Barnicot, N. A. 1966. A note on the structure of spindle fibers. J. Cell Sci., 1 217-222. Barton, R. 1969. Investigation of negatively stained plant flagellar microtubules by optical diffraction. J. CeU Biol., 41 637-641. 

---