The Effects of Magnesium on the Dynamic Instability of Individual Microtubules

Abstract

Microtubules have been shown to exhibit two contrasting states: slow growth and rapid shortening. Recently, we have characterized how the transitions between these phases change with tubulin concentration, and determined the rate constants for the slow elongation phase for each microtuble end (Walker et al., 1988). These studies, as well as the original study that proposed the existance of these two phases (Mitchison and Kirschner, 1984), were done in the presence of ImM magnesium (Mg). Recent work has demonstrated that Mg can have dramatic effects on both the assembly and disassembly phases of microtubule growth (O’Brien and Erickson, 1988; Gal et al., 1988), and that high Mg can promote bizarre microtubule behaviors, such as synchronous oscillations (Pirollet et al., 1987; Carlier et al., 1988; Mandelkow et al., 1988). However, these experiments were done in bulk solution, where the properties of individual microtubules, and differences between the ends, could not be determined. We therefore investigated the effects of Mg on the assembly and disassembly of individual microtubules.