Microtubules

(Lodish et al., 2000, Section 19.1 )

Microtubule structures are responsible for various cell movements:

Beating of cilia and flagella.
Transport of membrane vesicles in the cytoplasm.
Extension of surface membrane.
Alignment and separation of chromosomes during meiosis and mitosis.

Microtubules are made up of polymerized alpha and beta tubulin (Fig. 19.2a).

alpha tubulin binds GTP irreversibly.
beta tubulin hydrolyses GTP and exchanges GDP for GTP.
dimers of alpha and beta subunits bind head to tail (end to end) and associate 13 per single circumference of a microtubule (Fig. 19.2b).
protofilaments can fuse 10 subunits laterally to form doublet or triplet filaments (Fig. 19.3).

Short lived and dynamic microtubules.

cytosolic microtubule network -> spindle fibers during mitosis and miosis.

Stable microtubules.

axoneme in the flagellum.
axons in nerve cells.

Microtubules assemble from organizing centers and have a constant orientation relative to MTOCs.

Flagellum or cilium (Fig. 19.7a) Crossection (Fig. 19.28a).
Mitotic organizing center (Fig. 19.7b).
Nerve cell axon (Fig. 19.7c).
Vesicle transport (Golgi oriented MTOC).

Gamma-tubulin complex nucleates polymerization of butulin subunits (Fig. 19.8b).

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References:
Lodish, H., Berk, A., Zipursky, S.L., Matsudaira, P., Baltimore, D., Darnell, J., 2000, Molecular Cell Biology, 4th Ed., W.H. Freeman and Company, NY, New York. ISBN 0-7167-3136-3.