Microtubules organizing center

MTs are stabilized at their minus ends by the centrosome (also called microtubule organizing center, MTOC). Centrosomes are protein complexes containing among other proteins two centrioles (ringlike structures) and a-tubulin. Centrosomes serve as nucleation points for microtubular polymerization and constrain the lattice structure of an MT to 13 protofila-... 

Microtubule-organizing centers (MTOCs) serve to nucleate, stabilize, and organize the arrangement of microtubules, both in vivo and in vitro (Brinkley, 1985 Joshi, 1994). Two large morphological groups of MTOCs are identified ... 

Brinkley, B.R. (1985). Microtubule organizing centers. Atm. Rev. Cell Biol. 1,145-172. 

Joshi, H.C. (1994). Microtubule organizing centers and y-tubulin. Curr. Opin. Cell Biol. 6, 55-62. 

Verlhac MH, de Pennart H, Maro B, Cobb MH, Clarke HJ 1993 MAP kinase becomes stably activated at metaphase and is associated with microtubule-organizing centers during meiotic maturation of mouse oocytes. Dev Biol 158 330-340 Verlhac MH, Kubiak JZ, Clarke HJ, Maro B 1994 Microtubule and chromatin behavior follow MAP kinase activity but not MPF activity during meiosis in mouse oocytes. Development 120 1017-1025... 

Tassin, A.-M., Maro, B. and Bornens, M. (1985) Fate of microtubule organizing centers during myogenesis in vitro. Journal of Cell Biology 100, 35-46. 

Only microtubules are seen in this fibroblast cell. They originate from the microtubule organizing center (centrosome) and radiate out as far as the plasma membrane. 

The reattachment of two supramolecular polymers, such as that observed when a high concentration of sheared microtubules is permitted to incubate in vitro. End-to-end annealing of microtubules is an unlikely process in living cells, because (a) the viscosity of the cytoplasm is apt to reduce greatly the tumbling of these polymers and (b) the so-called minus ends of the microtubules are usually firmly attached to microtubule-organizing centers. 

Microtubule assembly in cells differs in some ways from assembly in vitro. In cells, nucleation of microtubules requires a third type of tubulin, which is called y-tubulin, that functions in concert with other proteins in the form of a y-tubulin ring complex. In most animal cells, the y-tubuIin ring complex is located at the pericentriolar region of the microtubule organizing center (or centrosome) where it nucleates microtubule assembly at the minus ends (7). The y-tubulin does not become incorporated into the microtubule, but rather it only localizes to the minus ends. Assembly of tubulin to form microtubules during the early stages of polymerization in vitro can be considered a pseudo first-order reaction. A steady state is eventually attained in which both the soluble tubulin concentration and the microtubule polymer mass attain stable plateaus (8). The critical concentration at apparent equilibrium (actually a steady state, see below) is the concentration of soluble tubulin in apparent equilibrium with the microtubule polymers. 

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