Modification of microtubule steady-state dynamics by phosphorylation of the microtubule-associated proteins.

Phosphorylation of purified microtubule-associated proteins (MAPs) inhibits the rate and extent of MAP-stimulated microtubule assembly. The extent of microtubule assembly is reduced as a result of a decrease in the fraction of tubulin polymerized, without a significant change in the critical protein concentration. The decreased rate of microtubule assembly using phosphorylated MAPs reflects a reduction in microtubule nucleation resulting in fewer, but 2-fold longer, microtubules at steady state. Analysis of microtubule (MT) dynamics at steady state reveals that the rate of directional incorporation of subunits (flux) is 22 subunits.MT-1.sec-1 with phosphorylated MAPs, compared to 10 subunits.MT-1.sec-1 with unphosphorylated MAPs. The initial rate of disassembly determined by isothermal dilution is 232 subunits.MT-1.sec-1 for microtubules assembled with phosphorylated MAPs, compared to 102 subunits.MT-1.sec-1 for microtubules assembled with unphosphorylated MAPs. By using these results, the directionality (the number of successful subunit additions relative to the total number of association events per unit time) for subunit addition is found to be 0.1 for microtubules assembled with either phosphorylated or unphosphorylated MAPs. These observations are interpreted in terms of a mechanism in which phosphorylation of MAPs increases the rate of steady-state subunit flux by an equivalent enhancement of the rates of subunit association and dissociation, such that the critical protein concentration and directionality remain unchanged.