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. One domain binds to tubulin polymers or unpolymerized tubulin. This speeds up polymerization, facilitates assembly and stabilizes the microtubules. The other end will bind to vesicles or granules.
MAPs vary with the cell type.
The best examples are found in neurons. Furthermore, it is believed that some of these MAPs may bind to special sites on the alpha tubulin that form after it is in the microtubule. These are sites where a specific molecule is acetylated or the tyrosine residue is removed from the carboxy terminal. These sites are important marker sites for stabilized microtubules, because they disappear when microtubules are depolymerized. This figure shows a 3-D view of a neuron with its processes containing microtubules.