Microtubule Organization and Dynamics
■ Tubulins belong to an ancient family of GTPases that polymerize to form microtubules, hollow cylindrical structures 25 nm in diameter.
■ Microtubules, like actin microfilaments, exhibit both structural and functional polarity.
■ Dimeric a^-tubulin subunits interact end-to-end to form protofilaments, which associate laterally into microtubules (see Figure 20-7).
■ Microtubules exhibit structural polarity. Subunits are added and lost preferentially at one end, the (+) end.
■ Assembly and disassembly of microtubules depends on the critical concentration, Cc, of a^-tubulin subunits. Above the Cc, microtubules assemble; below the Cc, microtubules disassemble.
■ Microtubules exhibit two dynamic phenomena that are pronounced at tubulin concentrations near the Cc: (1) tread-milling, the addition of subunits at one end and their loss at the other end, and (2) dynamic instability, the oscillation between lengthening and shortening (see Figure 20-9).
■ The balance between growth and shrinkage of unstable microtubules depends on whether the exchangeable GTP bound to ^-tubulin is present on the (+) end or whether it has been hydrolyzed to GDP (see Figure 20-11).
■ Microtubule-associated proteins (MAPs) organize mi-crotubules and affect their stability. Some MAPs prevent or promote cytosolic microtubule depolymerization; other MAPs organize microtubules into bundles or cross-link them to membranes and intermediate filaments or both (see Table 20-1).
■ Various drugs, including colchicine and taxol, disrupt mi-crotubule dynamics and have an antimitotic effect. Some of these drugs are useful in the treatment of certain cancers.
■ Cell polarity including the organization of cell organelles, direction of membrane trafficking, and orientation of microtubules is determined by microtubule-organizing centers (MTOCs). Most interphase animal cells contain a single, perinuclear MTOC from which cytosolic microtubules radiate (see Figure 20-13).
■ Because microtubule assembly is nucleated from MTOCs, the (—) end of most microtubules is adjacent to the MTOC and the (+) end is distal (see Figure 20-14).
■ A ^-tubulin-containing complex is a major component of the pericentriolar material and is able to nucleate the polymerization of tubulin subunits to form microtubules in vitro.
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