The Nucleation of Microtubules can occur Spontaneously In Vitro but Requires gTubulin In Vivo

The nucleation and polymerization of microtubules can occur spontaneously in vitro from pure tubulin in the presence of Mg2+ and GTP. However, in vivo, the process usually involves microtubule-organizing centers such as the centrosome of animal cells and the spindle pole body of yeasts, which result in a radial array of microtubules that are focused at their minus (slow growing) ends, with their fast-growing plus ends oriented outward (reviewed in [5]). Certain cell types employ other modes of producing microtubules. For example, in some animal cells containing centrosomes, non-centrosome-associated microtubules sometimes form [6]. In plants, microtubules do not grow out of one discrete site, but emanate from different points on the surface of the nucleus as well as on the cell cortex [7, 8]. Some meiotic cells also lack centrosomes yet produce microtubules [9].

The nucleating activity of the centrosome and spindle pole body is thought to depend on the tubulin isoform y-tubulin (see Chapter 2). A variety of genetic, antibody interference, and biochemical studies have implicated this protein as the major factor in microtubule nucleation (reviewed in [10]). Studies in animal cells of microtubules that did not originate from the centrosome found that the micro-tubules still grew from small cytoplasmic foci containing y-tubulin [6]. y-Tubulin's role is less clear in plants, but it most likely nucleates the unfocused microtubules that are distributed around the cell. Plant y-tubulin may also provide some other

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