Assembly MAPs bind to the sides of microtubules and project outward. They may cross-link adjacent microtubles and determine the spacing of microtubules. Some link microtubules to membranes or to intermediate filaments. Type I MAPs are found mainly in axons and dendrites of nerve cells, and in some non-neural cells. Micrographs in L p. 798, 808.
Some MAPs stabilize microtubules , inhibiting depolymerization.
Phosphorylation , via kinases involved in cell cycle regulation, may inhibit binding of MAPs to microtubules. For example, MAP kinase , which is activated by growth factor signal cascades, phosphorylates MAPs along with other proteins. Phosphatases that remove phosphate from MAPs are also regulated. At the onset of mitosis , microtubules depolymerize and tubulin dimers are used to form the mitotic spindle. At the end of mitosis, the microtubule array of interphase is reestablished.