Microtubule dynamics and the regulation by microtubule-associated proteins (MAPs).

Individual microtubules (MTs) repeat alternating phases of polymerization and depolymerization, a process known as "dynamic instability." The dynamic instability is regulated by various protein factors according to the requirement of cellular conditions. Heat-stable MAPs regulate the dynamic instability by increasing the rescue frequency. To explore the influence of MAP2, a heat-stable MAPs abundant in neuron, on in vitro MT dynamics, the distribution of MAP2 on individual MTs was correlated with the dynamic phase changes of the same MTs by optical microscopy. MAP2 distributed inhomogeneously along the length of MTs by forming high-density regions, clusters. Stops of depolymerization were always found to occur only at the cluster sites. Every cluster did not stop depolymerization, but depolymerization did always stop at a cluster site. We suggest that mode of distribution along MT is an important factor of the function of heat-stable MAPs.