The study investigates the mechanism by which the microtubule-associated protein (MAP) tau modulates microtubule assembly and disassembly. Using dark-field microscopy, the authors observed individual microtubules in real time to examine their growth and shrinkage dynamics. They found that tau increases the rate of microtubule polymerization, decreases the rate of depolymerization, and significantly reduces the frequency of transitions from the growing to the shrinking phase (catastrophe rate). The effect of tau on catastrophe rate is independent of its ability to increase the elongation rate. Phosphorylation of tau by MAP2 kinase, but not by CAM kinase, reduces its activity by lowering the affinity of tau for the microtubule lattice. These findings suggest that tau generates a partially stable but still dynamic state in microtubules, which is perturbed by phosphorylation, affecting all three activities of microtubule dynamics.The study investigates the mechanism by which the microtubule-associated protein (MAP) tau modulates microtubule assembly and disassembly. Using dark-field microscopy, the authors observed individual microtubules in real time to examine their growth and shrinkage dynamics. They found that tau increases the rate of microtubule polymerization, decreases the rate of depolymerization, and significantly reduces the frequency of transitions from the growing to the shrinking phase (catastrophe rate). The effect of tau on catastrophe rate is independent of its ability to increase the elongation rate. Phosphorylation of tau by MAP2 kinase, but not by CAM kinase, reduces its activity by lowering the affinity of tau for the microtubule lattice. These findings suggest that tau generates a partially stable but still dynamic state in microtubules, which is perturbed by phosphorylation, affecting all three activities of microtubule dynamics.