Key Concepts of Section 18.2

• Individual microtubule (+) ends can undergo dynamic instability, showing alternating periods of growth or rapid shrinkage (see Figure 18-10).

• Most of the β-tubulin in microtubules contains bound GDP. In growing microtubules, the (+) ends have a cap of a few dimers containing GTP-β-tubulin and GDP-Pi-β-tubulin and are blunt or slightly splayed out. Shrinking microtubules have lost the cap, causing the protofilaments at the end, containing GDP-β-tubulin, to peel outward and disassemble (see Figure 18-11).

• Growing microtubules store the energy derived from GTP hydrolysis in the microtubule lattice, so they have the potential to do work when disassembling.

• Microtubules assembled from the centrosome and exhibiting dynamic instability can “search” the cytoplasm for structures or organelles with appropriate targets and “capture” them, which results in stabilization of the microtubule (+) end. In this way, assembly coupled with “search and capture” can contribute to the overall distribution of microtubules in a cell.