Microtubule formation can be traced to hundreds of tubulin rings that form an integral part of the MTOC (Fig. 2.42). The and -tubulin dimers are added to a -tubulin ring in an end-to-end fashion (see Fig. 2.40). Polymerization of tubulin dimers requires the presence of guanosine triphosphate (GTP) and Mg2. Each tubulin molecule binds GTP before it is incorporated into the forming microtubule. The GTP–tubulin complex is then polymerized, and at some point GTP is hydrolyzed to guanosine diphosphate (GDP). As a result of this polymerization pattern, microtubules are polar because all of the dimers have the same orientation. Each microtubule possesses a minus (nongrowing) end that corresponds to -tubulin; in the cell, it is usually embedded in the MTOC. The plus (growing) end of microtubules corresponds to -tubulin and extends the cell periphery. Tubulin dimers dissociate from microtubules in the steady state, which adds a pool of free tubulin dimers to the cytoplasm. This pool is in equilibrium with the polymerized tubulin in the micro-tubules; therefore, polymerization and depolymerization are in equilibrium. The equilibrium can be shifted in the direction of depolymerization by exposing the cell or isolated microtubules to low temperatures or high pressure. Repeated exposure to alternating low and high temperature is the basis of the purification technique for tubulin and microtubules. The speed of polymerization or depolymerization can also be modified by interaction with specific microtubule-associated proteins (MAPs). These proteins, such as MAP-1, 2, 3, and 4, MAP-, and TOG , regulate microtubule assembly and anchor the microtubules to specific organelles. MAPs are also responsible for the existence of stable populations of nondepolymerizing microtubules in the cell, such as those found in cilia and ﬂagella.
The length of microtubules changes dynamically as tubulin dimers are added or removed in a process of dynamic instability.
Microtubules observed in cultured cells with real-time video microscopy appear to be constantly growing toward the cell periphery (by addition of tubulin dimers) and then suddenly shrink in the direction of the MTOC (by removal of tubulin dimers). This constant remodeling process, known as dynamic instability, is linked to a pattern of GTP hydrolysis during the microtubule assembly and disassembly process. The MTOC can be compared to a feeding chameleon, which fires its long, projectile tongue to make contact with potential food. The chameleon then retracts its tongue back into its mouth and repeats this process until it is successful in obtaining food. The same strategy of “firing” microtubules from the MTOC toward the cell periphery and subsequently retracting them enables the cell to establish an organized system of mi- ( end) ( end) tubulin dimer bound to GTP tubulin dimer bound to GDP 24 nm Pi Pi -tubulin -tubulin -tubulin capping proteins