MICROTUBULE FORMATION

The first stage of formation is called "nucleation". The process requires tubulin, Mg++ and GTP and also proceeds at 37 C. This stage is relatively slow until the microtubule is initially formed. Then the second phase, called "elongation" proceeds much more rapidly. 

During "nucleation", an alpha and a beta tubulin molecule join to form a heterodimer. Then these attach to other dimers to form oligomers which elongate to form protofilaments. Each dimer carries two GTP molecules. However the GTP that appears to function binds to the beta tubulin molecules. When a tubulin molecule adds to the microtubule, the GTP is hydrolyzed to GDP. Eventually the oligomers will join to form the ringed microtubule. The hydrolysis of GTP of course is facilitated at a temperature of 37 C and stopped at temperatures of 4 C. 

This figure shows that, as the oligomers assemble, they form a series of rings, 25 nm in diameter. In cross section, each ring consists of 13 beads. The rows of beads in longitudinal section are called protofilaments.

In the cell itself, microtubules are formed in an area near the nucleus called the "aster". Microtubules are polar with a plus end (fast growing) and a minus end (slow growing). Usually the minus end is the anchor point. In this figure, the plus end is shown to the left by the numerous tubulin dimers. This is the end that carries the GTP molecules which may be hydrolyzed to GDP. Hydrolysis is not necessary, however (see p 810 in text and discussion below). 

Tests have shown that microtubules will form normally with nonhydrolyzable GTP analog molecules attached. However, they will not be able to depolymerize (see below). Thus, the normal role of GTP hydrolysis may be to promote the constant growth of microtubules as they are needed by a cell. 

DYNAMIC INSTABILITY

Microtubules may vary in their rate of assembly and disassembly. Tubulin half life is nearly a full day, however, the half life of a given microtubule may be only 10 minutes. Thus, they are in a continued state of flux. This is believed to respond to the needs of the cell and is called "dynamic instability". Furthermore, there are regulatory processes that appear to control this in a cell. Microtubule growth would be promoted in a dividing or moving cell. However, microtubule growth would be more controlled in a stable, polarized cell. 

As described in your text, the cell can provide a GTP cap on the growing end of a microtubule to regulate further growth. This happens when the tubulin molecules are added faster than the GTP can be hydrolyzed. Thus, the microtubule becomes stable and does not depolymerize. It may also be encouraged to continue growing. Once the GTP is hydrolyzed, it begins to shrink, however. Another way of capping a microtubule is to put a structure at its end, such as a cell membrane.