Microtubules MTOCs

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 ... 

MTOCs containing highly regular, parallel arrays of microtubules, as seen in centrioles and basal bodies. 

Radial arrays are best exemplified by mitotic half-spindles, which have a central MTOC, the centrosome. The centrosome consists of two centrioles (which are homologous with the basal body) surrounded by dense pericentriolar material (Kalt and Schliwa, 1993). In plant cells, the MTOC of the mitotic spindle consists of dense material only without centrioles. The plus ends of microtubules of the mitotic spindle are directed toward the equator of the cell. Some are free, and others attach to kinetochores on chromatids (see Figure 4). 

The microtubule network of the inicrphase cell is depolvmerized and reformed to make the mitotic spindle. This requires two MTOCs. the first being the original centromere with its pair of centrioles. 

The second arises from replication of the centrioles during the S and G2 phases of interphase. During prophase, the two pairs of centrioles move apart from one another and the pericentriolar material surrounding them both can nucleate and stabilize microtubules. These two MTOCs become the spindle poles. 

Like microfilaments and Intermediate filaments, microtubules are not randomly distributed in cells. Rather, microtubules radiate from the centrosome, which is the primary microtubule-organizing center (MTOC) In animal cells (Fig ure 5-33). As detailed In Chapter 20, the two ends of a microtubule differ in their dynamic properties and are commonly designated as the (-h) and (—) ends. For this reason, microtubles can have two distinct orientations relative to one another and to other cell structures. In many nondividing animal cells, the MTOC is located at the center of the cell near the nucleus, and the radiating microtubules are all oriented with their (-f) ends directed toward the cell periphery. Although most interphase animal cells contain a single perinuclear MTOC, epithelial cells and plant cells contain hundreds of MTOCs. Both of these cell types exhibit distinct... 

A FIGURE 5-33 Fluorescence micrograph of a Chinese hamster ovary cell stained to reveal microtubles and the MTOC. The microtubules (green), detected with an antibody to tubulin, are seen to radiate from a central point, the microtubuleorganizing center (MTOC), near the nucleus. The MTOC (yellow) is detected with an antibody to a protein localized to the centrosome. [Courtesy of R. Kuriyame.]... 

A FIGURE 20-1 Microtubules (blue) organized around the MTOC and spindle poles ( ) establish an internal polarity to movements and structures in the interphase cell (/eft) and the mitotic cell right). Assembly and disassembly (H) cause microtubules to probe the cell cytoplasm and are harnessed at mitosis to move chromosomes. Long-distance movement of vesicles (B and El) are powered by kinesin and dynein motors. Both motors are critical in the assembly of the spindle and the separation of chromosomes in mitosis. 

Colchicine and a synthetic relative, colcemid, have long been used as mitotic inhibitors. In cells exposed to high concentrations of colcemid, cytosolic microtubules depolymer-ize, leaving an MTOC. However, when plant or animal cells are exposed to low concentrations of colcemid, the microtubules remain and the cells become blocked at meta-... 

MTOCs Orient Most Microtubules and Determine Cell Polarity... 

In an interphase fibroblast cell, cytosolic microtubules are arranged in a distinctive hub-and-spoke array that lies at the center of a cell (Figure 2 0-13a). The microtubule spokes radiate from a central site occupied by the centrosome, which is the primary microtubule-organizing center in many interphase cells. We will use the term MTOC to refer to any of the structures used by cells to nucleate and organize microtubules. In animal cells, the MTOC is usually a centrosome, a collection of microtubule-associated proteins that sometimes but not always contains a pair of centrioles (Figure... 

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